Flex Component Life Cycle

Posted by: Yogesh Puri on: May 9, 2008

• In: Flex Components
• 12 Comments

The lifecycle of component in flex is certainly somewhat similar to the old V2 framework where you create a component holder, craetechildren, set-sizes and dispatch events. Following four methods plays a role for component rendering

1. commitProperties()
2. measure()
3. layoutChrome()
4. updateDisplayList()

Here is what I found after doing some studies. The most of the following part is taken from Adobe flex help however what I am trying to do here is to elaborate it more by putting up some comments to each step to make it clearer.

The green comments shows the methods and events performed by or on component itself whereas the comments in blue represents the methods and events performed on or by the parent container.

Following steps are demonstrating the Button component life cycle

• Given if the component is going to be in a container then the parent property of component will be referred to the container(DisplayObjectContainer type)
  //– A property for UIComponent class
• Get the style settings of the component
• Dispatches the preinitialize event on the component.

//– The preinitialize event(mx.events.FlexEvent) is triggered when the UIComponent is in a very raw stage and there is no children in existence at that time

• Calls the component’s createChildren() method.

createChildren is a protected method of UIComponent class which we need to override when we create a subclass of the UIComponent. Also from within an override of the createChildren() method, you call the addChild() method to add each child object.

You do not call this method directly. Flex calls the createChildren() method in response to the call to the addChild() method to add the component to its parent.

• Calls the invalidateProperties(), invalidateSize(), and invalidateDisplayList() methods to trigger later calls to the commitProperties(), measure(), or updateDisplayList() methods during the next render event.

invalidateProperties() marks a component so that its commitProperties() method gets called during a later screen update.

invalidateSize () Marks a component so that its measure() method gets called during a later screen update.

invalidateDisplayList () Marks a component so that its updateDisplayList() method gets called during a later screen update.

The only exception to this rule is that Flex does not call the measure() method when the user sets the height and width of the component.

• Dispatches the initialize event on the component. At this time, all of the component’s children are initialized, but the component has not been sized or processed for layout. You can use this event to perform additional processing of the component before it is laid out.

Initizlize event gets dispatched when the component has finished its construction and has all initialization properties set.

After the initialization phase, properties are processed, the component is measured, laid out, and drawn. After which the creationComplete event is dispatched.

• Dispatches the childAdd event on the parent container.

The childAdd event is dispatched when the addChild() or addChildAt() method is called. At the time when this event is sent, the child object has been initialized, but its width and height have not yet been calculated, and the child has not been drawn on the screen. If you want to be notified when the child has been fully initialized and rendered, then register as a listener for the child’s creationComplete event.

• Dispatches the initialize event on the parent container.

Dispatched when the component has finished its construction and has all initialization properties set. After the initialization phase, properties are processed, the component is measured, laid out, and drawn, after which the creationComplete event is dispatched.

• During the next render event, Flex performs the following actions:
  • Calls the component’s commitProperties() method.
    commitProperties() processes the properties set on the component. You do not call this method directly. Flex calls the commitProperties() method when you use the addChild() method to add a component to a container, or when you call the invalidateProperties() method of the component. Calls to the commitProperties() method occur before calls to the measure() method. This lets you set property values that might be used by the measure() method.
  • Calls the component’s measure() method.
    Measure() calculates the default size, and optionally the default minimum size, of the component. This is an advanced method that you might override when creating a subclass of UIComponent.
    The default implementation of measure() sets measuredWidth, measuredHeight, measuredMinWidth, and measuredMinHeight to 0.
  • Calls the component’s (Adobe help mention this method is in UIComponent where as it is in container class) container’s layoutChrome() method.
    The Container class, and some subclasses of the Container class, use the layoutChrome() method to define the border area around the container.
  • Calls the component’s updateDisplayList() method.
    The updateDisplayList() method sizes and positions the children of your component based on all previous property and style settings, and draws any skins or graphic elements that the component uses. The parent container for the component determines the size of the component itself.
  • Dispatches the updateComplete event on the component.
    Dispatched when an object has had its commitProperties(), measure(), and updateDisplayList() methods called (if needed).
    This is the last opportunity to alter the component before it is displayed. All properties have been committed and the component has been measured and layed out.
• Flex dispatches additional render events if the commitProperties(), measure(), or updateDisplayList() methods call the invalidateProperties(), invalidateSize(), or invalidateDisplayList() methods.
• After the last render event occurs, Flex performs the following actions:
  • Makes the component visible by setting the visible property to true.
  • Dispatches the creationComplete event on the component. The component is sized and processed for layout. This event is only dispatched once when the component is created.
  • Dispatches the updateComplete event on the component. Flex dispatches additional updateComplete events whenever the layout, position, size, or other visual characteristic of the component changes and the component is updated for display.

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