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[操作系统]Handler机制(一)


Message:

定义:

public final class Message implements Parcelable 

Message类是个final类,就是说不能被继承,同时Message类实现了Parcelable接口,我们知道android提供了一种新的类型:Parcel。本类被用作封装数据的容器,是链表结构,有个属性next和sPool,这两个变量是不同的,具体什么不同看下文。

文档描述:






Defines a message containing a description and arbitrary data object that can be sent to a {@link Handler}. This object contains two extra int fields and an extra object field that allow you to not do allocations in many cases. 

定义一个包含任意类型的描述数据对象,此对象可以发送给Handler。对象包含两个额外的int字段和一个额外的对象字段,这样可以使得在很多情况下不用做分配工作。尽管Message的构造器是公开的,但是获取Message对象的最好方法是调用Message.obtain()或者Handler.obtainMessage(), 这样是从一个可回收对象池中获取Message对象。

1.看一下全局变量:有好多存数据的对象。

public int what;public int arg1; public int arg2;public Object obj;public Messenger replyTo;/*package*/ int flags;/*package*/ long when;/*package*/ Bundle data;/*package*/ Handler target;/*package*/ Runnable callback;// sometimes we store linked lists of these things/*package*/ Message next;private static final Object sPoolSync = new Object();private static Message sPool;private static int sPoolSize = 0;private static final int MAX_POOL_SIZE = 50;private static boolean gCheckRecycle = true;

  1. what:用户定义消息代码以便收件人可以识别这是哪一个Message。每个Handler用它自己的名称空间为消息代码,所以您不需要担心你的Handler与其他handler冲突。
  2. arg1、arg2:如果只是想向message内放一些整数值,可以使用arg1和arg2来代替setData方法。
  3. obj:发送给接收器的任意对象。当使用Message对象在线程间传递消息时,如果它包含一个Parcelable的结构类(不是由应用程序实现的类),此字段必须为非空(non-null)。其他的数据传输则使用setData(Bundle)方法。注意Parcelable对象是从FROYO版本以后才开始支持的。
  4. replyTo:指明此message发送到何处的可选Messenger对象。具体的使用方法由发送者和接受者决定。
  5. FLAG_IN_USE:判断Message是否在使用( default 包内可见)
  6. FLAG_ASYNCHRONOUS:如果设置message是异步的。
  7. FLAGS_TO_CLEAR_ON_COPY_FROM:明确在copyFrom方法
  8. 其他参数都比较简单,不详述

Obtain方法:

 

//从全局池中返回一个新的Message实例。在大多数情况下这样可以避免分配新的对象。//是一个静态方法public static Message obtain() {    synchronized (sPoolSync) {      if (sPool != null) {        Message m = sPool;        sPool = m.next;        m.next = null;        m.flags = 0; // clear in-use flag        sPoolSize--;        return m;      }    }    return new Message();  }

  在看它一系列的重载方法:

/**   * Same as {@link #obtain()}, but copies the values of an existing   * message (including its target) into the new one.   * @param orig Original message to copy.   * @return A Message object from the global pool.   */public static Message obtain(Message orig) {    Message m = obtain();    m.what = orig.what;    m.arg1 = orig.arg1;    m.arg2 = orig.arg2;    m.obj = orig.obj;    m.replyTo = orig.replyTo;    m.sendingUid = orig.sendingUid;    if (orig.data != null) {      m.data = new Bundle(orig.data);    }    m.target = orig.target;    m.callback = orig.callback;    return m;  } /**   设置target   */public static Message obtain(Handler h) {    Message m = obtain();    m.target = h;    return m;  } /**   * Same as {@link #obtain(Handler)}, but assigns a callback Runnable on   * the Message that is returned.   * @param h Handler to assign to the returned Message object's <em>target</em> member.   * @param callback Runnable that will execute when the message is handled.   * @return A Message object from the global pool.   */  public static Message obtain(Handler h, Runnable callback) {    Message m = obtain();    m.target = h;    m.callback = callback;    return m;  }/**   * Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>,   * <em>arg1</em>, <em>arg2</em>, and <em>obj</em> members.   。。。。   * @param obj The <em>obj</em> value to set.   * @return A Message object from the global pool.   */  public static Message obtain(Handler h, int what,       int arg1, int arg2, Object obj) {    Message m = obtain();    m.target = h;    m.what = what;    m.arg1 = arg1;    m.arg2 = arg2;    m.obj = obj;    return m;  }

还有几个没列举出来,都是先调用obtain()方法,然后把获取的Message实例加上各种参数。代码一目了然。。。

recycle():回收当前message到全局池

 

/**   * Return a Message instance to the global pool.   * <p>   * You MUST NOT touch the Message after calling this function because it has   * effectively been freed. It is an error to recycle a message that is currently   * enqueued or that is in the process of being delivered to a Handler.   * </p>   */  public void recycle() {    if (isInUse()) {      if (gCheckRecycle) {        throw new IllegalStateException("This message cannot be recycled because it "            + "is still in use.");      }      return;    }    recycleUnchecked();  }  /**   * Recycles a Message that may be in-use.   * Used internally by the MessageQueue and Looper when disposing of queued Messages.   */  void recycleUnchecked() {    // Mark the message as in use while it remains in the recycled object pool.    // Clear out all other details.    flags = FLAG_IN_USE;    what = 0;    arg1 = 0;    arg2 = 0;    obj = null;    replyTo = null;    sendingUid = -1;    when = 0;    target = null;    callback = null;    data = null;    synchronized (sPoolSync) {      if (sPoolSize < MAX_POOL_SIZE) {        next = sPool;        sPool = this;        sPoolSize++;      }    }  }

向全局池中返回一个Message实例。一定不能在调用此函数后再使用Message——它实际上已经被释放。

getWhen:

 /**   * Return the targeted delivery time of this message, in milliseconds.   */  public long getWhen() {    return when;  }

返回此消息的传输时间,以毫秒为单位。

 

 setTarget,getTarget:

//设置handler和返回handlerpublic void setTarget(Handler target) {    this.target = target;  }  /**   * Retrieve the a {@link android.os.Handler Handler} implementation that   * will receive this message. The object must implement   * {@link android.os.Handler#handleMessage(android.os.Message)   * Handler.handleMessage()}. Each Handler has its own name-space for   * message codes, so you do not need to   * worry about yours conflicting with other handlers.   */  public Handler getTarget() {    return target;  }

获取将接收此消息的Handler对象。此对象必须要实现Handler.handleMessage()方法。每个handler各自包含自己的消息代码,所以不用担心自定义的消息跟其他handlers有冲突。

setData:

 设置一个可以是任何类型值的bundle。

/**   * Sets a Bundle of arbitrary data values. Use arg1 and arg2 members   * as a lower cost way to send a few simple integer values, if you can.   * @see #getData()   * @see #peekData()   */  public void setData(Bundle data) {    this.data = data;  }

  getData,peekData

 public Bundle getData() {    if (data == null) {      data = new Bundle();    }        return data;  }public Bundle peekData() {    return data;}

发送消息的一些方法:

/**向Handler发送此消息,getTarget()方法可以获取此Handler。如果这个字段没有设置会抛出个空指针异常。   * Sends this Message to the Handler specified by {@link #getTarget}.   * Throws a null pointer exception if this field has not been set.   */  public void sendToTarget() {    target.sendMessage(this);  }  

构造方法:

 /** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).  */  public Message() {  }//推荐使用Message.obtain()

writeToParcel:

public void writeToParcel(Parcel dest, int flags) {    if (callback != null) {      throw new RuntimeException(        "Can't marshal callbacks across processes.");    }    dest.writeInt(what);    dest.writeInt(arg1);    dest.writeInt(arg2);    if (obj != null) {      try {        Parcelable p = (Parcelable)obj;        dest.writeInt(1);        dest.writeParcelable(p, flags);      } catch (ClassCastException e) {        throw new RuntimeException(          "Can't marshal non-Parcelable objects across processes.");      }    } else {      dest.writeInt(0);    }    dest.writeLong(when);    dest.writeBundle(data);    Messenger.writeMessengerOrNullToParcel(replyTo, dest);    dest.writeInt(sendingUid);  }

 将类的数据写入外部提供的Parcel中和从Parcel中读取数据。






Message结束。。。。