delegate

delegate( selector, event, handler(ev,newVal,oldVal,from) ) listen for changes in a child attribute from the parent. The child attribute does not have to exist.

// create an observable
var observe = new can.Map({
  foo : {
    bar : "Hello World"
  }
})

//listen to changes on a property
observe.delegate("foo.bar","change", function(ev, prop, how, newVal, oldVal){
  // foo.bar has been added, set, or removed
  this //->
});

// change the property
observe.attr('foo.bar',"Goodbye Cruel World")

Types of events

Delegate lets you listen to add, set, remove, and change events on property.

add

An add event is fired when a new property has been added.

var o = new can.Control({});
o.delegate("name","add", function(ev, value){
  // called once
  can.$('#name').show()
})
o.attr('name',"Justin")
o.attr('name',"Brian");

Listening to add events is useful for 'setup' functionality (in this case showing the #name element.

set

Set events are fired when a property takes on a new value. set events are always fired after an add.

o.delegate("name","set", function(ev, value){
  // called twice
  can.$('#name').text(value)
})
o.attr('name',"Justin")
o.attr('name',"Brian");

remove

Remove events are fired after a property is removed.

o.delegate("name","remove", function(ev){
  // called once
  $('#name').text(value)
})
o.attr('name',"Justin");
o.removeAttr('name');

Wildcards - matching multiple properties

Sometimes, you want to know when any property within some part of an observe has changed. Delegate lets you use wildcards to match any property name. The following listens for any change on an attribute of the params attribute:

var o = can.Control({
  options : {
    limit : 100,
    offset: 0,
    params : {
      parentId: 5
    }
  }
})
o.delegate('options.*','change', function(){
  alert('1');
})
o.delegate('options.**','change', function(){
  alert('2');
})

// alerts 1
// alerts 2
o.attr('options.offset',100)

// alerts 2
o.attr('options.params.parentId',6);

Using a single wildcard () matches single level properties. Using a double wildcard (*) matches any deep property.

Listening on multiple properties and values

Delegate lets you listen on multiple values at once. The following listens for first and last name changes:

var o = new can.Map({
  name : {first: "Justin", last: "Meyer"}
})

o.bind("name.first,name.last",
       "set",
       function(ev,newVal,oldVal,from){

})

Listening when properties are a particular value

Delegate lets you listen when a property is set to a specific value:

var o = new can.Map({
  name : "Justin"
})

o.bind("name=Brian",
       "set",
       function(ev,newVal,oldVal,from){

})

Parameters

1. selector {String}

   The attributes you want to listen for changes in.

   Selector should be the property or property names of the element you are searching. Examples:

   "name" - listens to the "name" property changing
   "name, address" - listens to "name" or "address" changing
   "name address" - listens to "name" or "address" changing
   "address.*" - listens to property directly in address
   "address.**" - listens to any property change in address
   "foo=bar" - listens when foo is "bar"
   

2. event {String}

   The event name. One of ("set","add","remove","change")

3. handler {function(ev, newVal, oldVal, prop)}

   The callback handler called with:

   • newVal - the new value set on the observe
   • oldVal - the old value set on the observe
   • prop - the prop name that was changed

Returns