handler訊息機制

標籤：

MessageQueue代碼：http://grepcode.com/file_/repository.grepcode.com/java/ext/com.google.android/android/5.1.1_r1/android/os/MessageQueue.java/?v=source

Handler代碼：

http://grepcode.com/file_/repository.grepcode.com/java/ext/com.google.android/android/5.1.1_r1/android/os/Handler.java/?v=source

Looper代碼：

http://grepcode.com/file_/repository.grepcode.com/java/ext/com.google.android/android/5.1.1_r1/android/os/Looper.java/?v=source

 

handler的訊息機制主要牽扯三方面：

1. 訊息寄件者；
2. 訊息佇列；
3. 訊息處理迴圈操作。

訊息發送：

public final boolean More ...sendMessage(Message msg)    {        return sendMessageDelayed(msg, 0);    }

往下走：

 public final boolean More ...sendMessageDelayed(Message msg, long delayMillis)    {        if (delayMillis < 0) {            delayMillis = 0;        }        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);    }

最後走到這裡:

public boolean More ...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);    }

調用 enqueueMessage 在訊息佇列中插入一條訊息，在 enqueueMessage總中，會把 msg.target 設定為當前的Handler 對象。

private boolean More ...enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {    msg.target = this;    if (mAsynchronous) {        msg.setAsynchronous(true);    }    return queue.enqueueMessage(msg, uptimeMillis);}

如下將訊息插入訊息佇列中去。並且可以很明顯看出訊息佇列是線性鏈表結構。

boolean enqueueMessage(Message msg, long when) {    if (msg.target == null) {        throw new IllegalArgumentException("Message must have a target.");    }    if (msg.isInUse()) {        throw new IllegalStateException(msg + " This message is already in use.");    }    synchronized (this) {        if (mQuitting) {            IllegalStateException e = new IllegalStateException(                msg.target + " sending message to a Handler on a dead thread");            Log.w("MessageQueue", e.getMessage(), e);            msg.recycle();            return false;        }        msg.markInUse();        msg.when = when;        Message p = mMessages;        boolean needWake;        if (p == null || when == 0 || when < p.when) {            // New head, wake up the event queue if blocked.            msg.next = p;            mMessages = msg;            needWake = mBlocked;        } else {            // Inserted within the middle of the queue.  Usually we don‘t have to wake            // up the event queue unless there is a barrier at the head of the queue            // and the message is the earliest asynchronous message in the queue.            needWake = mBlocked && p.target == null && msg.isAsynchronous();            Message prev;            for (;;) {                prev = p;                p = p.next;                if (p == null || when < p.when) {                    break;                }                if (needWake && p.isAsynchronous()) {                    needWake = false;                }            }            msg.next = p; // invariant: p == prev.next            prev.next = msg;        }        // We can assume mPtr != 0 because mQuitting is false.        if (needWake) {            nativeWake(mPtr);        }    }    return true;}

 

 

每個線程只能運行一個looper對象，建立 Looper 的時候，內部會建立一個訊息佇列MessageQueue;並且looper線上程裡面必須先prepare(),再loop來對訊息佇列messageQueue來進行遍曆迴圈操作。

public static void loop() {        final Looper me = myLooper();        //如果looper對象是空的，就提示必須在當前線程通過looper.prepare()來進行建立        if (me == null) {            throw new RuntimeException("No Looper; Looper.prepare() wasn‘t called on this thread.");        }        //如果looper不為null,就取出訊息佇列        final MessageQueue queue = me.mQueue;        Binder.clearCallingIdentity();        final long ident = Binder.clearCallingIdentity();        //遍曆訊息佇列        for (;;) {            Message msg = queue.next(); // might block            if (msg == null) {                // No message indicates that the message queue is quitting.                //沒有訊息中斷操作                return;            }            //----------------隊列不為null的操作----------------            // 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);            }            //----------------隊列不為null，分發資料出去----------------            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.recycleUnchecked();        }    } 

 

迴圈遍曆並分發訊息出去msg.target.dispatchMessage(msg)：

/**     * Handle system messages here.     *首先，如果訊息的 callback 不是空，便調用 handleCallback 處理。否則判斷 Handler 的 mCallback 是否為空白，不為空白則調用它的 handleMessage方法。如果仍然為空白，才調用 Handler 自身的 handleMessage，也就是我們建立 Handler 時重寫的方法     */public void dispatchMessage(Message msg) {    if (msg.callback != null) {        handleCallback(msg);    } else {        if (mCallback != null) {            if (mCallback.handleMessage(msg)) {                return;            }        }        //回調handler的handleMessage(msg)方法，從而接收資料        handleMessage(msg);    }}

 額，先寫到這，明天抽時間再看下，儘可能減少錯誤。

handler訊息機制