By default, components added by nonbase states aren’t instantiated
until the state is first requested. The MXML in Example 12-16 illustrates this. The
trace() statement outputs null because button is not yet defined when the application
first starts.
Example 12-16. Understanding object creation policies: Default policy
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute"
initialize="initializeHandler(event)">
<mx:Script>
<![CDATA[
private function initializeHandler(event:Event):void {
trace(button);
}
]]>
</mx:Script>
<mx:states>
<mx:State name="example">
<mx:AddChild>
<mx:Button id="button" label="Example" />
</mx:AddChild>
</mx:State>
</mx:states>
</mx:Application>However, you can manage when components added by states are
instantiated using a creation policy. The default creation policy setting
is auto, which means the component is
instantiated when the state is first requested. You can set creation
policies for each added component using the creationPolicy attribute of the <mx:AddChild> tag, or the creationPolicy
property of the AddChild class.
The possible values are auto (default), all, and none.
When you set the creation policy of an added component to all, the component is instantiated when the
application first starts. The MXML in Example 12-17 illustrates how that
works. Because the creation policy of the button is now set to all, the trace() statement outputs the reference to the
component.
Example 12-17. Understanding object creation policies: Policy all
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute"
initialize="initializeHandler(event)">
<mx:Script>
<![CDATA[
private function initializeHandler(event:Event):void {
trace(button);
}
]]>
</mx:Script>
<mx:states>
<mx:State name="example">
<mx:AddChild creationPolicy="all">
<mx:Button id="button" label="Example" />
</mx:AddChild>
</mx:State>
</mx:states>
</mx:Application>When the creation policy is set to none, the component isn’t instantiated until you explicitly call
the createInstance() method of the
AddChild object. If you’re defining the AddChild object using the <mx:AddChild> tag, you must assign an
id. Example 12-18 illustrates how the
none creation policy works. The first
trace() statement outputs null because the component hasn’t been
instantiated. The second trace()
statement outputs the reference to the component because it is called
immediately following the call to createInstance().
Example 12-18. Understanding object creation policy: Policy none
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute"
initialize="initializeHandler(event)">
<mx:Script>
<![CDATA[
private function initializeHandler(event:Event):void {
trace(button);
exampleAddChild.createInstance();
trace(button);
}
]]>
</mx:Script>
<mx:states>
<mx:State name="example">
<mx:AddChild creationPolicy="none" id="exampleAddChild"
>
<mx:Button id="button" label="Example" />
</mx:AddChild>
</mx:State>
</mx:states>
</mx:Application>In most applications, the default (auto) creation policy is the correct setting.
However, there are several reasons you might want to select
a different creation policy. The all
policy, for example, is useful in at least two scenarios:
- The added component requires a long time to initialize
If the component isn’t initialized until the state is requested, the user might experience a delay. By setting the policy to
all, the component is initialized when the application first starts; that should mitigate any issues related to component initialization times and delays when changing states.- You want to reference the added component before first requesting the state
For example, when you create a component with several states, you might want to reference added components via an accessor method.
The none policy may not seem
immediately useful; however, consider that the auto and all
policies are very black and white:
When you select
auto, components aren’t instantiated until the state is requested.When you select
all, components are instantiated when the application initializes.
There are reasons you might want to ensure that a component
initializes before a state is requested, but you don’t want to force the
component to instantiate when the application initializes. For example, a
complex application might contain many states, each having components that
take a long time to initialize. If you set the creation policy of all the
AddChild objects to all, the user might have to wait a long time
before the application initializes. However, it might be a better user
experience if the application starts right away while the components for
the rest of the states initialize in the background. Using the none creation policy allows you to do just that.
Simply call the createInstance() method
of the AddChild object to instantiate
the child component.