This is a post I've been wanting to write since April! Now that the major performance refactoring in the binding engine is complete we've been able to shift gears a little bit and start adding some of the "nice to have" items, including the feature that is the subject of this post: binding behaviors.

What is a binding behavior?

Binding behaviors are a category of view resource, just like a value converters, custom attributes and custom elements. Binding behaviors are most like value converters in that you use them declaratively in binding expressions to affect the binding.

The primary difference between a binding behavior and a value converter is binding behaviors have full access to the binding instance, throughout it's lifecycle. Contrast this with a value converter which only has the ability to intercept values passing from the model to the view and visa versa.

The additional "access" afforded to binding behaviors gives them the ability to change the behavior of the binding, enabling a lot of interesting scenarios which you'll see below.

throttle

Aurelia ships with a handful of behaviors out of the box to enable common scenarios. The first is the throttle binding behavior which limits the rate at which the view-model is updated in two-way bindings or the rate at which the view is updated in one-way binding scenarios.

By default throttle will only allow updates every 200ms. You can customize the rate of course. Here's a few examples.

<!-- 
the model's query property will be updated at most once every 200ms
-->
<input type="text" value.bind="query & throttle">

The first thing you probably noticed in the example above is the & symbol, which is used to declare binding behavior expressions. Binding behavior expressions use the same syntax pattern as value converter expressions:

Binding behaviors can accept arguments: firstName & myBehavior:arg1:arg2:arg3
A binding expression can contain multiple binding behaviors: firstName & behavior1 & behavior2:arg1.
Binding expressions can also include a combination of value converters and binding behaviors: ${foo | upperCase | truncate:3 & throttle & anotherBehavior:arg1:arg2}.

Here's another example using throttle, demonstrating the ability to pass arguments to the binding behavior:

<!-- 
the model's query property will be updated at most once every 850ms
-->
<input type="text" value.bind="query & throttle:850">

The throttle behavior is particularly useful when binding events to methods on your view-model. Here's an example with the mousemove event:

<!-- 
the model's mouseMove() method will be called at most once every 200ms
-->
<div mousemove.delegate="mouseMove($event) & throttle">
</div>

Here's a live example:

debounce

The debounce binding behavior is another rate limiting binding behavior. Debounce prevents the binding from being updated until a specified interval has passed without any changes.

A common use case is a search input that triggers searching automatically. You wouldn't want to make a search API on every change (every keystroke). It's more efficient to wait until the user has paused typing to invoke the search logic.

<!-- 
the model's query property will be updated after the user has stopped typing for 200ms
-->
<input type="text" value.bind="query & debounce">

<!-- 
the model's query property will be updated after the user has stopped typing for 850ms
-->
<input type="text" value.bind="query & debounce:850">

Like throttle, the debounce binding behavior really shines in event binding. Here's another example with the mousemove event:

<!-- 
call the model's mouseMove() method once the mouse has stopped moving for 500ms
-->
<div mousemove.delegate="mouseMove($event) & debounce:500">
</div>

Here's a live example:

updateTrigger

Update trigger allows you to override the input events that cause the element's value to be written to the view-model. The default events are change and input.

Here's how you would tell the binding to only update the model on blur:

<input value.bind="firstName & updateTrigger:'blur'>

Multiple events are supported:

<input value.bind="firstName & updateTrigger:'blur':'paste'>

Here's a live example:

signal

The signal binding behavior enables you to "signal" the binding to refresh. This is especially useful when a binding result is impacted by global changes that are outside of the observation path.

For example, if you have a "translate" value converter that converts a key to a localized string- eg ${'greeting-key' | translate} and your site allows users to change the current language, how would you refresh the bindings when that happens?

Another example is a value converter that uses the current time to convert a record's datetime to a "time ago" value: posted ${postDateTime | timeAgo}. The moment this binding expression is evaluated it will correctly result in posted a minute ago. As time passes, it will eventually become inaccurate. How can we refresh this binding periodically so that it correctly displays 5 minutes ago, then 15 minutes ago, an hour ago, etc?

Here's how you would accomplish this using the signal binding behavior:

export class TimeAgoValueConverter {
  toView(datetime) {
    // todo: compute the "time ago" string...
    // http://momentjs.com/docs/#/displaying/from/
  }
}

posted ${postDateTime | timeAgo & signal:'my-signal'}

In the binding expression above we're using the signal binding behavior to assign the binding a "signal name" of my-signal. Signal names are arbitrary, you can give multiple bindings the same signal name if you want to signal multiple bindings at the same time.

Here's how we can use the BindingSignaler to signal the bindings periodically:

import {BindingSignaler} from 'aurelia-templating-resources';

@inject(BindingSignaler)
export class App {
  constructor(signaler) {
    // signal the binding to refresh periodically:
    setInterval(() => signaler.signal('my-signal'), 5000);
  }
}

Here's a live example that signals the bindings when you press a button:

oneTime

In previous versions of Aurelia there was no way to express a one-time string interpolation binding. Now with the oneTime binding behavior you can write:

<span>${foo & oneTime}</span>

This is an important feature to expose because one-time bindings are the most efficient type of binding because they don't incur any property observation overhead.

There are also binding behaviors for oneWay and twoWay which you could use like this:

<!-- these have the same result -->
<input value.bind="foo & oneWay>
<input value.one-way="foo">

<!-- these have the same result -->
<input value.bind="foo & twoWay>
<input value.two-way="foo">

Custom binding behaviors

You can build custom binding behaviors just like you can build value converters. Instead of toView and fromView methods you'll create bind(binding, scope, [...args]) and unbind(binding, scope) methods. In the bind method you'll add your behavior to the binding and in the unbind method you should cleanup whatever you did in the bind method to restore the binding instance to it's original state.

Here's a custom binding behavior that calls a method on your view model each time the binding's updateSource / updateTarget and callSource methods are invoked.

const interceptMethods = ['updateTarget', 'updateSource', 'callSource'];

export class InterceptBindingBehavior {
  bind(binding, scope, interceptor) {
    let i = interceptMethods.length;
    while (i--) {
      let method = interceptMethods[i];
      if (!binding[method]) {
        continue;
      }
      binding[`intercepted-${method}`] = binding[method];
      let update = binding[method].bind(binding);
      binding[method] = interceptor.bind(binding, method, update);
    }
  }

  unbind(binding, scope) {
    let i = interceptMethods.length;
    while (i--) {
      let method = interceptMethods[i];
      if (!binding[method]) {
        continue;
      }
      binding[method] = binding[`intercepted-${method}`];
      binding[`intercepted-${method}`] = null;
    }
  }
}

Usage:

<template>
  <require from="./intercept-binding-behavior"></require>

  <div mousemove.delegate="mouseMove($event) & intercept:myFunc">
  </div>

  <input value.bind="foo & intercept:myFunc">
</template>

Live example:

Feedback? Questions?

Let us know what you think! Sound off in the comments below, or in the Aurelia gitter. If you find any issues with binding behaviors, please open a issue.

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Aurelia Binding Behaviors

What is a binding behavior?

throttle

debounce

updateTrigger

signal

oneTime

Custom binding behaviors

Feedback? Questions?