How to hook into a change in the model in a widget - dojo

Question

I'm trying to respond to a change in one of the properties of the model in a widget. To be clear, when the value of the property changes, I want to run some code to react to the change. In a parent widget I have a date picker which changes the date in the model.
I cannot get the custom setter to be called _setParentPropertyAttr...
If I include this in my widget
<span data-dojo-type="dojox/mvc/Output" data-dojo-props="value: at(rel:, 'ParentProperty')">
It works nicely. Changing the date picker outputs the current value to the page. So I can supply the value property to the output widget when the date changes in the model. But what I need to do (I think) is supply a custom property with the date property in the model when the date picker changes the value.
I realise this question is a bit vague but I can't provide the code as it's proprietary.
I've tried to break the problem down by setting a property manually within my widget as:
myProperty:0,
...
constructor
...
_setMyPropertyAttr: function(value):
{
console.log("setting myproperty");
}
....
this.set('myProperty', 5);
....
but that isn't working either.
If you set a property within a widget does that not call the custom setter?
I'm struggling a bit because there aren't so many dojo examples out there any help is much appreciated.

Answer 1

You can bind an event to be called when a widget's property is set/update or you can even use watch to do that.
But this only works using the set function, using someWidget.someProperty = 5; wont work.
Let me show you how dojo do it. The basic about the magic setters and getters is explained here.
_set: function(/*String*/ name, /*anything*/ value){
// summary:
// Helper function to set new value for specified property, and call handlers
// registered with watch() if the value has changed.
var oldValue = this[name];
this[name] = value;
if(this._created && !isEqual(oldValue, value)){
if(this._watchCallbacks){
this._watchCallbacks(name, oldValue, value);
}
this.emit("attrmodified-" + name, {
detail: {
prevValue: oldValue,
newValue: value
}
});
}
}
This peace of code is from dijit/_WidgetBase, the _set function is what dojo calls after a set is called, and is where it finally set the property value this[name] = value; and as you can see, it emit an event that will be called attrmodified-propertyName and also call a watchCallbacks.
For example, if in some place, we have this:
on(someWidget, 'attrmodified-myproperty', function(){
console.log(someWidget.get('myProperty'));
});
and then we use:
someWidget.set('myProperty', 'Hello World!');
The event will be triggered. Note that someWidget.myProperty = 'Hello World!' wont trigger the event registration. Also note that if in our widget we define the magic setter:
_setMyPropertyAttr: function(value) {
//do something here with value
// do more or less with other logic
//but some where within this function we need to cal "_set"
this._set('myProperty', value);
}
without calling _set, the magic wont happen.
As i said at the beginning, we can also use watch:
someWidget.watch('myProperty', function(){
console.log(someWidget.get('myProperty'));
});
Note that we can register to this events or the watch function within the same widget.
Also, as a plus, the magic setter can be triggered when creating the widget with just passing the property name in the constructor object param, this work for the declarative syntax too, for example:
var someWidget = new MyWidget({
'myProperty': 'Hello World!!'
});
and
<div data-dojo-type="MyWidget" data-dojo-props="myProperty: 'Hello World!!'"></div>
both will triggered a call to the _setMyPropertyAttr if exist, or dojo will use the magic setter in the case it doesn't exist.
Hope it helps

Answer 2

Consider using custom setter on your widget, where you can add your custom logic.
Example of definition of custom setter on your widget:
_setOpenAttr: function(/*Boolean*/ open){
// execute some custom logic here
this._set("open", open);
}
Example of setting a property on your widget:
widgetRef.set('open', true);
Alternatively you can could consider using dojo/store/Observable.
dojo/store/Observable store wrapper that adds support for notification of data changes.
You can read more about it on the followign link:
https://dojotoolkit.org/reference-guide/1.10/dojo/store/Observable.html

Answer 3

If figured out the problem. If I set a watch on the model I can then check if indiviual properties have changed in the watch function. I knew it would be something simple!