Diving Deep with the Flex Component Life Cycle

Diving Deep with the Flex Component Lifecycle Joshua Jamison EﬀectiveUI www.eﬀectiveui.com January 30, 2009

Who am I? ‣ Joshua Jamison Software Architect @ EﬀectiveUI •

Who are you (hopefully)? ‣ Beginner to intermediate level developers ‣ Anyone who doesn’t currently understand the lifecycle ‣ Anyone who wants a good review of the basics

What’s this about, anyway? ‣ Flex component lifecycle ‣ Flex frame cycle (“elastic racetrack”)

Flex Component Lifecycle ‣ What is it? The way the framework interacts with • every Flex component • A set of methods the framework calls to instantiate, control, and destroy components • Methods that make the most of the elastic racetrack

Elastic Racetrack: introduction image courtesy of Ted Patrick ‣ Flex component lifecycle is built on this frame model ‣ More on this later

A frame in AS3 image courtesy of Sean Christmann

Phases of the Lifecycle ‣ 3 Main Phases: ‣ BIRTH: construction, con guration, • attachment, initialization ‣ LIFE: • invalidation, validation, interaction ‣ DEATH: • detachment, garbage collection

Birth Congratulations: You’re about to have a component.

Construction Birth construction con guration attachment initialization Life Death

What is a constructor? ‣ A function called to instantiate (create in memory) a new instance of a class Birth construction con guration attachment initialization Life Death

How is a constructor invoked? Actionscript: var theLabel : Label = new Label(); MXML: <mx:Label id=quot;theLabelquot;/> Birth construction con guration attachment initialization Life Death

What does a constructor have access to? ‣ Properties on the class ‣ Methods on the class ‣ Children have not yet been created! Birth construction con guration attachment initialization Life Death

What does an ActionScript3 constructor look like? public function ComponentName() { super(); //blah blah blah } ‣ No required arguments (if it will be used in MXML); zero, or all optional ‣ Only one per class (no overloading!) ‣ No return type ‣ Must be public Birth construction con guration ‣ Calls super() to invoke superclass constructor; if attachment you don’t, the compiler will! initialization Life Death

What does an MXML constructor look like? ‣ No need to de ne one. In fact, if you try to put one in an <mx:Script> block, you’ll get an error. ‣ Why? Remember: MXML = Actionscript. A constructor is created by the compiler in the Actionscript generated from the MXML. ‣ Specify “-keep” in the Flex Builder Birth compiler arguments and look at the construction con guration generated code to verify this. attachment initialization Life Death

What should a constructor do? ‣ Not much. Since the component’s children have not yet been created, there’s not much that can be done. ‣ There are speci c methods (such as createChildren) that should be used for most of the things you’d be tempted to put in a constructor. ‣ A good place to add event listeners to the Birth object. construction con guration attachment initialization Life Death

Don’t create or attach children in the constructor ‣ It’s best to delay the cost of createChildren calls for added children until it’s necessary Birth construction con guration attachment initialization Life Death

Con guration Birth construction con guration attachment initialization Life Death

Con guration ‣ The process of assigning values to properties on objects ‣ In MXML, properties are assigned in this phase, before components are attached or initialized <local:SampleChild property1=quot;value!quot;/> Birth construction con guration attachment initialization Life Death

Hooray: Sample code! <mx:Application ...> ... <local:SampleChild property1=quot;value!quot;/> </mx:Application> Output: SampleChild constructor SampleChild.property1 setter Birth Adding child SampleChild4 construction con guration attachment initialization Life Death

Con guration and Containers ‣ Containers must not expect their children have to be instantiated when properties are set. <mx:Application ...> <local:SampleContainer property1=quot;value!quot;> <local:SampleChild property1=quot;value!quot;/> </local:SampleContainer> </mx:Application> SampleContainer constructor Birth SampleContainer.property1 setter construction SampleChild constructor con guration attachment SampleChild.property1 setter initialization Life Death

Con guration Optimization ‣ To avoid performance bottlenecks, make your setters fast and defer any real work until validation ‣ We’ll talk more about deferment in the validation / invalidation section Birth construction con guration attachment initialization Life Death

Attachment Birth construction con guration attachment initialization Life Death

What is attachment? ‣ Adding a component to the display list (addChild, addChildAt, MXML declaration) ‣ The component lifecycle is stalled after con guration until attachment occurs. Birth construction con guration attachment initialization Life Death

Consider this component: public class A extends UIComponent (It traces all of its methods.) { public function A() { trace( quot;CONSTRUCTORquot; ); super(); } override protected function createChildren() : void { trace( quot;CREATECHILDRENquot; ); super.createChildren(); } override protected function measure() : void { trace( quot;MEASUREquot; ); super.measure(); } override protected function updateDisplayList(width:Number, height:Number) : void { trace( quot;UPDATEDISPLAYLISTquot; ); super.updateDisplayList(width,height); } override protected function commitProperties():void { trace( quot;COMMITPROPERTIESquot; ); super.commitProperties(); }

And this application: <mx:Application ...> <mx:Script> <![CDATA[ override protected function createChildren() : void { super.createChildren(); var a : A = new A(); } ]]> </mx:Script> </mx:Application> Output: CONSTRUCTOR ‣ Without attachment, the rest of the lifecycle doesn’t happen.

But what about this application? <mx:Application ...> <mx:Script> <![CDATA[ override protected function createChildren() : void { super.createChildren(); var a : A = new A(); this.addChild( a ); } ]]> </mx:Script> Output: CONSTRUCTOR </mx:Application> CREATECHILDREN COMMITPROPERTIES MEASURE UPDATEDISPLAYLIST ‣ Moral of the story: don’t add components to the stage until you need them.

Initialization Birth construction con guration attachment initialization Life Death

Initialization ‣ 2 phases, 3 events: 1. ‘preInitialize’ dispatched Create 2. createChildren(); called 3. ‘initialize’ dispatched Validate 4. rst validation pass occurs 5. ‘creationComplete’ dispatched Birth construction con guration attachment initialization Life Death

createChildren() ‣ MXML uses the createChildren() method to add children to containers ‣ Override this method to add children using AS Follow MXML’s creation strategy: create, • con gure, attach override protected function createChildren():void { ... create textField = new UITextField(); textField.enabled = enabled; con gure textField.ignorePadding = true; textField.addEventListener(quot;textFieldStyleChangequot;, textField_textFieldStyleChangeHandler); ... ... attach addChild(DisplayObject(textField)); }

rst validation pass ‣ Invalidation is not part of initialization - only Validation ‣ Validation consists of 3 methods: • commitProperties() • measure() • updateDisplayList() ‣ more on these later Birth construction con guration attachment initialization Life Death

Life They grow up so fast.

Invalidation Birth Life invalidation validation interaction Death

Invalidation / Validation cycle ‣ Flex imposes deferred validation on the Flash API • goal: defer screen updates until all properties have been set ‣ 3 main method pairs to be aware of: • invalidateProperties() -> commitProperties() • invalidateSize() -> measure() • invalidateDisplayList() -> updateDisplayList()

Invalidation / Validation theory ‣ First, a little theory.

Deferment ‣ Deferment is the central concept to understand in the component Life-cycle ‣ Use private variables and boolean ags to defer setting any render-related properties until the proper validation method

Text-book example Bad: public function set text(value:String):void { myLabel.text = value; // Possible Error! during first config phase, // myLabel might not exist! } Good: private var _text:String = quot;quot;; override protected function public function set text(value:String):void { commitProperties():void{ { textSet = true; _text = value; if(textChanged){ myLabel.text = _text; textChanged = true; textChanged = false; } invalidateProperties(); invalidateSize(); super.commitProperties(); } invalidateDisplayList(); }

The Elastic Racetrack revisited image courtesy of Sean Christmann Invalidation occurs here

Invalidation methods ‣ invalidateProperties() Any property changes • ‣ invalidateSize() • Changes to width or height ‣ invalidateDisplayList() • Changes to child component size or position Birth Life invalidation validation interaction Death

Invalidation example 1 <mx:Application> <mx:Script> <![CDATA[ import mx.collections.ArrayCollection; [Bindable] public var arr : ArrayCollection = new ArrayCollection(); public function onClick() : void { var c : int = 0; while( c++ < 20 ) { arr.addItem( c ); } } ]]> </mx:Script> <mx:VBox> <mx:Button label=quot;Click me!quot; click=quot;onClick()quot;/> <test:BadList id=quot;theListquot; dataProvider=quot;{arr}quot;/> </mx:VBox> </mx:Application>

Invalidation example 2 public class BadList extends VBox { private var _dataProvider : ArrayCollection; public function set dataProvider( arr : ArrayCollection ) : void { this._dataProvider = arr; arr.addEventListener( CollectionEvent.COLLECTION_CHANGE, dataProviderChangeHandler ); } private function dataProviderChangeHandler( e : Event ) : void { this.removeAllChildren(); for each( var n : Number in this._dataProvider ) { var l : Label = new Label(); l.text = n.toString(); this.addChild( l ); } } public function BadList() {} } Result: dataProviderChangeHandler called 20 times

Invalidation example 3 public class GoodList extends VBox { private var _dataProvider : ArrayCollection; private var _dataProviderChanged : Boolean = false; public function set dataProvider( arr : ArrayCollection ) : void { this._dataProvider = arr; arr.addEventListener( CollectionEvent.COLLECTION_CHANGE, dataProviderChangeHandler ); this._dataProviderChanged = true; this.invalidateProperties(); } override protected function commitProperties():void { super.commitProperties(); if( this._dataProviderChanged ) { this.removeAllChildren(); for each( var n : Number in this._dataProvider ) { var l : Label = new Label(); l.text = n.toString(); Result: commitProperties this.addChild( l ); called only twice (once } during initialization) this._dataProviderChanged = false; } } private function dataProviderChangeHandler( e : Event ) : void { this._dataProviderChanged = true; this.invalidateProperties(); } public function GoodList() {} }

Validation Birth Life invalidation validation interaction Death

The Elastic Racetrack revisited Validation occurs here

Validation ‣ Apply the changes deferred during invalidation ‣ Update all visual aspects of the application in preparation for the render phase ‣ 3 methods: • commitProperties() • measure() Birth • updateDisplayList() Life invalidation validation interaction Death

commitProperties() ‣ Ely says: “Calculate and commit the eﬀects of changes to properties and underlying data.” ‣ Invoked rst - immediately before measurement and layout Birth Life invalidation validation interaction Death

commitProperties() cont. ‣ ALL changes based on property and data events go here ‣ Even creating and destroying children, so long as they’re based on changes to properties or underlying data ‣ Example: any list based component with empty renderers on the screen Birth Life invalidation validation interaction Death

measure() ‣ Component calculates its preferred (“default”) and minimum proportions based on content, layout rules, constraints. ‣ Measure is called bottom up - lowest children rst ‣ Caused by “invalidateSize()” ‣ NEVER called for explicitly sized Birth components Life invalidation validation interaction Death

overriding measure() ‣ Used for dynamic layout containers (VBox, etc.) ‣ Use getExplicitOrMeasuredWidth() (or height) to get child proportions ‣ ALWAYS called during initialization ‣ Call super.measure() rst! ‣ Set measuredHeight, measuredWidth for the default values; measuredMinHeight Birth and measuredMinWidth for the minimum. Life invalidation validation interaction Death

measure() cont. ‣ Not reliable - Framework optimizes away any calls to measure it deems “unecessary” Birth Life invalidation validation interaction Death

updateDisplayList() ‣ All drawing and layout code goes here, making this the core method for all container objects ‣ Caused by invalidateDisplayList(); ‣ Concerned with repositioning and resizing children ‣ updateDisplayList() is called top-down Birth Life invalidation validation interaction Death

Overriding updateDisplayList() ‣ Usually call super.updateDisplayList() rst • super() is optional - don’t call it if you’re overriding everything it does ‣ Size and lay out children using move(x,y) and setActualSize(w,h) if possible • I never have good luck with setActualSize() Birth Life invalidation validation interaction Death

Elastic Racetrack cont. ‣ User Actions Dispatch invalidation events • • Interact with any non-validation events from this frame (mouse movements, timers, etc.)

Elastic Racetrack Cont. ‣ Invalidate Action Process all validation calls • ‣ Render Action • Do the heavy lifting - actually draw on the screen

The Elastic Racetrack revisited Queued Invalidation Deferred Validation Render!

Interaction Birth Life invalidation validation interaction Death

How do objects know when something happens? ‣ Events: objects passed around when anything interesting goes on (clicks, moves, changes, timers...) ‣ If something happens to a component, it “ res” or “dispatches” the event ‣ If another component wants to know when something happens, it “listens” for events Birth ‣ Event-based architecture is loosely- Life invalidation coupled validation interaction Death

Bene ts of Loosely-Coupled Architectures ‣ Everything becomes more reusable ‣ Components don’t have to know anything about the components in which they’re used Birth Life invalidation validation interaction Death

Who can dispatch events? ‣ Subclasses of EventDispatcher EventDispatcher inherits directly from • Object ‣ Simply call dispatchEvent(event) to re oﬀ an event when something happens Birth Life invalidation validation interaction Death

How to tell events apart? ‣ Event class Diﬀerent classes allow for customized • payloads ‣ “type” eld: a constant Birth Life invalidation validation interaction Death

Common Events ‣ Event.CHANGE ‣ MouseEvent.CLICK ‣ FlexEvent.CREATION_COMPLETE ‣ Event.RESIZE ‣ MouseEvent.ROLL_OUT Birth Life invalidation validation interaction Death

Handling Events ‣ <mx:Button id=”theButton” click=”callThisFunction(event)”/> ‣ theButton.addEventListener( MouseEvent .CLICK, callThisFunction ) Birth Life invalidation validation interaction Death

Event Propagation ‣ Three phases: Capturing, Targeting, Bubbling Application Application Capturing Bubbling Phase Phase Target Targeting Phase Birth Life invalidation validation interaction Death

Event Propagation ‣ Three phases: Capturing, Targeting, Bubbling <mx:Application initialize=quot;onInitialize()quot;> <mx:Script> <![CDATA[ public function onInitialize() : void { this.addEventListener( MouseEvent.CLICK, clickHandler, true ); this.addEventListener( MouseEvent.CLICK, clickHandler, false ); outer.addEventListener( MouseEvent.CLICK, clickHandler, true ); outer.addEventListener( MouseEvent.CLICK, clickHandler, false ); inner.addEventListener( MouseEvent.CLICK, clickHandler, true ); inner.addEventListener( MouseEvent.CLICK, clickHandler, false ); button.addEventListener( MouseEvent.CLICK, clickHandler, true ); button.addEventListener( MouseEvent.CLICK, clickHandler, false ); } public function clickHandler( e : Event ) : void { trace(quot;----------------------------------------------------------quot;); trace(quot;TARGET: quot; + e.target.id ); trace(quot;CURRENT TARGET: quot; + e.currentTarget.id ); trace(quot;PHASE: quot; + ( e.eventPhase == 1 ? quot;CAPTUREquot; : ( e.eventPhase == 2 ? quot;TARGETquot; : quot;BUBBLEquot; ) ) ); } ]]> </mx:Script> <mx:VBox> <mx:Panel id=quot;outerquot;> <mx:TitleWindow id=quot;innerquot;> <mx:Button id=quot;buttonquot;/> </mx:TitleWindow> Birth </mx:Panel> </mx:VBox> Life </mx:Application> invalidation ‣ validation interaction Death

Event Propagation ---------------------------------------------------------- TARGET: button CURRENT TARGET: eventTest PHASE: CAPTURE ---------------------------------------------------------- TARGET: button CURRENT TARGET: outer PHASE: CAPTURE ---------------------------------------------------------- TARGET: button CURRENT TARGET: inner PHASE: CAPTURE ---------------------------------------------------------- TARGET: button CURRENT TARGET: button PHASE: TARGET ---------------------------------------------------------- TARGET: button CURRENT TARGET: inner PHASE: BUBBLE ---------------------------------------------------------- TARGET: button CURRENT TARGET: outer PHASE: BUBBLE ---------------------------------------------------------- Birth TARGET: button CURRENT TARGET: eventTest Life PHASE: BUBBLE invalidation validation interaction Death

Stopping events from propagating ‣ stopPropagation() : Prevents processing of any event listeners in nodes subsequent to the current node in the event ow ‣ stopImmediatePropagation() : Prevents processing of any event listeners in the current node and any subsequent nodes in the event ow Birth Life invalidation validation interaction Death

target vs. currentTarget ‣ target: the object that dispatched the event (doesn’t change) ‣ currentTarget: the object who is currently being checked for speci c event listeners (changes) Birth Life invalidation validation interaction Death

Dispatching events from custom components ‣ MXML: <mx:Metadata> [Event(name=quot;atePizzaquot;, type=quot;flash.events.JoshEventquot;)] </mx:Metadata> ‣ Actionscript: [Event(name=quot;atePizzaquot;, type=quot;flash.events.JoshEventquot;)] public class MyComponent extends UIComponent { ... } Birth Life invalidation validation interaction Death

Death All good things come to an end.

Detachment Birth Life Death detachment garbage collection

Detachment ‣ “Detachment” refers to the process of removing a child from the display list ‣ These children can be re-parented (brought back to life) or abandoned to die ‣ Abandoned components don’t get validation calls and aren’t drawn ‣ If an abandoned component has no more active references, it *should* be garbage- Birth collected Life Death detachment garbage collection

Detachment cont. ‣ Re-parenting isn’t cheap, but it’s cheaper than re-creating the same component twice ‣ Children do not need to be removed from their parent before being re-parented, but always should be ‣ Consider hiding rather than removing • set visible and includeInLayout to false Birth Life Death detachment garbage collection

Garbage Collection Birth Life Death detachment garbage collection

Garbage Collection ‣ The process by which memory is returned to the system ‣ Only objects with no remaining references to them will be gc’d • Set references to detached children to “null” to mark them for GC ‣ Talk to Grant Skinner about forcing GC Birth http://gskinner.com/blog/archives/2006/08/as3_resource_ma_2.html • Life Death detachment garbage collection

Conclusion ‣ Defer, Defer, DEFER! ‣ Use validation methods correctly ‣ Remember the elastic racetrack

References ‣ Ely Green eld: “Building a Flex Component” • http://www.on ex.org/ACDS/ BuildingAFlexComponent.pdf ‣ Cha c Kazoun, Joey Lott: “Programming Flex 2” by O’Reilly • http://oreilly.com/catalog/9780596526894/ ‣ Colin Moock: “Essential Actionscript 3.0” by O’Reilly • http://oreilly.com/catalog/9780596526948/ index.html

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Diving Deep with the Flex Component Life Cycle

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