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Binding: Delegate vs Trigger

When to use delegate and when to use trigger.

Delegate vs Trigger

A couple frequently asked questions are:

  • What's the difference between delegate and trigger?
  • When should I use delegate and when should I use trigger?

The short answer is: Use delegate except when you cannot use delegate.

Event delegation is a technique used to improve application performance. It drastically reduces the number of event subscriptions by leveraging the "bubbling" characteristic of most DOM events. With event delegation, handlers are not attached to individual elements. Instead, a single event handler is attached to a top-level node such as the body element. When an event bubbles up to this shared top-level handler the event delegation logic calls the appropriate handler based on the event's target .

To find out if event delegation can be used with a particular event, google mdn [event name] event. In fact, preceding any web platform related google search with mdn often returns a high quality result from the Mozilla Developer Network. Once you're on the event's MDN page, check whether the event bubbles. Only events that bubble can be used with Aurelia's delegate binding command. The blur, focus, load and unload events do not bubble so you'll need to use the trigger binding command to subscribe to these events.

Here's the MDN page for blur . It has further info on event delegation techniques for the blur and focus events.

Exceptions to the general guidance above:

Use trigger on buttons when the following conditions are met:

  1. You need to disable the button.
  2. The button's content is made up of other elements (as opposed to just text).

This will ensure clicks on disabled button's children won't bubble up to the delegate event handler. More info here .

Use trigger for click in certain iOS use-cases:

iOS does not bubble click events on elements other than a, button, input and select. If you're subscribing to click on a non-input element like a div and are targeting iOS, use the trigger binding command. More info here and here .

Event delegation with Capture

In most situations, delegate and trigger are enough for you to handle user interaction. However, delegate and trigger can be sometime too late to react to an event. Some other times, events may never reach to your delegate or trigger handlers, this is the case when you interact with content (elements) generated by 3rd party plugins.

delegate and trigger will fail in the following example:

  <div class='my-plugin-container' click.delegate='onClickPluginContainer()'>
      Content inside here is generated by a plugin
      That will call `event.stopPropagation()` on any click events

This is where you may need another way to listen to click event, via binding command capture:

  <div class='my-plugin-container' click.capture='onClickPluginContainer()'>
      Content inside here is generated by a plugin
      That will call `event.stopPropagation()` on any click events

In the 2nd example, onClickPluginContainer() is guaranteed to happen no matter event.stopPropagation() is called or not inside the container.

With the introduction of capture binding command, you may ask "Which is best command for event handling ?".

Well, the short answer above remains true, Use delegate except when you cannot use delegate. capture is not needed normally and may even be confusing as it's not how we normally works with browser events.