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Android异步消息处理机制实现原理详解

海绵般汲取 人气:0

本文着重讲解了Android异步消息处理机制实现原理详解,文中通过代码实例讲解的非常细致,对大家的工作和学习具有一定的参考学习价值,欢迎大家阅读和收藏

消息处理机制主要对象:Looper,Handler,Message(还有MessageQueue和Runnable)

Looper不断从MessageQueue消息队列中取出一个Message,然后传给Handle,如此循环往复,如果队列为空,那么它会进入休眠。

这些类的主要变量

Looper.java

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
  private static Looper sMainLooper; // guarded by Looper.class

  final MessageQueue mQueue;
  final Thread mThread;

Handler.java

final MessageQueue mQueue;
  final Looper mLooper;
  final Callback mCallback;
  final boolean mAsynchronous;
  IMessenger mMessenger;

Message.java

Handler target;每个消息只能对应一个handler
Runnable callback;回调接口

MessageQueue.java

Message mMessages;

Runnable是一个空接口类,没有变量

上一个书上的图:

Handler和Thread没有直接关系,但对应关系可以推理得到

每个Thread只对应一个Looper;

每个Looper只对应一个MessageQueue;

每个MessageQueue对应N个Message,每个Message只对应一个Handler

==》每个Thread对应N个Handler。

Handler是”真正处理事情“的地方,作用:处理消息,将Message压入MessageQueue中

带着一个问题看源码:创建handler对象的线程(ui/主线程除外)为什么,必须先调用Looper.prepare() ?

public Handler() {
    this(null, false);
  }
public Handler(Callback callback) {
    this(callback, false);
  }
public Handler(Looper looper) {
    this(looper, null, false);
  }
 public Handler(Looper looper, Callback callback) {
    this(looper, callback, false);
  }
 public Handler(boolean async) {
    this(null, async);
  }
public Handler(Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) {
      final Class<? extends Handler> klass = getClass();
      if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
          (klass.getModifiers() & Modifier.STATIC) == 0) {
        Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
          klass.getCanonicalName());
      }
    }

    mLooper = Looper.myLooper();
    if (mLooper == null) {
      throw new RuntimeException(
        "Can't create handler inside thread that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
  }
 public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
  }

初始化handler对象时(构造方法是Handler(),Handler(Callback callback))都间接调用Handler(Callback callback, boolean async)构造方法

主要代码是Looper.myLooper();

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();//这是在Looper类中的定义
public static Looper myLooper() {
    return sThreadLocal.get();//从当前线程中获得looper对象
  }
public static void prepare() {
prepare(true);
}

private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));//为当前线程设置looper对象
}

我们自己创建线程必须通过Looper.prepare()方法为当前线程设置looper对象才可以通过Looper.myLooper()方法返回looper对象,这样在非UI线程创建handler对象时才不会报错。"Can't create handler inside thread that has not called Looper.prepare()"

ps:prepare(boolean quitAllowed)(这个不用我们关心,略过。。)

这个quitAlowed参数是定义消息队列用了,看的源代码是android4.4

Looper.javaprivate Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mRun = true;
    mThread = Thread.currentThread();
  }MessageQueue.java
// True if the message queue can be quit.  private final boolean mQuitAllowed;//true消息队列可以被quit,false消息队列不能被quit。

主线程/UI线程的MessageQueue不能被销毁掉。看源码(销毁调用Looper.quit())

public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
      if (sMainLooper != null) {
        throw new IllegalStateException("The main Looper has already been prepared.");
      }
      sMainLooper = myLooper();
    }
  }

偏离太远了

所以得出结论:创建handler对象的线程(ui/主线程除外),必须先调用Looper.prepare()

Handler作用1:处理消息

在Looper类中处理消息是通过msg.target.dispatchMessage(msg);target就是handler对象(Message类的内部变量Handler target)将消息转发到处理消息的对应的handler对象上,然后这个target即handler对象会在处理消息前做一个检查

public void dispatchMessage(Message msg) {
    if (msg.callback != null) {//如果msg有绑定callback回调接口Runaable不为空,则执行Runnable的run方法
      handleCallback(msg);
    } else {
      if (mCallback != null) {//如果handler的内置接口类Callback不为空,则执行boolean handleMessage(Message msg)这个方法
        if (mCallback.handleMessage(msg)) {执行完成则return
          return;
        }
      }
      handleMessage(msg);//最后才执行handler本身的方法
    }
  }
 private static void handleCallback(Message message) {    message.callback.run();  }
public interface Callback {//handler的内置接口类Callback
public boolean handleMessage(Message msg);
  }

Handler作用2:将Message压入MessageQueue中

handler中提供的很多发送message的方法,除了sendMessageAtFrontOfQueue()方法(直接调用enqueueMessage(queue, msg, 0);)之外,其它的发送消息方法最终都会辗转调用到sendMessageAtTime()方法

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
    MessageQueue queue = mQueue;
    if (queue == null) {
      RuntimeException e = new RuntimeException(
          this + " sendMessageAtTime() called with no mQueue");
      Log.w("Looper", e.getMessage(), e);
      return false;
    }
    return enqueueMessage(queue, msg, uptimeMillis);
  }

sendMessageAtTime()方法也是调用Handler中的enqueueMessage(queue, msg, uptimeMillis)方法

和sendMessageAtFrontOfQueue()方法两者最后都会调用enqueueMessage(queue, msg, uptimeMillis)方法

区别是需要延迟uptimeMillis时间后才将Message压入MessageQueue中

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;//给msg的target赋值为handler自身然后加入MessageQueue中
    if (mAsynchronous) {
      msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
  }

最终所有的方法都是调用MessageQueue中的enqueueMessage(msg, uptimeMillis);方法,是不是感觉两个方法差不多啊,注意参数!!

MessageQueue的使用是在Looper中

Handler的作用整理完毕(好像我现在已经可以把Handler源码完整默写下来了。哈哈^.^记忆力真不行)

Looper类

作用:与当前线程绑定,保证一个线程只会有一个Looper实例,同时一个Looper实例也只有一个MessageQueue。

对于Looper主要是prepare()和loop()两个方法

prepare()将普通线程转化为looper线程,

loop()方法,不断从MessageQueue中去取消息,交给消息的target属性的dispatchMessage去处理。

public static void loop() {
    final Looper me = myLooper();
    if (me == null) {
      throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue;

    // Make sure the identity of this thread is that of the local process,
    // and keep track of what that identity token actually is.
    Binder.clearCallingIdentity();
    final long ident = Binder.clearCallingIdentity();

    for (;;) {
      Message msg = queue.next(); // might block
      if (msg == null) {
        // No message indicates that the message queue is quitting.
        return;
      }

      // This must be in a local variable, in case a UI event sets the logger
      Printer logging = me.mLogging;
      if (logging != null) {
        logging.println(">>>>> Dispatching to " + msg.target + " " +
            msg.callback + ": " + msg.what);
      }

      msg.target.dispatchMessage(msg);

      if (logging != null) {
        logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
      }

      // Make sure that during the course of dispatching the
      // identity of the thread wasn't corrupted.
      final long newIdent = Binder.clearCallingIdentity();
      if (ident != newIdent) {
        Log.wtf(TAG, "Thread identity changed from 0x"
            + Long.toHexString(ident) + " to 0x"
            + Long.toHexString(newIdent) + " while dispatching to "
            + msg.target.getClass().getName() + " "
            + msg.callback + " what=" + msg.what);
      }

      msg.recycle();
    }
  }

27行就是上面提到了,handler进行消息处理的关键代码了

看着上面的分析很复杂,总结下

1、首先Looper.prepare()为在当前线程中保存一个Looper实例(sThreadLocal.set()),然后该实例中保存一个MessageQueue对象;因为Looper.prepare()在一个线程中只能调用一次,所以MessageQueue在一个线程中只会存在一个。

2、Looper.loop()会让当前线程进入一个无限循环,不端从MessageQueue的实例中读取消息,然后回调msg.target.dispatchMessage(msg)方法。

3、Handler的构造方法,会首先得到当前线程中保存的Looper实例,进而与Looper实例中的MessageQueue想关联。

4、Handler的sendMessage方法,会给msg的target赋值为handler自身,然后加入MessageQueue中。

5、在构造Handler实例时,我们会重写handleMessage方法,也就是msg.target.dispatchMessage(msg)最终调用的方法。

好了,总结完成,大家可能还会问,那么在Activity中,我们并没有显示的调用Looper.prepare()和Looper.loop()方法,为啥Handler可以成功创建呢,这是因为在Activity的启动代码中,已经在当前UI线程调用了Looper.prepare()和Looper.loop()方法。

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