javascript - @bindable changeHandler fires before bindings are done updating

Question

The Code:

App.js

export class App {
  constructor() {
    this.widgets = [{ name: 'zero'}, {name: 'one'}, {name:'two'}];
    this.shipment = { widget: this.widgets[1] };
  }
}

App.html

<template>
  <require from="./widget-picker"></require>
  <require from="./some-other-component"></require>

  <widget-picker widget.bind="shipment.widget" widgets.bind="widgets"></widget-picker>
  <some-other-component widget.bind="shipment.widget"/>
</template>

widget-picker.js

import {bindable, bindingMode} from 'aurelia-framework';

export class WidgetPicker {
  @bindable({ defaultBindingMode: bindingMode.twoWay, changeHandler: 'widgetChanged'  }) 
  widget;

  @bindable widgets;

  widgetChanged(widget) {
      // Use an Event Aggregator to send a message to SomeOtherComponent
      // to say that they should check their widget binding for updates.
  }
}

widget-picker.html

<select value.bind="widget">
  <option repeat.for="widget of widgets" model.bind="widget">${widget.name}</option>
</select>

The Problem:

The @bindable's changeHandler fires the widgetChanged event before the binding gets updated to App.js and its this.shipment.widget.

So when the Event Aggregator message goes out, the previous value is still set on `this.shipment.widget'.

Question:

Is there a way to make @bindable's changeHandler wait until all the bindings that will be updated for @bindable are done?

Or is there another callback I can use? Maybe a changedHandler (past tense)?

I did try to add change.delegate="widgetChanged" to the select, hoping that the delegate option would make it slower, but it still fires before the update is fully rolled out.

Answer 1

You could push the work you need to do onto the micro task queue:

import {bindable, bindingMode, inject, TaskQueue} from 'aurelia-framework';

@inject(TaskQueue)
export class WidgetPicker {
  @bindable({ defaultBindingMode: bindingMode.twoWay, changeHandler: 'widgetChanged'  }) 
  widget;
  @bindable widgets;

  constructor(taskQueue) {
    this.taskQueue = taskQueue;
  }

  widgetChanged(widget) {
    this.taskQueue.queueMicroTask(
      () => {
        // Use an Event Aggregator to send a message to SomeOtherComponent
        // to say that they should check their widget binding for updates.
      });
  }
}

This will ensure it occurs during the same "turn" of the event loop (as opposed to doing something like setTimeout(...)).