編輯:關於Android編程
一、簡介
Android的消息機制主要是指Handler的運行機制,那麼什麼是Handler的運行機制那?通俗的來講就是,使用Handler將子線程的Message放入主線程的Messagequeue中,在主線程使用。
二、學習內容
學習Android的消息機制,我們需要先了解如下內容。
平常我們接觸的大多是Handler和Message,今天就讓我們來深入的了解一下他們。
三、代碼詳解
一般而言我們都是這樣使用Handler的
xxHandler.sendEmptyMessage(xxx);
當然還有其他表示方法,但我們深入到源代碼中,會發現,他們最終都調用了一個方法
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,但這依然不是結束,我們可以看到最後一句enqueueMessage(queue, msg, uptimeMillis);按字面意思來說插入一條消息,那麼疑問來了,消息插入了哪裡。
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; }
進入源代碼,我們發現,我們需要了解一個新類Messagequeue。
雖然我們一般把他叫做消息隊列,但是通過研究,我們發下,它實際上是一種單鏈表的數據結構,而我們對它的操作主要是插入和讀取。
看代碼33-44,學過數據結構,我們可以輕松的看出,這是一個單鏈表的插入末尾的操作。
這樣就明白了,我們send方法實質就是向Messagequeue中插入這麼一條消息,那麼另一個問題隨之而來,我們該如何處理這條消息。
處理消息我們離不開一個重要的,Looper。那麼它在消息機制中又有什麼樣的作用那?
Looper扮演著消息循環的角色,具體而言它會不停的從MessageQueue中查看是否有新消息如果有新消息就會立刻處理,否則就已知阻塞在那裡,現在讓我們來看一下他的代碼實現。
首先是構造方法
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
可以發現,它將當前線程對象保存了起來。我們繼續
Looper在新線程創建過程中有兩個重要的方法looper.prepare() looper.loop
new Thread(){ public void run(){ Looper.prepare(); Handler handler = new Handler(); Looper.loop(); } }.start();
我們先來看prepare()方法
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)); }
咦,我們可以看到這裡面又有一個ThreadLocal類,我們在這簡單了解一下,他的特性,set(),get()方法。
首先ThreadLocal是一個線程內部的數據存儲類,通過它可以在指定的線程中存儲數據,數據存儲後,只有在制定線程中可以獲取存儲的數據,對於其他線程而言則無法獲取到數據。簡單的來說。套用一個列子:
private ThreadLocal<Boolean> mBooleanThreadLocal = new ThreadLocal<Boolean>();// mBooleanThreadLocal.set(true); Log.d(TAH,"Threadmain"+mBooleanThreadLocal.get()); new Thread("Thread#1"){ public void run(){ mBooleanThreadLocal.set(false); Log.d(TAH,"Thread#1"+mBooleanThreadLocal.get()); }; }.start(); new Thread("Thread#2"){ public void run(){ Log.d(TAH,"Thread#2"+mBooleanThreadLocal.get()); }; }.start();
上面的代碼運行後,我們會發現,每一個線程的值都是不同的,即使他們訪問的是同意個ThreadLocal對象。
那麼我們接下來會在之後分析源碼,為什麼他會不一樣。現在我們跳回prepare()方法那一步,loop()方法源碼貼上
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(); } }
首先loop()方法,獲得這個線程的Looper,若沒有拋出異常。再獲得新建的Messagequeue,在這裡我們有必要補充一下Messagequeue的next()方法。
Message next() { // Return here if the message loop has already quit and been disposed. // This can happen if the application tries to restart a looper after quit // which is not supported. final long ptr = mPtr; if (ptr == 0) { return null; } int pendingIdleHandlerCount = -1; // -1 only during first iteration int nextPollTimeoutMillis = 0; for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } nativePollOnce(ptr, nextPollTimeoutMillis); synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); Message prevMsg = null; Message msg = mMessages; if (msg != null && msg.target == null) { // Stalled by a barrier. Find the next asynchronous message in the queue. do { prevMsg = msg; msg = msg.next; } while (msg != null && !msg.isAsynchronous()); } if (msg != null) { if (now < msg.when) { // Next message is not ready. Set a timeout to wake up when it is ready. nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); } else { // Got a message. mBlocked = false; if (prevMsg != null) { prevMsg.next = msg.next; } else { mMessages = msg.next; } msg.next = null; if (DEBUG) Log.v(TAG, "Returning message: " + msg); msg.markInUse(); return msg; } } else { // No more messages. nextPollTimeoutMillis = -1; } // Process the quit message now that all pending messages have been handled. if (mQuitting) { dispose(); return null; } // If first time idle, then get the number of idlers to run. // Idle handles only run if the queue is empty or if the first message // in the queue (possibly a barrier) is due to be handled in the future. if (pendingIdleHandlerCount < 0 && (mMessages == null || now < mMessages.when)) { pendingIdleHandlerCount = mIdleHandlers.size(); } if (pendingIdleHandlerCount <= 0) { // No idle handlers to run. Loop and wait some more. mBlocked = true; continue; } if (mPendingIdleHandlers == null) { mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)]; } mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers); } // Run the idle handlers. // We only ever reach this code block during the first iteration. for (int i = 0; i < pendingIdleHandlerCount; i++) { final IdleHandler idler = mPendingIdleHandlers[i]; mPendingIdleHandlers[i] = null; // release the reference to the handler boolean keep = false; try { keep = idler.queueIdle(); } catch (Throwable t) { Log.wtf(TAG, "IdleHandler threw exception", t); } if (!keep) { synchronized (this) { mIdleHandlers.remove(idler); } } } // Reset the idle handler count to 0 so we do not run them again. pendingIdleHandlerCount = 0; // While calling an idle handler, a new message could have been delivered // so go back and look again for a pending message without waiting. nextPollTimeoutMillis = 0; } }
從24-30我們可以看到,他遍歷了整個queue找到msg,若是msg為null,我們可以看到50,他把nextPollTimeoutMillis = -1;實際上是等待enqueueMessage的nativeWake來喚醒。較深的源碼涉及了native層代碼,有興趣可以研究一下。簡單來說next()方法,在有消息是會返回這條消息,若沒有,則阻塞在這裡。
我們回到loop()方法27msg.target.dispatchMessage(msg);我們看代碼
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
msg.target實際上就是發送這條消息的Handler,我們可以看到它將msg交給dispatchMessage(),最後調用了我們熟悉的方法handleMessage(msg);
三、總結
到目前為止,我們了解了android的消息機制流程,但它實際上還涉及了深層的native層方法.
以上就是本文的全部內容,希望本文的內容對大家的學習或者工作能帶來一定的幫助,同時也希望多多支持本站!
JSON的定義: 一種輕量級的數據交換格式,具有良好的可讀和便於快速編寫的特性。業內主流技術為其提供了完整的解決方案(有點類似於正則表達式 ,獲得了當今大部分語言的支持)
Android Studio原生支持使用Gradle來構建項目,使用動態語言Groovy定義項目構建的過程,避免了build.xml文件繁瑣的定義。然而使用的時候,卻有著
這一章將重點討論怎麼在應用中加入ffmpeg組件。所有測試都將在 Android Studio工具中進行。測試例子源地址:https://github.com/roman
Android中JNI的作用,就是讓Java能夠去調用由C/C++實現的代碼,為了實現這個功能,需要用到Anrdoid提供的NDK工具包,在這裡不講如何配