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The "throttle" extender

Normally, computed observables are re-evaluated synchronously, as soon as each of their dependencies change. The throttle extender, however, causes a computed observable to delay re-evaluation until its dependencies have stopped changing for a specified period of time. Throttled computed observables therefore update asychronously.

The main uses cases for throttling are:

  • Making things respond after a certain delay
  • Combining multiple changes into a single re-evaluation (also known as “atomic updates”)

You’ll find examples of these below.

Example 1: The basics

Consider the computed observable in the following code:

var name = ko.observable('Bert');

var upperCaseName = ko.computed(function() {
    return name().toUpperCase();
});

Normally, if you update name as follows:

name('The New Bert');

… then upperCaseName will be recomputed immediately, before your next line of code runs. But if you had instead defined upperCaseName using throttle as follows:

var upperCaseName = ko.computed(function() {
    return name().toUpperCase();
}).extend({ throttle: 500 });

… then upperCaseName would not be recomputed immediately when name changes — instead, it would wait for 500 milliseconds (half a second) before recomputing its value and then notifying any associated UI. Each time name changes, that timeout is reset back to zero, so the re-evaluation only occurs once name has stopped changing for at least half a second.

Example 2: Doing something when the user stops typing

In this live example, there’s an instantaneousValue observable that reacts immediately when you press a key. This is then wrapped inside a throttledValue computed observable that’s configured to react only when you stop typing for at least 400 milliseconds.

Try it:

Type stuff here:

Current throttled value:

Stuff you have typed:

Source code: View

<p>Type stuff here: <input data-bind="value: instantaneousValue, valueUpdate: 'afterkeydown'" /></p>
<p>Current throttled value: <b data-bind="text: throttledValue"> </b></p>

<div data-bind="visible: loggedValues().length > 0">
    <h3>Stuff you have typed:</h3>
    <ul data-bind="foreach: loggedValues">
        <li data-bind="text: $data"></li>
    </ul>
</div>

Source code: View model

function AppViewModel() {
    this.instantaneousValue = ko.observable();
    this.throttledValue = ko.computed(this.instantaneousValue)
                            .extend({ throttle: 400 });

    // Keep a log of the throttled values
    this.loggedValues = ko.observableArray([]);
    this.throttledValue.subscribe(function (val) {
        if (val !== '')
            this.loggedValues.push(val);
    }, this);
}

ko.applyBindings(new AppViewModel());

Example 3: Avoiding multiple Ajax requests

The following model represents data that you could render as a paged grid:

function GridViewModel() {
    this.pageSize = ko.observable(20);
    this.pageIndex = ko.observable(1);
    this.currentPageData = ko.observableArray();

    // Query /Some/Json/Service whenever pageIndex or pageSize changes,
    // and use the results to update currentPageData
    ko.computed(function() {
        var params = { page: this.pageIndex(), size: this.pageSize() };
        $.getJSON('/Some/Json/Service', params, this.currentPageData);
    }, this);
}

Because the ko.computed evaluates both pageIndex and pageSize, it becomes dependent on both of them. So, this code will use jQuery’s $.getJSON function to reload currentPageData when the GridViewModel is first instantiated and whenever the pageIndex or pageSize properties are later changed.

This is very simple and elegant (and it’s trivial to add yet more observable query parameters that also trigger a refresh automatically whenever they change), but there is a potential efficiency problem. What if you want to change both pageIndex and pageSize at once? You might add the following function to GridViewModel:

this.setPageSize = function(newPageSize) {
    // Whenever you change the page size, we always reset the page index to 1
    this.pageSize(newPageSize);
    this.pageIndex(1);
}

The problem is that this will cause two simultaneous Ajax requests: the first one will start when you update pageSize, and the second one will start immediately afterwards when you update pageIndex. This is a waste of bandwidth and server resources, and a source of unpredictable race conditions.

Throttling is an elegant solution. You can add an arbitrarily short but nonzero throttle timeout (e.g., 1 millisecond), and then any sequence of synchronous changes to dependencies will only trigger one re-evaluation of your computed observable. For example,

    ko.computed(function() {
        // This evaluation logic is exactly the same as before
        var params = { page: this.pageIndex(), size: this.pageSize() };
        $.getJSON('/Some/Json/Service', params, this.currentPageData);
    }, this).extend({ throttle: 1 });

Now you can update pageIndex and pageSize as many times as you like, and the Ajax call will only happen once at the end of that sequence. It doesn’t matter if your thread continually makes changes for longer than 1 millisecond, because re-evaluation won’t start until you release your thread back to the JavaScript runtime.

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