Flex Metadata in ActionScript3_keep actionscript metadata-优快云博客

本文链接：https://blog.youkuaiyun.com/llmlx/article/details/5317900

本文介绍了元数据（metadata）的概念及其在不同编程语言中的应用，包括.NET、Java和ActionScript，并详细解析了元数据如何与反射机制相结合，实现代码的特殊处理。

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metadata最先开始于dotnet吧，叫做Attribute；然后java也紧随其后，叫做Annotaion；actionscript则叫做Metadata。其中dotnet与ActionScript的语法格式上都是相似的，如AS中的[Bindable]。在Java中则是用@XXX这样的语法，如@override。不过目的都是一个，给类或方法或属性增加meta data标记，从而实现对这些被标记的代码进行一些特殊处理。譬如FlexUnit4，不再要求从测试用例中的测试方法必须以test开头，而是在方法之前用[Test]进行标记。这个跟JUnit 4是一致的，因为FlexUnit4应该是参考了JUnit的。在dotnet中可以有如下的例子（该例子引自http://ondotnet.com/pub/a/dotnet/excerpt/prog_csharp_ch18/index.html?page=4 ，该文章是一篇很好的关于介绍dotnet Attribute的文章）

namespace Programming_CSharp { using System; using System.Reflection; // create custom attribute to be assigned to class members [AttributeUsage(AttributeTargets.Class | AttributeTargets.Constructor | AttributeTargets.Field | AttributeTargets.Method | AttributeTargets.Property, AllowMultiple = true)] public class BugFixAttribute : System.Attribute { // attribute constructor for // positional parameters public BugFixAttribute (int bugID, string programmer, string date) { this.bugID = bugID; this.programmer = programmer; this.date = date; } // accessor public int BugID { get { return bugID; } } // property for named parameter public string Comment { get { return comment; } set { comment = value; } } // accessor public string Date { get { return date; } } // accessor public string Programmer { get { return programmer; } } // private member data private int bugID; private string comment; private string date; private string programmer; } // ********* assign the attributes to the class ******** [BugFixAttribute(121,"Jesse Liberty","01/03/05")] [BugFixAttribute(107,"Jesse Liberty","01/04/05", Comment="Fixed off by one errors")] public class MyMath { public double DoFunc1(double param1) { return param1 + DoFunc2(param1); } public double DoFunc2(double param1) { return param1 / 3; } } public class Tester { public static void Main( ) { MyMath mm = new MyMath( ); Console.WriteLine("Calling DoFunc(7). Result: {0}", mm.DoFunc1(7)); } } }

Java中的Annotation类似dotnet，你可以在这里找到关于Annotation的例子，这个例子很能帮助你理解测试框架中用到的Test Metadata。

从以上可以看出metadata通常都与reflection紧密相连。在AS中亦是如此，不同的是在AS中并不像dotnet和java一样需要事先定义一个metadata相关的类。AS中通过解析flash.utils.describeType来获取metadata信息，而这也是reflection的一部分。一下我们给出一个代码片段，该代码源自http://www.quietlyscheming.com/blog/components/a-couple-of-samples-from-max-no-not-that-one/

在UI_Code中引用了Bind和HandleEvent两个metadata标记。并且在构造函数中调用了ASBinder.InitObject(this)从而解析这些metadata标记。这些标记中引用的source或者destination中对应的都是UIExample中的控件。

package { import flash.events.Event; import mx.containers.Canvas; import mx.controls.TextInput; // Bind metadata on the class tells the framework to set up a binding from the source to the destination. The source // and destination can be dotted expressions, evaluated in the context of the object instance. // direction can be either 'forward' or 'both'. if forward, the binding is from the source to the destination. If both, the binding // is bidirectional. If omitted, it defaults to forward. [Bind(source="tiOne.text",destination="tiTwo.text",direction="both")] public class UI_Code extends Canvas { public var tiSix:TextInput; public var tiFive:TextInput; public var clickCount:int = 0; public function UI_Code() { super(); // an object needs to initialize itself in its constructor using a single call to ASBinder.InitObject() ASBinder.InitObject(this); } // Bind metadata on a method tells the framework to call that function whenever the source value of the metadata changes. // the source property can be any dotted expression, evaluated in the context of the object being defined. i.e., the // example below is equivalent to 'this.tiFour.text'. [Bind(source="tiFour.text")] public function tiFourText_Changed(newValue:String):void { if(newValue != null && tiFive != null) tiFive.text = "text length is " + newValue.length; } // HandleEvent metadata on a method tells the framework to add this method as an event handler to the target/event as // described in the metadata. dispatcher can be any dotted expression, evaluated in the context of the object being defined. // i.e., the example below is equivalent to 'this.clickButton'. [HandleEvent(dispatcher="clickButton", event="click")] public function button_click_Handler(e:Event):void { tiSix.text = 'button clicked ' + ++clickCount + " times."; } } }

<?xml version="1.0" encoding="utf-8"?> <UI_Code xmlns="*" xmlns:mx="http://www.adobe.com/2006/mxml" width="502" height="300"> <mx:TabNavigator right="0" top="0" bottom="0" left="0"> <mx:Canvas label="Declarative Binding" width="100%" height="100%"> <mx:TextInput x="10" y="71" width="300" id="tiOne"/> <mx:TextInput x="10" y="110" width="300" id="tiTwo"/> <mx:Text y="10" text="The text properties of these two text inputs were bound together declaratively using actionscript metadata. The binding is two way. Try typing in either one, and see that the other one changes." height="53" right="10" left="10"/> <mx:Canvas y="140" height="117" right="10" left="10" borderColor="#FFAA00" backgroundColor="#FFFFCC" borderStyle="solid"> <mx:Text text="[Bind(source="tiOne.text",destination="tiTwo.text",direction="both")] public class UI_Code extends Canvas { public var tiSix:TextInput; ... " width="100%" height="100%" fontWeight="normal" fontFamily=""Courier New"" fontSize="12"/> </mx:Canvas> </mx:Canvas> <mx:Canvas label="Call Function on Change" width="100%" height="100%"> <mx:TextInput x="10" y="125" width="300" id="tiFive"/> <mx:TextInput x="10" y="95" width="300" id="tiFour"/> <mx:Text x="10" y="10" text="The text property of the first text input below is bound declaratively to an actionscript function via metadata. When you type into it, the function computes the length and assigns the result to the output" width="378" height="77"/> <mx:Canvas y="150" height="117" right="10" left="10" borderColor="#FFAA00" backgroundColor="#FFFFCC" borderStyle="solid"> <mx:Text width="100%" height="100%" condenseWhite="false"> <mx:text>  </mx:text> </mx:Text> </mx:Canvas> </mx:Canvas> </mx:TabNavigator> </UI_Code>

package { import flash.events.IEventDispatcher; import flash.system.ApplicationDomain; import flash.utils.describeType; import flash.utils.getQualifiedClassName; import flash.utils.getQualifiedSuperclassName; import mx.binding.utils.BindingUtils; import mx.binding.utils.ChangeWatcher; public class ASBinder { private static var directivesByClassName:Object = {}; private static function directivesForClass(className:String):ClassDirective { var directives:ClassDirective = directivesByClassName[className]; if(directives != null) return directives; var ty:Class = ApplicationDomain.currentDomain.getDefinition(className) as Class; directives = new ClassDirective(); var baseClassName:String = getQualifiedSuperclassName(ty); if(baseClassName != null) { var baseClassDirectives:ClassDirective = directivesForClass(baseClassName); directives.bindDirectives = baseClassDirectives.bindDirectives.concat(); // methods are inherited down in describeTypeData. //directives.handleEventDirectives = baseClassDirectives.handleEventDirectives.concat(); } var des:XML = flash.utils.describeType(ty); var metadata:XMLList = des.factory.metadata.(@name == "Bind"); for (var i:int = 0; i < metadata.length(); i++) { var bindingDirective:BindDirective = new BindDirective(); bindingDirective.source = metadata[i].arg.(@key == "source").@value; bindingDirective.destination = metadata[i].arg.(@key == "destination").@value; bindingDirective.twoWay = (metadata[i].arg.(@key == "direction").@value == "both"); directives.bindDirectives.push(bindingDirective); } metadata = des.factory.method.metadata.(@name == "Bind"); for (i = 0; i < metadata.length(); i++) { bindingDirective = new BindDirective(); bindingDirective.source = metadata[i].arg.(@key == "source").@value; bindingDirective.handler = metadata[i].parent().@name; directives.bindDirectives.push(bindingDirective); } metadata = des.factory.method.metadata.(@name == "HandleEvent"); for (i = 0; i < metadata.length(); i++) { var handlerDirective:HandleEventDirective = new HandleEventDirective(); handlerDirective.dispatcher = metadata[i].arg.(@key == "dispatcher").@value; handlerDirective.event = metadata[i].arg.(@key == "event").@value; handlerDirective.handler = metadata[i].parent().@name; directives.handleEventDirectives.push(handlerDirective); } directivesByClassName[className] = directives; return directives; } private static function directivesForObject(target:Object):ClassDirective { var cname:String = getQualifiedClassName(target); return directivesForClass(cname); } public static function InitObject(target:Object):void { var directives:ClassDirective = directivesForObject(target); var bindDirectives:Array = directives.bindDirectives; for (var i:int = 0; i < bindDirectives.length; i++) { createBinding(target,bindDirectives[i]); } var handlerDirectives:Array = directives.handleEventDirectives; for (i=0; i < handlerDirectives.length; i++) { bindHandler(target,handlerDirectives[i]); } } private static function bind(target:Object,source:String,destination:String):void { var executeBinding:Function = function(newValue:*):void { var bindDest:Object = target; try { var destObjects:Array = destination.split("."); while(destObjects.length > 1) { bindDest = bindDest[destObjects.shift()]; } bindDest[destObjects.shift()] = newValue; } catch(e:*) {} }; BindingUtils.bindSetter(executeBinding,target,source.split(".")); } private static function createBinding(target:Object,directive:BindDirective):void { if(directive.destination != null && directive.destination != "") { bind(target,directive.source,directive.destination); if(directive.twoWay) bind(target,directive.destination,directive.source); } if(directive.handler != null && directive.handler != "") { var handlerFunc:Function = target[directive.handler]; BindingUtils.bindSetter(handlerFunc,target,directive.source.split(".")); } } private static function bindHandler(target:Object,directive:HandleEventDirective):void { var watcher:ChangeWatcher; var assignHandler:Function = function(value:IEventDispatcher):void { if(value == null) return; value.addEventListener(directive.event,target[directive.handler]); watcher.unwatch(); } watcher = BindingUtils.bindSetter(assignHandler,target,directive.dispatcher.split(".")); } } } class ClassDirective { public var bindDirectives:Array = []; public var handleEventDirectives:Array = []; } class BindDirective { public var source:String; public var destination:String; public var handler:String; public var twoWay:Boolean = false; } class HandleEventDirective { public var dispatcher:String; public var event:String; public var handler:String; }