總結(jié)

handler.PNG

• 其中綠色的部分與藍(lán)色可能在一個(gè)線程钉汗，也可能不在一個(gè)線程

• 初始化handler時(shí)依溯，指定了對(duì)應(yīng)的Looper以及消息隊(duì)列MessageQueue

  public Handler(Callback callback, boolean async) {
        //可能的內(nèi)存泄露
        //所以盡可能用static 的handler,用弱引用引用外部類
        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());
            }
        }
  
        //所以必須在一個(gè)調(diào)用了Looper.prepare()的線程中初始化handler
        //android的主線程在啟動(dòng)是就已經(jīng)調(diào)用過了
        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;
    }
  

• 在主線程或工作線程中滤愕，將要傳遞的數(shù)據(jù)或者要執(zhí)行的Runable包裝成一個(gè)Message對(duì)象茎刚，然后Handler將這個(gè)Message對(duì)象加到主線程的消息隊(duì)列中

  //Handler 代碼
  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);
    }
  

• Message除了包含必要的信息外堤如，還有一個(gè)target，這個(gè)target就是這個(gè)Handler對(duì)象本身

  private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
       //指定了消息處理者target,也就是handler本身
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }
  

• Looper不斷的從消息隊(duì)列中取出消息忽媒，然后調(diào)用消息的target去分發(fā)消息争拐，也就是調(diào)用Handler去處理消息，處理完后晦雨，將Message放到Message對(duì)象池中架曹，循環(huán)使用

  //Looper代碼
  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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }
  
            final long traceTag = me.mTraceTag;
            if (traceTag != 0) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                //取出消息，然后調(diào)用target去處理消息闹瞧，實(shí)際也就是handler去處理消息
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
  
            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.recycleUnchecked();
        }
    }
  

  • handler處理分發(fā)消息

    public void dispatchMessage(Message msg) {
       //如果消息的callback不為null,實(shí)際上就是這個(gè)消息是包裝了一個(gè)Runable
       //就去執(zhí)行這個(gè)Runable
       if (msg.callback != null) {
           handleCallback(msg);
       } else {
           //如果handler的callback不為空音瓷，并且返回true，表示處理了消息夹抗，就結(jié)束了
           //否則就由自身的handleMessage函數(shù)去處理
           //handler的callback主要目的是不用重載函數(shù)，只用傳入一個(gè)callback對(duì)象就行
           if (mCallback != null) {
               if (mCallback.handleMessage(msg)) {
                   return;
               }
           }
           handleMessage(msg);
       }
    }