flex 常用元标签讲解

本文转自 flex 常用元标签讲解

Flex 引入了元数据标签的概念，大多数人都使用过的 [Bindable] 标签便是其中的 meta tag 之一，它在代码中的作用就是向编译器提供如何编译程序的信息。实际上，这些标签并没有被编译到生成的 SWF 文件中，而只是告诉编译器如何生成 SWF 文件。

adobe 官网 http://livedocs.adobe.com/flex/3/html/help.html?content=metadata_3.html详细讲解了 16种meta 的使用，简单如下表所示。

Tag	Description
[ArrayElementType]	Defines the allowed data type of each element of an Array.
[Bindable]	Identifies a property that you can use as the source of a data binding expression.
[DefaultProperty]	Defines the name of the default property of the component when you use the component in an MXML file.
[Deprecated]	Marks a class or class element as deprecated so that the compiler can recognize it and issue a warning when the element is used in an application.
[Effect]	Defines the MXML property name for the effect.
[Embed]	Imports JPEG, GIF, PNG, SVG, and SWF files at compile time. Also imports image assets from SWC files.
[Event]	Defines the MXML property for an event and the data type of the event object that a component emits.
[Exclude]	Omits the class element from the Flex Builder tag inspector. The syntax is as follows:[Exclude(name="label", kind="property")]
[ExcludeClass]	Omits the class from the Flex Builder tag inspector. This is equivalent to the @private tag in ASDoc when applied to a class.
[IconFile]	Identifies the filename for the icon that represents the component in the Insert bar of Adobe Flex Builder.
[Inspectable]	Defines an attribute exposed to component users in the attribute hints and Tag inspector of Flex Builder. Also limits allowable values of the property.
[InstanceType]	Specifies the allowed data type of a property of type IDeferredInstance.
[NonCommittingChangeEvent]	Identifies an event as an interim trigger.
[RemoteClass]	Maps the ActionScript object to a Java object.
[Style]	Defines the MXML property for a style property for the component.
[Transient]	Identifies a property that should be omitted from data that is sent to the server when an ActionScript object is mapped to a Java object using [RemoteClass].

本人认为，这其中常用的 Bindable ， Embed ， RemoteClass 这 3 种（本人是从开发业务程序角度来考虑， effect 等 ui 效果就暂且忽略了）。
1、 Bindable 标签
Bindable 标签应该是使用率最高的标签，其功效就是将某个变量或者属性绑定到目标对象上，比如业务应用经常要操作 datagrid ，此时就需将变量绑定到 datagrid.dataprovider 上，通过变量操作来显示 datagrid 的数据变动。
[ Bindable ]
private var transWayLists:ArrayCollection;
...
<mx:AdvancedDataGrid dataProvider="{ transWayLists }"/>
2、 RemoteClass 标签
该标签用以映射 flex object 到 java object ，因此在写 flex 程序与 java 程序交互之时特别有用，比如 flex 登录程序总需发送登录信息到 java 登录服务，然后 java 登录服务通过验证返回登录用户信息，此时就需 flex user映射到 java user ，如下：
flex ：
[ Bindable ]
[RemoteClass ( alias="com. marcus .User" )]
public class User
{
public var userId:String;
public var businessId:String;
public var userName:String;
public var password:String;
public var passTimeout:Date;
...
}
java:
package com.marcus;
import java.util.Date;
public class User {
private String userId;
private String businessId;
private String userName;
private String password;
private Date passTimeout;
...
}
3、 Embed 标签
该标签用以嵌入 jpg 等资源文件，并在编译的时候生成到 swf 文件中，因此嵌入资源文件过大时，会影响swf 加载速度。
[Embed(source='home.png')] // 绑定图片home.png 给home_icon 类
private var home_icon:Class;

4、 ArrayElementType 标签
该标签用以标识数组中元素的数据类型，该 meta tag 并未列入本人的常用 meta tag 之一，为何还要强调的原因就是本人认为 ArrayElementType 标签就是一个鸡肋标签，狗屁不是，建议大家慎用。