編輯:關於Android編程
注:這裡只是說一下sendmessage的一個過程,post就類似的
如果我們需要發送消息,會調用sendMessage方法
public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); }
這個方法會調用如下的這個方法
public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); }
接下來設定延遲時間,然後繼續調用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); }
這裡獲得了消息隊列,檢查隊列是否存在,然後返回enqueMessage的方法的執行結果,這個結果是說明消息能否進入隊列的一個布爾值
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
這裡是對消息進行入隊處理,下面就是在MessageQueue中對消息進行入隊
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(TAG, 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; }
就是對傳遞過來的消息進行一些封裝然後放到隊列中,至此我們的sendMessage處理完畢,返回的結果是進隊是否成功的布爾值,那麼究竟消息之後是如何被處理的呢?
我們可以看到在Handler構造的時候記錄了一個Looper對象,也記錄了一個回掉函數
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; }
這裡的myLooper方法返回的是當前線程關聯的一個Looper對象
public static @Nullable Looper myLooper() { return sThreadLocal.get(); }
當Looper實例化了以後會執行自己的prepare方法然後執行loop方法,loop方法就是不斷的讀取消息隊列中的消息然後執行相應的操作的方法,因為是在其他線程中執行的循環所以不會影響其他線程
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.recycleUnchecked(); } }
在循環中如果讀取到了消息,就會執行dispatchMessage方法,然後分派完消息之後再執行一次recycleUnchecked方法來重用這個Message,我們看到dispatchMessage方法
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
這裡看到直接執行了一個handlerMessage方法,這個方法是一個回調方法,我們是必須實現的,否則Handler什麼都不會做,為什麼呢?還記得剛剛說構造Handler的時候我們記錄了一個CallBack的回掉嗎?Handler中的這個handlerMessage方法是一個空方法,如果我們重寫了這個方法,在回調的時候就會執行我們先寫下的代碼,也就是接收到消息之後要做什麼。
public interface Callback { public boolean handleMessage(Message msg); } public void handleMessage(Message msg) { }
這裡簡單說下整個過程:
當我們實例化一個Handler的子類並重寫handleMessage方法之後,這個時候系統已經幫我們做了幾個事情
1.實例化了一個消息隊列MessageQueue
2.實例化了一個關聯的Looper對象,並讓Looper不斷的讀取消息隊列
3.把我們重寫的handleMessage方法記錄為我們需要回調的方法
當我們執行Handler的sendMessage方法的時候,系統會把我們傳過去的Message對象添加到消息隊列,這個時候如果Looper讀取到了消息,就會把消息派發出去,然後回調handleMessage方法,執行我們設定的代碼。
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