Ember applications have long life-cycles. A user may navigate to several pages and use many different features before they leave the application. This makes JavaScript and Ember development unlike Rails development, where the lifecycle of a request is short and the environment disposed of after each request. It makes Ember development much more like iOS or video game development than traditional server-side web development.
It is good to note that this isn't something inherent to just Ember. Any single-page app framework or solution (Angular, React, Vue, Backbone...) must deal with this lifecycle of objects, and specifically with how async tasks can be entangled with a lifecycle.
There is a fantastic Ember addon, ember-concurrency that solves these problems in a very exciting and simple way. It is largely inspired by RxJS and the Observable pattern, both of which concern themselves with creating life-cycle-free async that, in practice, tend to be hard for developers to learn.
This addon introduces several functional utility methods to help manage async, object lifecycles, and the Ember runloop. These tools should provide a simple developer experience that allows engineers to focus on the business domain, and think less about the weird parts of working in a long-lived app.
ember install ember-lifeline
Ember Lifeline supports a functional API that enables entanglement - the association of async behavior to object instances. This allows you to write async code in your classes that can be automatically cleaned up for you when the object is destroyed.
The API is divided into two main parts:
- Run loop entanglement
- DOM event handler entanglement
Additionally, lifeline exposes a primative, disposables, that allows you to entangle functionality of your choosing.
runDisposables during your object's destroy method. This ensures lifeline will correctly dispose of any remaining async work.
import Component from '@ember/component';
import { runTask, runDisposables } from 'ember-lifeline';
export default Component.extend({
// use `runTask` method somewhere in this component
destroy() {
runDisposables(this); // ensure that lifeline will clean up any remaining async work
this._super(...arguments);
}
})Lifeline provides mixins that conveniently implement destroy, correctly calling runDisposables.
Lifeline also exposes a QUnit test helper to ensure you've correctly implemented runDisposables within your objects. Please see the Testing section below.
tl;dr Call runTask(obj, fn, timeout) on any object to schedule work.
Use runTask where you might use setTimeout, setInterval, or
Ember.run.later.
runTask will handle three common issues with the above APIs.
First, setTimeout and setInterval do not use the runloop. Ember uses
a work queuing mechanism called the runloop .
In order for the queues to flush without autoruns (a feature that helps devs
be lazy in development but is disabled in tests and harms performance), a
runloop must be added around a callstack. For example:
import Component from '@ember/component';
import { run } from '@ember/runloop';
export default Component.extend({
init() {
this._super(...arguments);
window.setTimeout(() => {
run(() => {
this.set('date', new Date);
});
}, 500);
}
});There are several ways to add runloops in the Ember API docs,
but regardless it is less than ideal to need to remember and reason about this.
Often Ember.run.later is used instead of setTimeout, for this reason. However
that still has issues.
Second, none of setTimeout, setInterval or Ember.run.later bind the
timeout to the lifecycle of the context object. If the example above is
re-written to use Ember.run.later...
import Component from '@ember/component';
import { run } from '@ember/runloop';
export default Component.extend({
init() {
this._super(...arguments);
run.later(() => {
this.set('date', new Date);
}, 500);
}
});We're still making a dangerous assumption that this component instance
still exists 500ms from now. In practice, especially with tests, objects
scheduling timers may be destroyed by the time the timer fires. This causes
a number of unexpected errors. To fix this, the codebase is littered
with checks for isDestroyed state on objects retained after destruction:
import Component from '@ember/component';
import { run } from '@ember/runloop';
export default Component.extend({
init() {
this._super(...arguments);
run.later(() => {
// First, check if this object is even valid
if (this.isDestroyed) { return; }
this.set('date', new Date);
}, 500);
}
});The code above is correct, but again, less than simple to write.
Instead, always use runTask. runTask entangles a timer with the
lifecycle of the object scheduling the work. When the object is destroyed,
the task is also cancelled.
Using runTask, the above can be written as:
import Component from '@ember/component';
import { runTask, runDisposables } from 'ember-lifeline';
export default Component.extend({
init() {
this._super(...arguments);
runTask(this, () => {
this.set('date', new Date);
}, 500);
},
destroy() {
runDisposables(this);
this._super(...arguments);
}
});Once you've ensured your object calls runDisposables in its destroy method, there's no need to worry about cancellation or the isDestroyed status of the object itself.
tl;dr Call scheduleTask(obj, queueName, fnOrMethodName, args*) on any object to schedule work on the run loop.
Use scheduleTask where you might use Ember.run.schedule.
Like runTask, scheduleTask avoids common pitfalls of deferred work.
Ember.run.schedule does not bind the scheduled work to the lifecycle of the context object.
import Component from '@ember/component';
import { run } from '@ember/runloop';
export default Component.extend({
init() {
this._super(...arguments);
run.schedule('actions', this, () => {
this.set('date', new Date);
});
}
});There's a chance that objects scheduling work may be destroyed by the time the
queue is flushed. Leaving behavior to chance invites flakiness. This manifests
as a number of unexpected errors. Fixing this issue requires checks for
isDestroyed state on objects retained after destruction:
import Component from '@ember/component';
import { run } from '@ember/runloop';
export default Component.extend({
init() {
this._super(...arguments);
run.schedule('actions', this, () => {
// First, check if this object is even valid
if (this.isDestroyed) { return; }
this.set('date', new Date);
});
}
});The code above is correct, but less than ideal. Instead, always use
scheduleTask. scheduleTask entangles a scheduled task with the lifecycle of
the object scheduling the work. When the object is destroyed, the task is also
cancelled.
Using scheduleTask, the above can be written as:
import Component from '@ember/component';
import { scheduleTask, runDisposables } from 'ember-lifeline';
export default Component.extend({
init() {
this._super(...arguments);
scheduleTask(this, 'actions', () => {
this.set('date', new Date);
});
},
destroy() {
runDisposables(this);
this._super(...arguments);
}
});Scheduling work on the afterRender queue has well known, negative performance implications.
Therefore, scheduleTask is prohibited from scheduling work on the afterRender queue.
tl;dr Call debounceTask(obj, methodName, args*, wait, immediate) on any object to debounce work.
Debouncing is a common async pattern often used to manage user input. When a task is debounced with a timeout of 100ms, it first schedules the work for 100ms later. Then if the same task is debounced again with (again) a timeout of 100ms, the first timer is cancelled and a new one made for 100ms after the second debounce request. If no request to debounce that task is made for 100ms, the task executes.
Here is a good blog post about debounce and throttle patterns: jQuery throttle / debounce: Sometimes, less is more!
Debouncing is a pattern for managing scheduled work over time, and so it
falls prey to some of the same faults as setTimeout. Again Ember provides
Ember.run.debounce to handle the runloop aspect, but does not provide
a simple solution for cancelling work when the object is destroyed.
Enter debounceTask. For example, no matter how quickly you click on this
component, it will only report the time if you have stopped clicking
for 500ms:
import Component from '@ember/component';
import { debounceTask, runDisposables } from 'ember-lifeline';
export default Component.extend({
click() {
debounceTask(this, 'reportTime', 500);
},
reportTime() {
this.set('time', new Date());
},
destroy() {
runDisposables(this);
this._super(...arguments);
}
});However if the component is destroyed, any pending debounce task will be cancelled.
tl;dr Call throttleTask(obj, methodName, args*, spacing, immediate) on any object to throttle work.
When a task is throttled, it is executed immediately. For the length of the
timeout, additional throttle calls are ignored. Again, like debounce, throttle
falls prey to many issues shared by setTimeout, though fewer since the
work itself is always run immediately. Regardless even just for
consistency the API of throttleTask is presented:
import Component from '@ember/component';
import { throttleTask, runDisposables } from 'ember-lifeline';
export default Component.extend({
click() {
throttleTask(this, 'reportTime', 500);
},
reportTime() {
this.set('time', new Date());
},
destroy() {
runDisposables(this);
this._super(...arguments);
}
});In this example, the first click will update time, but clicks after that
for 500ms will be disregarded. Then, the next click will fire and start
a timeout window of its own.
Often it is desired to pass additional arguments to the throttle task. We also need to reference the same function in order for throttling to work. In order to acheive this it is recommended to make use of instance variables. This enables the throttle function to use the arguments in the state they are in at the time the task is executed:
import Component from '@ember/component';
import { throttleTask, runDisposables } from 'ember-lifeline';
export default Component.extend({
click(evt) {
this._evt = evt;
throttleTask(this, 'updateClickedEl', 500);
},
updateClickedEl() {
this.set('lastClickedEl', this._evt.target);
this._evt = null;
},
destroy() {
runDisposables(this);
this._super(...arguments);
}
});tl;dr call pollTask(obj, fn [, token]) on any object to setup polling.
Use pollTask where you might reach for recursive runTask(obj, fn, ms), Ember.run.later, setTimeout, and/or setInterval.
When using recursive runTask or run.later invocations causes tests to pause forever. This is due to the fact
that the Ember testing helpers automatically wait for all scheduled tasks in the run loop to finish before
resuming execution in the normal test context.
And as a reminder, setInterval should never be used. Say you setInterval(fn, 20);.
Regardless of how long fn takes, a new call will be scheduled every
20ms. For example if fn took 80ms to run (not uncommon), then four
new fn calls would be in the browser's event queue waiting to fire
immediately. This causes memory issues (the queue may never flush)
and performance problems. Instead, you should be scheduling new work
after the previous work was done. For example:
import Component from '@ember/component';
import { runTask, runDisposables } from 'ember-lifeline';
export default Component.extend({
init() {
this._super(...arguments);
this.updateTime();
},
updateTime() {
this.set('date', new Date());
runTask(this, () => this.updateTime(), 20);
},
destroy() {
runDisposables(this);
this._super(...arguments);
}
});In this way the true delay between setting date is 20ms + time the rendering took.
However, more work is still needed since when used in an acceptance test, the snippet above will cause the test to never complete.
To avoid this testing "freezing" behavior, we would need to update the component to have different behavior when testing than when running in normal development / production. Typically, this is done something like:
import Ember from 'ember';
import Component from '@ember/component';
import { runTask, runDisposables } from 'ember-lifeline';
export default Component.extend({
init() {
this._super(...arguments);
this.updateTime();
},
updateTime() {
this.set('date', new Date());
if (!Ember.testing) {
runTask(this, () => this.updateTime(), 20);
}
},
destroy() {
runDisposables(this);
this._super(...arguments);
}
});Unfortunately, this makes it very difficult to actually test that the polling is happening, and often times the polling behavior is itself either fundamental to the objects purpose or difficult enough to warrant its own tests.
This is where pollTask really shines. You could rewrite the above example to use pollTask
like this:
import Component from '@ember/component';
import { inject } from '@ember/service';
import { runTask, pollTask, runDisposables } from 'ember-lifeline';
export default Component.extend({
time: inject(),
init() {
this._super(...arguments);
// you can optionally provide a user defined token as a third argument
this._pollToken = pollTask(this, 'updateTime');
},
updateTime(next) {
let time = this.get('time');
this.set('date', time.now());
runTask(this, next, 20);
},
destroy() {
runDisposables(this);
this._super(...arguments);
}
});In development and production, the updateTime method is executed initially during the components
init and then recursively called every 20ms after its processing finishes. When the component is
destroyed (e.g. no longer rendered on screen) any pending timers from runTask or debounceTask
calls are properly canceled (as usual with those methods).
If you want to stop polling at any time, you will need to call cancelPoll
instead of runTask(this, next, delay) - cancelPoll cleans up internal data
associated with the poll, avoiding a memory leak. You can also call cancelPoll
from outside the poll loop.
In testing, the updateTime method would execute initially during the components instantiation (just like
in development and production environments), but would not automatically start polling. This allows
tests that are not related to the polling behavior to continue uninterrupted. To test the actual polling
functionality, use the provided pollTaskFor helper:
import moduleForComponent from 'ember-qunit';
import wait from 'ember-test-helpers/wait';
import { pollTaskFor } from 'ember-lifeline';
import Service from '@ember/service';
let fakeNow;
moduleForComponent('updating-time', {
integration: true,
beforeEach() {
this.register('service:time', Service.extend({
now() {
return fakeNow;
}
}));
}
});
test('updating-time updates', function(assert) {
fakeNow = new Date(2016);
this.render(hbs`
{{#updating-time as |time|}}
{{time}}
{{/updating-time}}
`);
assert.equal(this.$().text().trim(), fakeNow);
return wait()
.then(() => {
fakeNow = new Date(2017);
// you can optionally provide a user defined token
pollTaskFor(this._pollToken);
return wait();
})
.then(() => {
assert.equal(this.$().text().trim(), fakeNow);
});
});Note: If nothing has been queued for the given token, calling pollTaskFor(token) will trigger an error.
tl;dr call registerDisposable(obj, fn) on any object to register a function you want to run when the object is destroying.
Use registerDisposable as a replacement for explictly disposing of any externally managed resources. A disposable is a function that disposes of resources that are outside of Ember's lifecyle. This essentially means you can register a function that you want to run to automatically tear down any resources when the Ember object is destroyed.
Example:
It's common to see code written to explicitly unbind event handlers from external libraries.
// app/components/foo-bar.js
import Component from '@ember/component';
import { run } from '@ember/runloop';
import DOMish from 'some-external-library';
export default Component.extend({
init() {
this._super(...arguments);
this.DOMish = new DOMish();
this.bindEvents();
},
destroy() {
this._super(...arguments);
this.unbindEvents();
},
bindEvents() {
this.DOMish.on('foo', run.bind(this.respondToDomEvent));
},
unbindEvents() {
this.DOMish.off('foo');
}
respondToDOMEvent() {
// do something
}
});This not only adds verbosity to code, but also requires that you symetrically tear down any bindings you setup. By utilizing the registerDisposable API, ember-lifeline will ensure your registered disposable function will run when the object is destroyed, provided that you call runDisposables during your objects destruction.
// app/components/foo-bar.js
import Component from '@ember/component';
import { run } from '@ember/runloop';
import { registerDisposable, runDisposables } from 'ember-lifeline';
import DOMish from 'some-external-library';
export default Component.extend({
init() {
this._super(...arguments);
this.DOMish = new DOMish();
this.bindEvents();
},
destroy() {
runDisposables(this);
this._super(...arguments);
}
bindEvents() {
let onFoo = run.bind(this.respondToDomEvent);
this.DOMish.on('foo', onFoo);
this.domFooToken = registerDisposable(this, () => this.DOMish.off('foo', onFoo));
},
respondToDOMEvent() {
// do something
}
});The registerDisposable method returns a disposable, which is an object with the following interface:
interface IDisposable {
dispose: function;
disposed: boolean;
}tl;dr call addEventListener(obj, element, eventName, fn, options) on a
component or route to add a DOM event listener that will be automatically
removed when the component is un-rendered.
Event listeners pose similar but different challenges. They likewise must have a
runloop added around their callback, and are entangled with an object's lifecycle, in
this case to the detachment of that component from the DOM
(willDestroyElement). For example this is an idiomatic and correct way to add
an event listener to the window in Ember:
import Component from '@ember/component';
import { run } from '@ember/runloop';
export default Component.extend({
didInsertElement() {
this._super(...arguments);
$(window).on(`scroll.${this.elementId}`, (e) => {
run(() => {
this.set('windowScrollOffset', e.clientY);
});
});
},
willDestroyElement() {
$(window).off(`scroll.${this.elementId}`);
this._super(...arguments);
}
});This verbosity, and the need to do so many things right by hand, is very
unfortunate. With addEventListener the above example can be re-written as:
import Component from '@ember/component';
import { addEventListener } from 'ember-lifeline';
export default Component.extend({
didInsertElement() {
this._super(...arguments);
addEventListener(this, window, 'scroll', (e) => {
this.set('windowScrollOffset', e.clientY);
});
},
destroy() {
runDisposables(this);
this._super(...arguments);
}
});addEventListener will provide the runloop and remove the
listener when destroy is called, provided runDisposables is called.
addEventListener provides several ways to specify an element:
// Attach to an element inside this component
addEventListener(this, '.someClass', 'scroll', fn);
// Attach to the component's element itself
addEventListener(this, 'scroll', fn);
// Attach to a DOM node
addEventListener(this, document.body, 'click', fn);
// Attach to window
addEventListener(this, window, 'scroll', fn);tl;dr call removeEventListener(obj, element, eventName, fn, options) on a
component or route to actively remove a DOM event listener previously added by a
call to addEventListener.
Although any listener added by a call to addEventListener will be teared down when the route or component is being
destroyed, there might be cases where you want to actively remove an existing event listener even during the active
lifecycle, for example when temporarily dealing with high volume events like scroll or mousemove.
Be sure to pass the identical arguments used when calling addEventListener!
Ember lifeline also provides mixins, which extend the object's methods to include lifeline's functions.
To use any of the above mentioned functions in your component, route or service, simply import and apply one or all of these mixins to your class:
ContextBoundTasksMixinfor using any of the *Task methodsContextBoundEventListenersMixinfor using addEventListenerDisposableMixinfor using registerDisposable and runDisposable
Lifeline's entire purpose is to help ensure you've entangled and ultimately disposed of any outstanding async work in your applications. To help ensure this has occurred, and that you don't have any remaining queued async work, a test helper is provided which will assert that all async is disposed of.
To use the helper using the new ember-qunit module syntax:
// test-helper.js
import { module, test } from 'qunit';
import { setupTest } from 'ember-qunit';
import setupLifelineValidation from 'ember-lifeline/test-support';
module('module', function(hooks) {
setupLifelineValidation(hooks); // should be called before other setup functions
setupTest(hooks);
setupRenderingTest(hooks);
test('test', function(assert) {
...
})
})If a failure occurs, lifeline will output an array containing the module names that were the cause of the async leakage.
This addon was developed internally at Twitch, written originally by @mixonic and @rwjblue.
The name ember-lifeline was suggested by @nathanhammod.