Saturday, 6 April 2019

Lightning Web Component Animated Progress Bar

Lightning Web Component Animated Progress Bar


Lightning Web Components went GA in Spring 19, to the delight of some and consternation of others. Rather than a critique, which would be pretty short at the moment given how little real life usage I’ve had out of them, I decided to take one of my Aura Components (remember, Lightning Components are renamed Aura Components to avoid confusion with Lightning Web Components) and convert that to a Lightning Web Component.

The component in question is from my Animated Lightning Progress Bar blog post, which as the name suggests animates a progress bar from the current value to the desired value. 

Tracking Values

Properties of the Lightning Web Component annotated with @track annotation are tracked (no kidding!) by the framework and the contents of the component are automatically re-rendered when a tracked value changes. In this example I have one tracked property:

  • value - the current value being displayed in the progress bar

As mentioned in the original blog post, animating a progress bar is simply a matter of making small changes to the current value via a timer function, until it matches the desired value, and redrawing the progress bar every time the current value changes. By binding the progress bar element to the value property, every time I change it in JavaScript the progress bar is redrawn. Nice.

The other great thing about this is that I don’t have to use any special functions to read or write the value of the tracked property - I just use it or set it as I need to and the framework takes care of the rest, simply because of the annotation. I don’t think I’ll miss the component.get/set(‘v.<name>’) type functions that are liberally sprinkled over my Aura Component controllers.


One aspect immediately flagged up as different - I’m using the standard  lightning-input component with an onchange handler to fire when the user changes the value. The change handler is called every time the user types something, rather than when they tab out etc. Initially I was starting the animation as soon as the change event was received, but if the user then continued typing it wasn’t the greatest experience. To solve this, I delayed taking any action until 300ms after the user last pressed a key, via the standard JavaScript setTimeout function. When the change handler fires, it clears any existing timeout and detects if the value input by the user has changed. If it has, a  timeout is scheduled for 300ms time to start the animation. If the user continues typing then the timeout is continually cleared and rescheduled for another 300ms time. When the user finally stops typing (or stops for 300ms!) the timer fires and the animation is started.

The example animation increments or decrements the current value until it matches the amount input by the user, decrementing being used if a desired value is entered that is smaller than the current value. Another standard JavaScript function, setInterval, is used to execute a function every 100ms that updates the current value.  Once the final value matches the current value, the interval is cancelled.



What’s Different?

There are a few differences compared to the Aura component:

  • As mentioned above, property/attribute access is much cleaner. I also don’t have to pollute my markup with declarations of attributes.
  • In the Aura Component when the timer function fired it was outside the framework lifecycle, so needed to use the $A.getCallback function. In Lightning Web Components there’s no need to do this, I can just treat it as any other asynchronous Javascript function. This is very much the experience that I’ve been having with Lightning Web Components - everything feels a lot closer to standard JavaScript rather than programming against a library or framework.
  • My event handlers don’t have to have a “c.” prefix, again it looks like I’ve defined a regular JavaScript function and am using it.
  • Only one JavaScript file! No more JavaScript controllers that delegate to helpers, or business logic striped across these two and a renderer.
  • To make the component available to the Lightning App Builder, I add a targets stanza to the XML metadata for the Lightning Web Component, again avoiding polluting my markup with information for the framework.

The Code

There are three elements to this component, available at the following Gists:

One More Thing

In true Columbo style, there’s one more thing to mention that you’ll see in the component markup. The ESLint for Lightning Web Components will complain about setTimeout and setInterval. As I am using them for good reason, I don’t need to hear any more of it’s whining. I can stop this by adding a comment before the line of code that it dislikes:

// eslint-disable-next-line @lwc/lwc/no-async-operation
let timeoutRef = setInterval(function() {…});

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