I've been exploring Aurelia javascript UI framework recently to get some experience needed for our next big project. One thing that I couldn't implement out of the box was a kind of animation.

I have a grid of values bound to View Model. View Model communicates to server, receives any updates of data and the grid got immediately updated, all that works great with Aurelia. Now I want to highlight the cell which has just received an updated value with a small background animation, like this:

Updated cell animation

Aurelia has a library called aurelia-animator-css with a helper class to run CSS animation. If you use it directly in your View Model, you will end up with the code like

this.newMessageReceived =
    msg => {
        this.data.filter(i => i.id === msg.id).forEach(t => {
            let editedItemIdx = this.data.indexOf(i);
            var elem = this.element.querySelectorAll('tbody tr')[editedItemIdx + 1]
                                   .querySelectorAll('td')[3];

            this.animator.addClass(elem, 'background-animation').then(() => {
                this.animator.removeClass(elem, 'background-animation');
            });
        });
    };

So we get a new message, find the related item in our data, then find the index of that data. Then we use this index in query selector to get the exact row that needs animation, find the cell by hard coded index, and finally use animator to highlight the background.

Ouch... That smells. We spoiled our view model with view details, and all this code is very ugly and fragile.

Good news: we can improve the solution with the Aurelia's feature called Custom Attributes. Let's create a new javascript file and call it animateonchange.js:

import {customAttribute} from 'aurelia-framework';

@customAttribute('animateonchange')
export class AnimateOnChangeCustomAttribute {

}

I declared a class for our new attribute, so far it's empty. I imported customAttribute decorator from Aurelia framework: that the way we can define a name for our custom attribute. This can be avoided: if I change the name to AnimateonchangeCustomAttribute, Aurelia will infer the name from class name, but I want to stay explicit and keep the class name readable. Note that capital letters are not allowed in attribute name.

Now, let's declare the constructor of the new class and inject all the dependencies:

import {inject, customAttribute} from 'aurelia-framework';
import {CssAnimator} from 'aurelia-animator-css';

@customAttribute('animateonchange')
@inject(Element, CssAnimator)
export class AnimateOnChangeCustomAttribute {

    constructor(element, animator) {
        this.element = element;
        this.animator = animator;
    }

}

I used dependency injection to get attribute's element and CSS animator and save them into class fields. Here's how to use them:

import {inject, customAttribute} from 'aurelia-framework';
import {CssAnimator} from 'aurelia-animator-css';

@customAttribute('animateonchange')
@inject(Element, CssAnimator)
export class AnimateOnChangeCustomAttribute {
    constructor(element, animator) {
        this.element = element;
        this.animator = animator;
        this.initialValueSet = false;
    }

    valueChanged(newValue){
        if (this.initialValueSet) {
            this.animator.addClass(this.element, 'background-animation').then(() => {
                this.animator.removeClass(this.element, 'background-animation');
            });
        }
        this.initialValueSet = true;
    }
}

The new method valueChanged will be called every time the bound value changes. I want to ignore the first value (it's not an update yet), so I did that with initialValueSet flag. Then I just run CSS animator. No DOM-related queries!

Here is how we use the custom attribute from a view:

<template>

    <require from="./animateonchange"></require>

    <table class="table">
        <tr repeat.for="item of data">
            <td>${item.value1}</td>
            <td>${item.value2}</td>
            <td animateonchange.bind="item.value3ToUpdate">${item.value3ToUpdate}</td>
            <td>${item.value4}</td>
        </tr>
    </table>

</template>

First, we use require element to import custom attribute definition (make sure the path is correct and no .js extension is present).

Second, we use animateonchange.bind to bind the value to the custom attributes. And it works!

Of course, you need to define the CSS class, e.g.

.background-animation-add {
    -webkit-animation: changeBack 0.5s;
    animation: changeBack 0.5s;
}
.background-animation-remove {
    -webkit-animation: fadeIn 0.5s;
    animation: fadeIn 0.5s;
}
@-webkit-keyframes changeBack {
    0% { background-color: white; }
    50% { background-color: lightgreen; }
    100% { background-color: white; }
}
@keyframes changeBack {
    0% { background-color: white; }
    50% { background-color: lightgreen; }
    100% { background-color: white; }
}

Here is a plunkr link to a complete example

Happy coding!

Useful links: