Post pobrano z: Simple Swipe With Vanilla JavaScript

I used to think implementing swipe gestures had to be very difficult, but I have recently found myself in a situation where I had to do it and discovered the reality is nowhere near as gloomy as I had imagined.

This article is going to take you, step by step, through the implementation with the least amount of code I could come up with. So, let’s jump right into it!

The HTML Structure

We start off with a .container that has a bunch of images inside:

<div class='container'>
  <img src='img1.jpg' alt='image description'/>
  ...
</div>

Basic Styles

We use display: flex to make sure images go alongside each other with no spaces in between. align-items: center middle aligns them vertically. We make both the images and the container take the width of the container’s parent (the body in our case).

.container {
  display: flex;
  align-items: center;
  width: 100%;
  
  img {
    min-width: 100%; /* needed so Firefox doesn't make img shrink to fit */
    width: 100%; /* can't take this out either as it breaks Chrome */
  }
}

The fact that both the .container and its child images have the same width makes these images spill out on the right side (as highlighted by the red outline) creating a horizontal scrollbar, but this is precisely what we want:

Given that not all the images have the same dimensions and aspect ratio, we have a bit of white space above and below some of them. So, we’re going to trim that by giving the .container an explicit height that should pretty much work for the average aspect ratio of these images and setting overflow-y to hidden:

.container {
  /* same as before */
  overflow-y: hidden;
  height: 50vw;
  max-height: 100vh;
}

The result can be seen below, with all the images trimmed to the same height and no empty spaces anymore:

Alright, but now we have a horizontal scrollbar on the .container itself. Well, that’s actually a good thing for the no JavaScript case.

Otherwise, we create a CSS variable --n for the number of images and we use this to make .container wide enough to hold all its image children that still have the same width as its parent (the body in this case):

.container {
  --n: 1;
  width: 100%;
  width: calc(var(--n)*100%);
  
  img {
    min-width: 100%;
    width: 100%;
    width: calc(100%/var(--n));
  }
}

Note that we keep the previous width declarations as fallbacks. The calc() values won’t change a thing until we set --n from the JavaScript after getting our .container and the number of child images it holds:

const _C = document.querySelector('.container'), 
      N = _C.children.length;

_C.style.setProperty('--n', N)

Now our .container has expanded to fit all the images inside:

Switching Images

Next, we get rid of the horizontal scrollbar by setting overflow-x: hidden on our container’s parent (the body in our case) and we create another CSS variable that holds the index of the currently selected image (--i). We use this to properly position the .container with respect to the viewport via a translation (remember that % values inside translate() functions are relative to the dimensions of the element we have set this transform on):

body { overflow-x: hidden }

.container {
  /* same styles as before */
  transform: translate(calc(var(--i, 0)/var(--n)*-100%));
}

Changing the --i to a different integer value greater or equal to zero, but smaller than --n, brings another image into view, as illustrated by the interactive demo below (where the value of --i is controlled by a range input):

See the Pen by thebabydino (@thebabydino) on CodePen.

Alright, but we don’t want to use a slider to do this.

The basic idea is that we’re going to detect the direction of the motion between the "touchstart" (or "mousedown") event and the "touchend" (or "mouseup") and then update --i accordingly to move the container such that the next image (if there is one) in the desired direction moves into the viewport.

function lock(e) {};

function move(e) {};

_C.addEventListener('mousedown', lock, false);
_C.addEventListener('touchstart', lock, false);

_C.addEventListener('mouseup', move, false);
_C.addEventListener('touchend', move, false);

Note that this will only work for the mouse if we set pointer-events: none on the images.

.container {
  /* same styles as before */

  img {
    /* same styles as before */
    pointer-events: none;
  }
}

Also, Edge needs to have touch events enabled from about:flags as this option is off by default:

Before we populate the lock() and move() functions, we unify the touch and click cases:

function unify(e) { return e.changedTouches ? e.changedTouches[0] : e };

Locking on "touchstart" (or "mousedown") means getting and storing the x coordinate into an initial coordinate variable x0:

let x0 = null;

function lock(e) { x0 = unify(e).clientX };

In order to see how to move our .container (or if we even do that because we don’t want to move further when we have reached the end), we check if we have performed the lock() action, and if we have, we read the current x coordinate, compute the difference between it and x0 and resolve what to do out of its sign and the current index:

let i = 0;

function move(e) {
  if(x0 || x0 === 0) {
    let dx = unify(e).clientX - x0, s = Math.sign(dx);
  
    if((i > 0 || s < 0) && (i < N - 1 || s > 0))
      _C.style.setProperty('--i', i -= s);
	
    x0 = null
  }
};

The result on dragging left/ right can be seen below:

The above is the expected result and the result we get in Chrome for a little bit of drag and Firefox. However, Edge navigates backward and forward when we drag left or right, which is something that Chrome also does on a bit more drag.

In order to override this, we need to add a "touchmove" event listener:

_C.addEventListener('touchmove', e => {e.preventDefault()}, false)

Alright, we now have something functional in all browsers, but it doesn’t look like what we’re really after… yet!

Smooth Motion

The easiest way to move towards getting what we want is by adding a transition:

.container {
  /* same styles as before */
  transition: transform .5s ease-out;
}

And here it is, a very basic swipe effect in about 25 lines of JavaScript and about 25 lines of CSS:

Sadly, there’s an Edge bug that makes any transition to a CSS variable-depending calc() translation fail. Ugh, I guess we have to forget about Edge for now.

Refining the Whole Thing

With all the cool swipe effects out there, what we have so far doesn’t quite cut it, so let’s see what improvements can be made.

Better Visual Cues While Dragging

First off, nothing happens while we drag, all the action follows the "touchend" (or "mouseup") event. So, while we drag, we have no indication of what’s going to happen next. Is there a next image to switch to in the desired direction? Or have we reached the end of the line and nothing will happen?

To take care of that, we tweak the translation amount a bit by adding a CSS variable --tx that’s originally 0px:

transform: translate(calc(var(--i, 0)/var(--n)*-100% + var(--tx, 0px)))

We use two more event listeners: one for "touchmove" and another for "mousemove". Note that we were already preventing backward and forward navigation in Chrome using the "touchmove" listener:

function drag(e) { e.preventDefault() };

_C.addEventListener('mousemove', drag, false);
_C.addEventListener('touchmove', drag, false);

Now let’s populate the drag() function! If we have performed the lock() action, we read the current x coordinate, compute the difference dx between this coordinate and the initial one x0 and set --tx to this value (which is a pixel value).

function drag(e) {
  e.preventDefault();

  if(x0 || x0 === 0)  
    _C.style.setProperty('--tx', `${Math.round(unify(e).clientX - x0)}px`)
};

We also need to make sure to reset --tx to 0px at the end and remove the transition for the duration of the drag. In order to make this easier, we move the transition declaration on a .smooth class:

.smooth { transition: transform .5s ease-out; }

In the lock() function, we remove this class from the .container (we’ll add it again at the end on "touchend" and "mouseup") and also set a locked boolean variable, so we don’t have to keep performing the x0 || x0 === 0 check. We then use the locked variable for the checks instead:

let locked = false;

function lock(e) {
  x0 = unify(e).clientX;
  _C.classList.toggle('smooth', !(locked = true))
};

function drag(e) {
  e.preventDefault();
  if(locked) { /* same as before */ }
};

function move(e) {
  if(locked) {
    let dx = unify(e).clientX - x0, s = Math.sign(dx);

    if((i > 0 || s < 0) && (i < N - 1 || s > 0))
    _C.style.setProperty('--i', i -= s);
    _C.style.setProperty('--tx', '0px');
    _C.classList.toggle('smooth', !(locked = false));
    x0 = null
  }
};

The result can be seen below. While we’re still dragging, we now have a visual indication of what’s going to happen next:

Fix the transition-duration

At this point, we’re always using the same transition-duration no matter how much of an image’s width we still have to translate after the drag. We can fix that in a pretty straightforward manner by introducing a factor f, which we also set as a CSS variable to help us compute the actual animation duration:

.smooth { transition: transform calc(var(--f, 1)*.5s) ease-out; }

In the JavaScript, we get an image’s width (updated on "resize") and compute for what fraction of this we have dragged horizontally:

let w;

function size() { w = window.innerWidth };

function move(e) {
  if(locked) {
    let dx = unify(e).clientX - x0, s = Math.sign(dx), 
        f = +(s*dx/w).toFixed(2);

    if((i > 0 || s < 0) && (i < N - 1 || s > 0)) {
      _C.style.setProperty('--i', i -= s);
      f = 1 - f
    }
		
    _C.style.setProperty('--tx', '0px');
    _C.style.setProperty('--f', f);
    _C.classList.toggle('smooth', !(locked = false));
    x0 = null
  }
};

size();

addEventListener('resize', size, false);

This now gives us a better result.

Go back if insufficient drag

Let’s say that we don’t want to move on to the next image if we only drag a little bit below a certain threshold. Because now, a 1px difference during the drag means we advance to the next image and that feels a bit unnatural.

To fix this, we set a threshold at let’s say 20% of an image’s width:

function move(e) {
  if(locked) {
    let dx = unify(e).clientX - x0, s = Math.sign(dx), 
        f = +(s*dx/w).toFixed(2);

    if((i > 0 || s < 0) && (i < N - 1 || s > 0) && f > .2) {
      /* same as before */
    }
		
    /* same as before */
  }
};

The result can be seen below:

Maybe Add a Bounce?

This is something that I’m not sure was a good idea, but I was itching to try anyway: change the timing function so that we introduce a bounce. After a bit of dragging the handles on cubic-bezier.com, I came up with a result that seemed promising:

transition: transform calc(var(--f)*.5s) cubic-bezier(1, 1.59, .61, .74);

How About the JavaScript Way, Then?

We could achieve a better degree of control over more natural-feeling and more complex bounces by taking the JavaScript route for the transition. This would also give us Edge support.

We start by getting rid of the transition and the --tx and --f CSS variables. This reduces our transform to what it was initially:

transform: translate(calc(var(--i, 0)/var(--n)*-100%));

The above code also means --i won’t necessarily be an integer anymore. While it remains an integer while we have a single image fully into view, that’s not the case anymore while we drag or during the motion after triggering the "touchend" or "mouseup" events.

We then update the JavaScript to replace the code parts where we were updating these CSS variables. First, we take care of the lock() function, where we ditch toggling the .smooth class and of the drag() function, where we replace updating the --tx variable we’ve ditched with updating --i, which, as mentioned before, doesn’t need to be an integer anymore:

function lock(e) {
  x0 = unify(e).clientX;
  locked = true
};

function drag(e) {
  e.preventDefault();
	
  if(locked) {
    let dx = unify(e).clientX - x0, 
      f = +(dx/w).toFixed(2);
		
    _C.style.setProperty('--i', i - f)
  }
};

Before we also update the move() function, we introduce two new variables, ini and fin. These represent the initial value we set --i to at the beginning of the animation and the final value we set the same variable to at the end of the animation. We also create an animation function ani():

let ini, fin;

function ani() {};

function move(e) {
  if(locked) {
    let dx = unify(e).clientX - x0, 
        s = Math.sign(dx), 
        f = +(s*dx/w).toFixed(2);
		
    ini = i - s*f;

    if((i > 0 || s < 0) && (i < N - 1 || s > 0) && f > .2) {
      i -= s;
      f = 1 - f
    }

    fin = i;
    ani();
    x0 = null;
    locked = false;
  }
};

This is not too different from the code we had before. What has changed is that we’re not setting any CSS variables in this function anymore but instead set the ini and the fin JavaScript variables and call the animation ani() function.

ini is the initial value we set --i to at the beginning of the animation that the "touchend"/ "mouseup" event triggers. This is given by the current position we have when one of these two events fires.

fin is the final value we set --i to at the end of the same animation. This is always an integer value because we always end with one image fully into sight, so fin and --i are the index of that image. This is the next image in the desired direction if we dragged enough (f > .2) and if there is a next image in the desired direction ((i > 0 || s < 0) && (i < N - 1 || s > 0)). In this case, we also update the JavaScript variable storing the current image index (i) and the relative distance to it (f). Otherwise, it’s the same image, so i and f don’t need to get updated.

Now, let’s move on to the ani() function. We start with a simplified linear version that leaves out a change of direction.

const NF = 30;

let rID = null;

function stopAni() {
  cancelAnimationFrame(rID);
  rID = null
};

function ani(cf = 0) {
  _C.style.setProperty('--i', ini + (fin - ini)*cf/NF);
	
  if(cf === NF) {
    stopAni();
    return
  }
	
  rID = requestAnimationFrame(ani.bind(this, ++cf))
};

The main idea here is that the transition between the initial value ini and the final one fin happens over a total number of frames NF. Every time we call the ani() function, we compute the progress as the ratio between the current frame index cf and the total number of frames NF. This is always a number between 0 and 1 (or you can take it as a percentage, going from 0% to 100%). We then use this progress value to get the current value of --i and set it in the style attribute of our container _C. If we got to the final state (the current frame index cf equals the total number of frames NF, we exit the animation loop). Otherwise, we just increment the current frame index cf and call ani() again.

At this point, we have a working demo with a linear JavaScript transition:

However, this has the problem we initially had in the CSS case: no matter the distance, we have to have to smoothly translate our element over on release ("touchend" / "mouseup") and the duration is always the same because we always animate over the same number of frames NF.

Let’s fix that!

In order to do so, we introduce another variable anf where we store the actual number of frames we use and whose value we compute in the move() function, before calling the animation function ani():

function move(e) {
  if(locked) {
    let dx = unify(e).clientX - x0, 
      s = Math.sign(dx), 
      f = +(s*dx/w).toFixed(2);
		
    /* same as before */

    anf = Math.round(f*NF);
    ani();

    /* same as before */
  }
};

We also need to replace NF with anf in the animation function ani():

function ani(cf = 0) {
  _C.style.setProperty('--i', ini + (fin - ini)*cf/anf);
	
  if(cf === anf) { /* same as before */ }
	
  /* same as before */
};

With this, we have fixed the timing issue!

Alright, but a linear timing function isn’t too exciting.

We could try the JavaScript equivalents of CSS timing functions such as ease-in, ease-out or ease-in-out and see how they compare. I’ve already explained in a lot of detail how to get these in the previously linked article, so I’m not going to go through that again and just drop the object with all of them into the code:

const TFN = {
  'linear': function(k) { return k }, 
  'ease-in': function(k, e = 1.675) {
    return Math.pow(k, e)
  }, 
  'ease-out': function(k, e = 1.675) {
    return 1 - Math.pow(1 - k, e)
  }, 
  'ease-in-out': function(k) {
    return .5*(Math.sin((k - .5)*Math.PI) + 1)
  }
};

The k value is the progress, which is the ratio between the current frame index cf and the actual number of frames the transition happens over anf. This means we modify the ani() function a bit if we want to use the ease-out option for example:

function ani(cf = 0) {
  _C.style.setProperty('--i', ini + (fin - ini)*TFN['ease-out'](cf/anf));
	
  /* same as before */
};

We could also make things more interesting by using the kind of bouncing timing function that CSS cannot give us. For example, something like the one illustrated by the demo below (click to trigger a transition):

See the Pen by thebabydino (@thebabydino) on CodePen.

The graphic for this would be somewhat similar to that of the easeOutBounce timing function from easings.net.

The process for getting this kind of timing function is similar to that for getting the JavaScript version of the CSS ease-in-out (again, described in the previously linked article on emulating CSS timing functions with JavaScript).

We start with the cosine function on the [0, 90°] interval (or [0, π/2] in radians) for no bounce, [0, 270°] ([0, 3·π/2]) for 1 bounce, [0, 450°] ([0, 5·π/2]) for 2 bounces and so on… in general it’s the [0, (n + ½)·180°] interval ([0, (n + ½)·π]) for n bounces.

See the Pen by thebabydino (@thebabydino) on CodePen.

The input of this cos(k) function is in the [0, 450°] interval, while its output is in the [-1, 1] interval. But what we want is a function whose domain is the [0, 1] interval and whose codomain is also the [0, 1] interval.

We can restrict the codomain to the [0, 1] interval by only taking the absolute value |cos(k)|:

See the Pen by thebabydino (@thebabydino) on CodePen.

While we got the interval we wanted for the codomain, we want the value of this function at 0 to be 0 and its value at the other end of the interval to be 1. Currently, it’s the other way around, but we can fix this if we change our function to 1 - |cos(k)|:

See the Pen by thebabydino (@thebabydino) on CodePen.

Now we can move on to restricting the domain from the [0, (n + ½)·180°] interval to the [0, 1] interval. In order to do this, we change our function to be 1 - |cos(k·(n + ½)·180°)|:

See the Pen by thebabydino (@thebabydino) on CodePen.

This gives us both the desired domain and codomain, but we still have some problems.

First of all, all our bounces have the same height, but we want their height to decrease as k increases from 0 to 1. Our fix in this case is to multiply the cosine with 1 - k (or with a power of 1 - k for a non-linear decrease in amplitude). The interactive demo below shows how this amplitude changes for various exponents a and how this influences the function we have so far:

See the Pen by thebabydino (@thebabydino) on CodePen.

Secondly, all the bounces take the same amount of time, even though their amplitudes keep decreasing. The first idea here is to use a power of k inside the cosine function instead of just k. This manages to make things weird as the cosine doesn’t hit 0 at equal intervals anymore, meaning we don’t always get that f(1) = 1 anymore which is what we’d always need from a timing function we’re actually going to use. However, for something like a = 2.75, n = 3 and b = 1.5, we get a result that looks satisfying, so we’ll leave it at that, even though it could be tweaked for better control:

This is the function we try out in the JavaScript if we want some bouncing to happen.

const TFN = {
  /* the other function we had before */
  'bounce-out': function(k, n = 3, a = 2.75, b = 1.5) {
    return 1 - Math.pow(1 - k, a)*Math.abs(Math.cos(Math.pow(k, b)*(n + .5)*Math.PI))
  }
};

Hmm, seems a bit too extreme in practice:

Maybe we could make n depend on the amount of translation we still need to perform from the moment of the release. We make it into a variable which we then set in the move() function before calling the animation function ani():

const TFN = {
  /* the other function we had before */
  'bounce-out': function(k, a = 2.75, b = 1.5) {
    return 1 - Math.pow(1 - k, a)*Math.abs(Math.cos(Math.pow(k, b)*(n + .5)*Math.PI))
  }
};

var n;

function move(e) {
  if(locked) {
    let dx = unify(e).clientX - x0, 
      s = Math.sign(dx), 
      f = +(s*dx/w).toFixed(2);
    
    /* same as before */
		
    n = 2 + Math.round(f)
    ani();
    /* same as before */
  }
};

This gives us our final result:

There’s definitely still room for improvement, but I don’t have a feel for what makes a good animation, so I’ll just leave it at that. As it is, this is now functional cross-browser (without have any of the Edge issues that the version using a CSS transition has) and pretty flexible.

The post Simple Swipe With Vanilla JavaScript appeared first on CSS-Tricks.

Post pobrano z: List Rendering and Vue’s v-for Directive

List rendering is one of the most commonly used practices in front-end web development. Dynamic list rendering is often used to present a series of similarly grouped information in a concise and friendly format to the user. In almost every web application we use, we can see lists of content in numerous areas of the app.

In this article we’ll gather an understanding of Vue’s v-for directive in generating dynamic lists, as well as go through some examples of why the key attribute should be used when doing so.

Since we’ll be explaining things thoroughly as we start to write code, this article assumes you’ll have no or very little knowledge with Vue (and/or other JavaScript frameworks).

Case Study: Twitter

We’re going to use Twitter as the case study for this article.

When logged in and in the main index route of Twitter we’re presented with a view similar to this:

On the homepage, we’ve become accustomed to seeing a list of trends, a list of tweets, a list of potential followers, etc. The content displayed in these lists depends on a multitude of factors—our Twitter history, who we follow, our likes, etc. As a result, we can say all this data is dynamic.

Though this data is dynamically obtained, the way this data is shown remains the same. This is in part due to using reusable web components.

For example; we can see the list of tweets as a list of single tweet-component items. We can think of tweet-component as a shell that takes data of sorts, such as the username, handle, tweet and avatar, among other pieces, and that simply displays those pieces in a consistent markup.

Let’s say we wanted to render a list of components (e.g. a list of tweet-component items) based on a large data source obtained from a server. In Vue, the first thing that should come to mind to accomplish this is the v-for directive.

The v-for directive

The v-for directive is used to render a list of items based on a data source. The directive can be used on a template element and requires a specific syntax along the lines of:

Let’s see an example of this in practice. First, we’ll assume we’ve already obtained a collection of tweet data:

const tweets = [
  {
    id: 1,
    name: 'James',
    handle: '@jokerjames',
    img: 'https://semantic-ui.com/images/avatar2/large/matthew.png',
    tweet: "If you don't succeed, dust yourself off and try again.",
    likes: 10,
  },
  { 
    id: 2,
    name: 'Fatima',
    handle: '@fantasticfatima',
    img: 'https://semantic-ui.com/images/avatar2/large/molly.png',
    tweet: 'Better late than never but never late is better.',
    likes: 12,
  },
  {
    id: 3,
    name: 'Xin',
    handle: '@xeroxin',
    img: 'https://semantic-ui.com/images/avatar2/large/elyse.png',
    tweet: 'Beauty in the struggle, ugliness in the success.',
    likes: 18,
  }
]

tweets is a collection of tweet objects with each tweet containing details of that particular tweet—a unique identifier, the name/handle of the account, tweet message, etc. Let’s now attempt to use the v-for directive to render a list of tweet components based on this data.

First and foremost, we’ll create the Vue instance—the heart of the Vue application. We’ll mount/attach our instance to a DOM element of id app and assign the tweets collection as part of the instance’s data object.

new Vue({
  el: '#app',
  data: {
    tweets
  }
});

We’ll now go ahead and create a tweet-component that our v-for directive will use to render a list. We’ll use the global Vue.component constructor to create a component named tweet-component:

Vue.component('tweet-component', {
  template: `  
    <div class="tweet">
      <div class="box">
        <article class="media">
          <div class="media-left">
            <figure class="image is-64x64">
              <img :src="tweet.img" alt="Image">
            </figure>
          </div>
          <div class="media-content">
            <div class="content">
              <p>
                <strong>{{tweet.name}}</strong> <small>{{tweet.handle}}</small>
                <br>
                {{tweet.tweet}}
              </p>
            </div>
              <div class="level-left">
                <a class="level-item">
                  <span class="icon is-small"><i class="fas fa-heart"></i></span>
                  <span class="likes">{{tweet.likes}}</span>
                </a>
              </div>
          </div>
        </article>
      </div>
    </div>  
  `,
  props: {
    tweet: Object
  }
});

A few interesting things to note here.

1. The tweet-component expects a tweet object prop as seen in the prop validation requirement (props: {tweet: Object}). If the component is rendered with a tweet prop that is not an object, Vue will emit warnings.
2. We’re binding the properties of the tweet object prop on to the component template with the help of the Mustache syntax: {{ }}.
3. The component markup adapts Bulma’s Box element as it represents a good resemblance to a tweet.

In the HTML template, we’ll need to create the markup where our Vue app will be mounted (i.e. the element with the id of app). Within this markup, we’ll use the v-for directive to render a list of tweets. Since tweets is the data collection we’ll be iterating over, tweet will be an appropriate alias to use in the directive. In each rendered tweet-component, we’ll also pass in the iterated tweet object as props for it to be accessed in the component.

<div id="app" class="columns">
  <div class="column">
    <tweet-component v-for="tweet in tweets" :tweet="tweet"/>
  </div>
</div>

Regardless of how many more tweet objects would be introduced to the collection; or how they’ll change over time—our set up will always render all the tweets in the collection in the same markup we expect.

With the help of some custom CSS, our app will look something like this:

See the Pen Simple Twitter Feed #1 by Hassan Dj (@itslit) on CodePen.

Though everything works as expected, we may be prompted with a Vue tip in our browser console:

[Vue tip]: <tweet-component v-for="tweet in tweets">: component lists rendered with v-for should have explicit keys...

You may not be able to see the warning in the browser console when running the code through CodePen.

Why is Vue telling us to specify explicit keys in our list when everything works as expected?

key

It’s common practice to specify a key attribute for every iterated element within a rendered v-for list. This is because Vue uses the key attribute to create unique bindings for each node’s identity.

Let’s explain this some more—if there were any dynamic UI changes to our list (e.g. order of list items gets shuffled), Vue will opt towards changing data within each element instead of moving the DOM elements accordingly. This won’t be an issue in most cases. However, in certain instances where our v-for list depends on DOM state and/or child component state, this can cause some unintended behavior.

Let’s see an example of this. What if our simple tweet component now contained an input field that will allow the user to directly respond to the tweet message? We’ll ignore how this response could be submitted and simply address the new input field itself:

We’ll include this new input field on to the template of tweet-component:

Vue.component('tweet-component', {
  template: `
    <div class="tweet">
      <div class="box">
        // ...
      </div>
      <div class="control has-icons-left has-icons-right">
        <input class="input is-small" placeholder="Tweet your reply..." />
        <span class="icon is-small is-left">
          <i class="fas fa-envelope"></i>
        </span>
      </div>
    </div>
  `,
  props: {
    tweet: Object
  }
});

Assume we wanted to introduce another new feature into our app. This feature would involve allowing the user to shuffle a list of tweets randomly.

To do this; we can first include a “Shuffle!” button in our HTML template:

<div id="app" class="columns">
  <div class="column">
    <button class="is-primary button" @click="shuffle">Shuffle!</button>
    <tweet-component  v-for="tweet in tweets" :tweet="tweet"/>
  </div>
</div>

We’ve attached a click event listener on the button element to call a shuffle method when triggered. In our Vue instance; we’ll create the shuffle method responsible in randomly shuffling the tweets collection in the instance. We’ll use lodash’s _shuffle method to achieve this:

new Vue({
  el: '#app',
  data: {
    tweets
  },
  methods: {
    shuffle() {
      this.tweets = _.shuffle(this.tweets)
    }
  }
});

Let’s try it out! If we click shuffle a few times; we’ll notice our tweet elements get randomly assorted with each click.

See the Pen Simple Twitter Feed #2 by Hassan Dj (@itslit) on CodePen.

However, if we type some information in the input of each component then click shuffle; we’ll notice something peculiar happening:

Since we haven’t opted to using the key attribute, Vue has not created unique bindings to each tweet node. As a result, when we’re aiming to reorder the tweets, Vue takes the more performant saving approach to simply change (or patch) data in each element. Since the temporary DOM state (i.e. the inputted text) remains in place, we experience this unintended mismatch.

Here’s a diagram that shows us the data that gets patched on to each element and the DOM state that remains in place:

To avoid this; we’ll have to assign a unique key to every tweet-component rendered in the list. We’ll use the id of a tweet to be the unique identifier since we should safely say a tweet’s id shouldn’t be equal to that of another. Because we’re using dynamic values, we’ll use the v-bind directive to bind our key to the tweet.id:

<div id="app" class="columns">
  <div class="column">
    <button class="is-primary button" @click="shuffle">Shuffle!</button>
    <tweet-component  v-for="tweet in tweets" :tweet="tweet" :key="tweet.id" />
  </div>
</div>

Now, Vue recognizes each tweet’s node’s identity; and thus will reorder the components when we intend on shuffling the list.

Since each tweet component is now being moved accordingly, we can take this a step further and use Vue’s transition-group to show how the elements are being reordered.

To do this, we’ll add the transition-group element as a wrapper to the v-for list. We’ll specify a transition name of tweets and declare that the transition group should be rendered as a div element.

<div id="app" class="columns">
  <div class="column">
    <button class="is-primary button" @click="shuffle">Shuffle!</button>
    <transition-group name="tweets" tag="div">
      <tweet-component  v-for="tweet in tweets" :tweet="tweet" :key="tweet.id" />
    </transition-group>
  </div>
</div>

Based on the name of the transition, Vue will automatically recognize if any CSS transitions/animations have been specified. Since we aim to invoke a transition for the movement of items in the list; Vue will look for a specified CSS transition along the lines of tweets-move (where tweets is the name given to our transition group). As a result, we’ll manually introduce a .tweets-move class that has a specified type and time of transition:

#app .tweets-move {
  transition: transform 1s;
}

This is a very brief look into applying list transitions. Be sure to check out the Vue docs for detailed information on all the different types of transitions that can be applied!

Our tweet-component elements will now transition appropriately between locations when a shuffle is invoked. Give it a try! Type some information in the input fields and click “Shuffle!” a few times.

See the Pen Simple Twitter Feed #3 by Hassan Dj (@itslit) on CodePen.

Pretty cool, right? Without the key attribute, the transition-group element can’t be used to create list transitions since the elements are patched in place instead of being reordered.

Should the key attribute always be used? It’s recommended. The Vue docs specify that the key attribute should only be omitted if:

• We intentionally want the default manner of patching elements in place for performance reasons.
• The DOM content is simple enough.

Conclusion

And there we have it! Hopefully this short article portrayed how useful the v-for directive is as well as provided a little more context to why the key attribute is often used. Let me know if you may have any questions/thoughts whatsoever!

---

If you liked my style of writing and are potentially interested in learning how to build apps with Vue.js, you may like the book Fullstack Vue: The Complete Guide to Vue.js that I helped publish! The book covers numerous facets of Vue including but not restricted to routing, simple state management, form handling, Vuex, server persistence, and testing. If you’re interested, you can get more information from our website, https://fullstack.io/vue.

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Post pobrano z: Going Offline

Jeremy Keith has written a new book all about service workers and offline functionality that releases at the end of the month. The first chapter is posted on A List Apart. Now that the latest versions of iOS and macOS Safari support service workers, I can’t think of a better time to learn about how progressive web apps work under the hood. In fact, here’s an example of a simple offline site and a short series on making web apps work offline.

News of Jeremy’s book had me going back through his previous book, Resilient Web Design, where I half-remembered this super interesting quote from Chapter 4:

If you build something using web technologies, and someone visits with a web browser, you can’t be sure how many of the web technologies will be supported. It probably won’t be 100%. But it’s also unlikely to be 0%. Some people will visit with iOS devices. Others will visit with Android devices. Some people will get 80% or 90% of what you’ve designed. Others will get just 20%, 30%, or 50%. The web isn’t a platform. It’s a continuum.

I love this idea of the web as a continuum that’s constantly improving and growing over time and so I’m sure Jeremy’s latest book will be just as fun and interesting.

Direct Link to Article — Permalink

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Post pobrano z: Displaying the Weather With Serverless and Colors

I like to jog. Sometimes it’s cold out. Sometimes it’s cold out, but it looks like it isn’t. The sun is shining, the birds are chirping. Then you step outside in shorts and a t-shirt and realize you have roughly 2 minutes before exposure sets in.

I decided to solve this first world problem using a lightbulb to display a certain color based on what the temperature outside is. It works better than I expected, and that’s saying something because usually nothing works out like I want it to.

This was a fun project to build, and since it is essentially a hosted service running on a timer, it’s a perfect use case for Serverless.

Now you might be thinking, “um, wouldn’t it be easier to just check the weather?” Well, it would, but then I wouldn’t have an excuse to buy an expensive lightbulb or write an article with the word “Serverless.”

So let’s look at how you can build your own Weather Bulb. The final code is not complicated, but it does have some interesting pieces that are worth noting. By the way, did I mention that it’s Serverless?

Building the Weather Bulb

The first thing you are going to need is the bulb. You can’t have a Weather Bulb sans bulb. Say the word “bulb” out loud about 10 times and you’ll notice what a bizarre word it is. Bulb, bulb, bulb, bulb — see? Weird.

I am using the LIFX Mini Color. It’s not *too* expensive, but more importantly, it’s got an API that is wide open.

The API has two methods of authentication. The first contains the word “OAuth” and I’m already sorry that you had to read that. Don’t worry, there is an easier way that doesn’t involve OAu…. that which won’t be named.

The second way is to register an application with LIFX. You get back a key and all you have to do is pass that key with any HTTP request. That’s what I’m using for this demo.

For instance, if we wanted to change the bulb color to blue, we can just pass color: blue to the /state endpoint.

The API supports a few different color formats, including named colors (like red, blue), hex values, RBG, Kevlin, hue brightness and saturation. This is important because it factors into what proved to be the hardest part of this project: turning temperature into color.

Representing Temperature With Color

If you’ve ever watched a weather report, you’ll be familiar with the way that meteorology represents weather conditions with color on a map.

Usually, this is done to visualize precipitation. You have probably seen that ominous green strip of storms bearing down on you on a weather map while you try to figure out if you should get in the bathtub because you’re in the path of a tornado. Or maybe that’s just all of us unlucky souls here in America’s Tornado Alley.

Color is also used to represent temperature. This is precisely what I wanted to do with the bulb. The tough thing is that there doesn’t seem to be a standardized way to do this. Some maps show it as solid colors in bands. In this case, blue might represent the band from 0℉ – 32℉.

Others have it as a gradient scale which is more precise. This is what I was after for the Weather Bulb.

My first stab at solving this was just to Google “temperature color scale” and other various iterations of that search term. I got back a lot of information about Kelvin.

Kelvin is a representation of the temperature of a color. Literally. For any light source (light bulb, the sun, ect) the actual temperature of that source will affect the color of the light it emits. A fire burns a yellowish red color. The hotter that fire gets, the more it moves towards white. Hence the saying, “white hot”. So if someone ever says “red hot,” you can correct them in front of everyone because who doesn’t love a pedantic jerk?

The LIFX bulb supports Kelvin, so you might think that this would work. After all, this is the Kelvin scale….

The problem is that there is simply not enough color variation because these are not actual colors, but rather the tinge of color that a light is emitting based on it’s “temperature.” Here is the Kelvin color wheel that comes with the LIFX app.

These colors are barely distinguishable from one another on the bulb. Not exactly what I was after.

That leaves me with trying to convert the color to either Hex, RGB or some other format. This is tough because where do you begin? I spent an embarrassing amount of time adjust RGB scale values between blue for cold (0, 0, 255) and red for hot (255, 0, 0). It was about this time that it dawned on me that maybe HSL would be a better way to go here. Why? Because hue is a whole lot easier to understand.

Hue

Hue is a representation of color on a scale between 0 and 360. This is why we often see color represented on a wheel (360°). That’s a vast oversimplification, but unless you want me to start talking about wavelengths, let’s go with that definition.

The hue color wheel looks like this….

If we flatten it out, it’s easier to reason about.

We’re ready to convert temperature to color. The first thing we need to do is figure out a set temperature range. I went with 0℉ to 100℉. We can’t work with infinite temperature color combinations. Numbers go on forever, colors do not. It can only get so hot before our bulb is just bright red all the time, and that’s 100℉. The same is true for cold.

If light blue represents 0℉, I can start at about the 200 mark on the hue scale. Red will represent 100℉. You can see that red is at both extremes, so I can move either left OR right, depending on what colors I want to use to represent the temperature. It’s not the same as the colors they use in actual weather programs, but who cares? Obviously not me.

I chose to go right because there is no pink on the left and pink is my favorite color. I also felt like pink represents warm a bit better than green. Green is rain and tornadoes.

Now we can back into a hue based on temperature. Ready? Here we go.

Let’s pretend it’s a brisk 50℉ outside.

If 100℉ is the hottest we go (360) and 0℉ is the coldest (200), then we have a color scale of 160 points. To figure out where in that 160 point range we need to be, we can divide the current temperature by the upper bound of 100℉ which will give us the exact percentage we need to move in our range, or 50%. If we move 50% of the way into a 160 point range, that leaves us at 80. Since we are starting at 200, that gives us a hue of 280.

That sounds complicated, but only because word problems in math SUCK. Here’s how the code looks when it’s all said and done…

let hue = 200 + (160 * ( temperature / 100 ));

OK! We’ve got a dynamic color scale based on hue, and wouldn’t you know it, we can just pass the hue to LIFX as simply as we pass a named color.

Now we just need to find out what the current temperature is, back into a hue and do that every few minutes. Serverless, here we come!

Serverless Timer Functions

Serverless is all the rage. It’s like HTML5 used to be: it doesn’t matter what it is, it only matters that you know the word and are not afraid to use it in a blog post.

For this example, we’ll use Azure Functions because there is support for timer triggers, and we can test those timer triggers locally before we deploy using VS Code. One of the things about Serverless that irritates me to no end is when I can’t debug it locally.

Using the Azure Functions Core Tools and the Azure Functions Extension for VS Code, I can create a new Serverless project and select a Timer Trigger.

Timer Triggers in Azure Functions are specified as Cron Expressions. Don’t worry, I didn’t know what that was either.

Cron Expressions allow you to get very specific with interval definition. Cron breaks things down into second, minute, hour, day, month, year. So if you wanted to run something every second of every minute of every hour of every day of every year, your expression would look like this…

* * * * * *

If you wanted to run it every day at 10:15, it would look like this…

* 15 10 * * *

If you wanted to run it every 5 minutes (which is what Azure defaults to), you specify that by saying “when minutes is divisible by 5.”

0 */5 * * * *

For the purposes of this function, we set it to 2 minutes.

I am using a 2 minute interval because that’s how often we can call the weather API for free 💰.

Getting the Forecast From DarkSky

DarkSky has a wonderful weather API that you can call up to 1,000 times per day for free. If there are 1,440 minutes in a day (and there are), that means we can call DarkSky every 1.44 minutes per day and stay in the free zone. I just rounded up to 2 minutes because temperature doesn’t change that fast.

This is what our function looks like when we call the DarkSky API. All of my tokens, keys, latitude and longitude settings are in environment variables so they aren’t hardcoded. Those are set in the local.settings.json file. I used axios for my HTTP requests because it is a magical, magical package.

const axios = require('axios');

module.exports = function (context, myTimer) {
  // build up the DarkSky endpoint
  let endpoint = `${process.env.DS_API}/${process.env.DS_SECRET}/${process.env.LAT}, 
                  ${process.env.LNG}`;
  
  // use axios to call DarkSky for weather
  axios
    .get(endpoint)
    .then(response => {
      let temp = Math.round(response.data.currently.temperature);
    
      // TODO: Set the color of the LIFX bulb
    })
    .catch(err => {
      context.log(err.message);
    });
};

Now that I have the temperature, I need to call the LIFX API. And wouldn’t you know it, someone has already created an npm package to do this called lifx-http-api. This is why you love JavaScript.

Setting the Bulb Hue

After the weather result comes back, I need to use the LIFX API instance and call the setState method. This method returns a promise which means that we need to nest promises. Nesting promises can get out of hand and could land us right back in callback hell, which is what we’re trying to avoid with promises in the first place.

Instead, we’ll handle the first promise and then return Promise.all which we can handle at another top-level then. This just prevents us from nesting then statements.

Remember kids, promises are just socially acceptable callbacks.

const axios = require('axios');
const LIFX = require('lifx-http-api');

let client = new LIFX({
  bearerToken: process.env.LIFX_TOKEN
});

module.exports = function (context, myTimer) {
  // build up the DarkSky endpoint
  let endpoint = <code>${process.env.DS_API}/${process.env.DS_SECRET}/${
    process.env.LAT
    },${process.env.LNG}<code>;
    
  // use axios to call DarkSky for weather
  axios
    .get(endpoint)
    .then(response => {
      let temp = Math.round(response.data.currently.temperature);
      
      // make sure the temp isn't above 100 because that's as high as we can go
      temp = temp < 100 ? temp : 100;
      
      // determine the hue
      let hue = 200 + (160 * (temp / 100));

      // return Promise.all so we can resolve at the top level
      return Promise.all([
        data,
        client.setState('all', { color: <code>hue:${hue}<code> })
      ]);
    })
    .then(result => {
      // result[0] contains the darksky result
      // result[1] contains the LIFX result
      context.log(result[1]);
    })
    .catch(err => {
      context.log(err.message);
    });
};

Now we can run this thing locally and watch our timer do it’s thang.

That’s it! Let’s deploy it.

Deploying Weather Bulb

I can create a new Functions project from the VS Code extension.

I can right-click that to “Open in portal” where I can define a deployment source so it sucks my code in from Github and deploys it. This is ideal because now whenever I push a change to Github, my application automatically gets redeployed.

All Hail the Weather Bulb

Now just sit back and behold the soft glow of the Weather Bulb! Why look at the actual temperature when you can look at this beautiful shade of hot pink instead?

Can you guess what the temperature is based on what you know from this article? The person who leaves a comment and gets the closest will get a free LIFX lightbulb from me (because I ❤️ all of you), or the cost of the bulb if you are outside the U.S. (~$40).

You can grab all of the code for this project from Github.

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