Android異步消息機制
阿新 • • 發佈:2019-05-17
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目錄介紹
- 1.Handler的常見的使用方式
- 2.如何在子線程中定義Handler
- 3.主線程如何自動調用Looper.prepare()
- 4.Looper.prepare()方法源碼分析
- 5.Looper中用什麽存儲消息
- 6.Handler發送消息如何運作
- 7.Looper.loop()方法源碼分析
- 8.runOnUiThread如何實現子線程更新UI
- 9.Handler的post方法和view的post方法
- 10.主線程中Looper的輪詢死循環為何沒阻塞主線程
- 11.得出部分結論
好消息
- 01.基礎組件(9篇)
- 02.IPC機制(0篇)
- 03.View原理(7篇)
- 04.動畫機制(2篇)
- 05.View事件(9篇)
- 06.消息機制(6篇)
- 07.多媒體(9篇)
- 08.View事件(4篇)
- 09.多線程(4篇)
- 10.Window(11篇)
- 11.WebView(4篇)
- 12.網絡相關(7篇)
- 13.註解(14篇)
- 14.音視頻(13篇)
- 15.優化相關(8篇)
- 16.設計模式(4篇)
- 20.零碎筆記(12篇)
- 21.kotlin學習(1篇)
- 22.源碼分析(11篇)
- 23.架構技術(13篇)
- 25.RecyclerView(21篇)
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1.Handler的常見的使用方式
- handler機制大家都比較熟悉呢。在子線程中發送消息,主線程接受到消息並且處理邏輯。如下所示
- 一般handler的使用方式都是在主線程中定義Handler,然後在子線程中調用mHandler.sendXx()方法,這裏有一個疑問可以在子線程中定義Handler嗎?
public class MainActivity extends AppCompatActivity { private TextView tv ; /** * 在主線程中定義Handler,並實現對應的handleMessage方法 */ public static Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what == 101) { Log.i("MainActivity", "接收到handler消息..."); } } }; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); tv = (TextView) findViewById(R.id.tv); tv.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { new Thread() { @Override public void run() { // 在子線程中發送異步消息 mHandler.sendEmptyMessage(1); } }.start(); } }); } }
2.如何在子線程中定義Handler
- 直接在子線程中創建handler,看看會出現什麽情況?
- 運行後可以得出在子線程中定義Handler對象出錯,難道Handler對象的定義或者是初始化只能在主線程中?其實不是這樣的,錯誤信息中提示的已經很明顯了,在初始化Handler對象之前需要調用Looper.prepare()方法
tv.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { new Thread() { @Override public void run() { Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what == 1) { Log.i(TAG, "在子線程中定義Handler,接收並處理消息"); } } }; } }.start(); } });
- 如何正確運行。在這裏問一個問題,在子線程中可以吐司嗎?答案是可以的,只不過又條件,詳細可以看這篇文章02.Toast源碼深度分析
- 這樣程序已經不會報錯,那麽這說明初始化Handler對象的時候我們是需要調用Looper.prepare()的,那麽主線程中為什麽可以直接初始化Handler呢?難道是主線程創建handler對象的時候,會自動調用Looper.prepare()方法的嗎?
tv.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { new Thread() { @Override public void run() { Looper.prepare(); Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what == 1) { Log.i(TAG, "在子線程中定義Handler,接收並處理消息"); } } }; Looper.loop(); } }.start(); } });
3.主線程如何自動調用Looper.prepare()
- 首先直接可以看在App初始化的時候會執行ActivityThread的main方法中的代碼,如下所示
- 可以看到Looper.prepare()方法在這裏調用,所以在主線程中可以直接初始化Handler了。
public static void main(String[] args) { //省略部分代碼 Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }
- 並且可以看到還調用了:Looper.loop()方法,可以知道一個Handler的標準寫法其實是這樣的
Looper.prepare(); Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what == 101) { Log.i(TAG, "在子線程中定義Handler,並接收到消息"); } } }; Looper.loop();
4.Looper.prepare()方法源碼分析
- 源碼如下所示
- 可以看到Looper中有一個ThreadLocal成員變量,熟悉JDK的同學應該知道,當使用ThreadLocal維護變量時,ThreadLocal為每個使用該變量的線程提供獨立的變量副本,所以每一個線程都可以獨立地改變自己的副本,而不會影響其它線程所對應的副本。
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)); }
- 思考:Looper.prepare()能否調用兩次或者多次
- 如果運行,則會報錯,並提示prepare中的Excetion信息。由此可以得出在每個線程中Looper.prepare()能且只能調用一次
//這裏Looper.prepare()方法調用了兩次 Looper.prepare(); Looper.prepare(); Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what == 1) { Log.i(TAG, "在子線程中定義Handler,並接收到消息。。。"); } } }; Looper.loop();
5.Looper中用什麽存儲消息
- 先看一下下面得源代碼
- 看Looper對象的構造方法,可以看到在其構造方法中初始化了一個MessageQueue對象。MessageQueue也稱之為消息隊列,特點是先進先出,底層實現是單鏈表數據結構
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)); } private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
- 得出結論
- Looper.prepare()方法初始話了一個Looper對象並關聯在一個MessageQueue對象,並且一個線程中只有一個Looper對象,只有一個MessageQueue對象。
6.Handler發送消息如何運作
- 首先看看構造方法
- 可以看出在Handler的構造方法中,主要初始化了一下變量,並判斷Handler對象的初始化不應再內部類,靜態類,匿名類中,並且保存了當前線程中的Looper對象。
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; }
- 看handler.sendMessage(msg)方法
- 關於下面得源碼,是步步追蹤,看enqueueMessage這個方法,原來msg.target就是Handler對象本身;而這裏的queue對象就是我們的Handler內部維護的Looper對象關聯的MessageQueue對象。
handler.sendMessage(message); //追蹤到這一步 public final boolean sendMessage(Message msg){ return sendMessageDelayed(msg, 0); } 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); } private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
- 看MessageQueue對象的enqueueMessage方法
- 看到這裏MessageQueue並沒有使用列表將所有的Message保存起來,而是使用Message.next保存下一個Message,從而按照時間將所有的Message排序
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; }
7.Looper.loop()方法源碼分析
- 看看裏面得源碼,如下所示
- 看到Looper.loop()方法裏起了一個死循環,不斷的判斷MessageQueue中的消息是否為空,如果為空則直接return掉,然後執行queue.next()方法
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; 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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs; final long traceTag = me.mTraceTag; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis(); final long end; try { msg.target.dispatchMessage(msg); end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis(); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } if (slowDispatchThresholdMs > 0) { final long time = end - start; if (time > slowDispatchThresholdMs) { Slog.w(TAG, "Dispatch took " + time + "ms on " + Thread.currentThread().getName() + ", h=" + msg.target + " cb=" + msg.callback + " msg=" + msg.what); } } 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(); } }
- 看queue.next()方法源碼
- 大概的實現邏輯就是Message的出棧操作,裏面可能對線程,並發控制做了一些限制等。獲取到棧頂的Message對象之後開始執行:msg.target.dispatchMessage(msg)
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; } }
- 那麽msg.target是什麽呢?通過追蹤可以知道就是定義的Handler對象,然後查看一下Handler類的dispatchMessage方法:
- 可以看到,如果我們設置了callback(Runnable對象)的話,則會直接調用handleCallback方法
- 在初始化Handler的時候設置了callback(Runnable)對象,則直接調用run方法。
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } private static void handleCallback(Message message) { message.callback.run(); }
8.runOnUiThread如何實現子線程更新UI
- 看看源碼,如下所示
- 如果msg.callback為空的話,會直接調用我們的mCallback.handleMessage(msg),即handler的handlerMessage方法。由於Handler對象是在主線程中創建的,所以handler的handlerMessage方法的執行也會在主線程中。
- 在runOnUiThread程序首先會判斷當前線程是否是UI線程,如果是就直接運行,如果不是則post,這時其實質還是使用的Handler機制來處理線程與UI通訊。
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } @Override public final void runOnUiThread(Runnable action) { if (Thread.currentThread() != mUiThread) { mHandler.post(action); } else { action.run(); } }
9.Handler的post方法和view的post方法
- Handler的post方法實現很簡單,如下所示
mHandler.post(new Runnable() { @Override public void run() { } }); public final boolean post(Runnable r){ return sendMessageDelayed(getPostMessage(r), 0); }
- view的post方法也很簡單,如下所示
- 可以發現其調用的就是activity中默認保存的handler對象的post方法
public boolean post(Runnable action) { final AttachInfo attachInfo = mAttachInfo; if (attachInfo != null) { return attachInfo.mHandler.post(action); } ViewRootImpl.getRunQueue().post(action); return true; } public void post(Runnable action) { postDelayed(action, 0); } public void postDelayed(Runnable action, long delayMillis) { final HandlerAction handlerAction = new HandlerAction(action, delayMillis); synchronized (this) { if (mActions == null) { mActions = new HandlerAction[4]; } mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction); mCount++; } }
10.主線程中Looper的輪詢死循環為何沒阻塞主線程
- 造成ANR的原因
- 造成ANR的原因一般有兩種:
- 當前的事件沒有機會得到處理(即主線程正在處理前一個事件,沒有及時的完成或者looper被某種原因阻塞住了)
- 當前的事件正在處理,但沒有及時完成
- 為了避免ANR異常,android使用了Handler消息處理機制。讓耗時操作在子線程運行。
- 造成ANR的原因一般有兩種:
- 問題描述
- 在處理消息的時候使用了Looper.loop()方法,並且在該方法中進入了一個死循環,同時Looper.loop()方法是在主線程中調用的,那麽為什麽沒有造成阻塞呢?
- ActivityThread中main方法
- ActivityThread類的註釋上可以知道這個類管理著我們平常所說的主線程(UI線程)
- 首先 ActivityThread 並不是一個 Thread,就只是一個 final 類而已。我們常說的主線程就是從這個類的 main 方法開始,main 方法很簡短
public static final void main(String[] args) { ... //創建Looper和MessageQueue Looper.prepareMainLooper(); ... //輪詢器開始輪詢 Looper.loop(); ... }
- Looper.loop()方法無限循環
- 看看Looper.loop()方法無限循環部分的代碼
while (true) { //取出消息隊列的消息,可能會阻塞 Message msg = queue.next(); // might block ... //解析消息,分發消息 msg.target.dispatchMessage(msg); ... }
- 為什麽這個死循環不會造成ANR異常呢?
- 因為Android 的是由事件驅動的,looper.loop() 不斷地接收事件、處理事件,每一個點擊觸摸或者說Activity的生命周期都是運行在 Looper.loop() 的控制之下,如果它停止了,應用也就停止了。只能是某一個消息或者說對消息的處理阻塞了 Looper.loop(),而不是 Looper.loop() 阻塞它。
- 處理消息handleMessage方法
- 如下所示
- 可以看見Activity的生命周期都是依靠主線程的Looper.loop,當收到不同Message時則采用相應措施。
- 如果某個消息處理時間過長,比如你在onCreate(),onResume()裏面處理耗時操作,那麽下一次的消息比如用戶的點擊事件不能處理了,整個循環就會產生卡頓,時間一長就成了ANR。
public void handleMessage(Message msg) { if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what)); switch (msg.what) { case LAUNCH_ACTIVITY: { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart"); final ActivityClientRecord r = (ActivityClientRecord) msg.obj; r.packageInfo = getPackageInfoNoCheck(r.activityInfo.applicationInfo, r.compatInfo); handleLaunchActivity(r, null); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break; case RELAUNCH_ACTIVITY: { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart"); ActivityClientRecord r = (ActivityClientRecord) msg.obj; handleRelaunchActivity(r); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break; case PAUSE_ACTIVITY: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause"); handlePauseActivity((IBinder) msg.obj, false, (msg.arg1 & 1) != 0, msg.arg2, (msg.arg1 & 2) != 0); maybeSnapshot(); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case PAUSE_ACTIVITY_FINISHING: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause"); handlePauseActivity((IBinder) msg.obj, true, (msg.arg1 & 1) != 0, msg.arg2, (msg.arg1 & 1) != 0); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; ........... } }
- loop的循環消耗性能嗎?
- 主線程Looper從消息隊列讀取消息,當讀完所有消息時,主線程阻塞。子線程往消息隊列發送消息,並且往管道文件寫數據,主線程即被喚醒,從管道文件讀取數據,主線程被喚醒只是為了讀取消息,當消息讀取完畢,再次睡眠。因此loop的循環並不會對CPU性能有過多的消耗。
- 得出結論
- 簡單的來說:ActivityThread的main方法主要就是做消息循環,一旦退出消息循環,那麽你的程序也就可以退出了。
11.得出部分結論
- 得出得結論如下所示
- 1.主線程中定義Handler對象,ActivityThread的main方法中會自動創建一個looper,並且與其綁定。如果是子線程中直接創建handler對象,則需要手動創建looper。不過手動創建不太友好,需要手動調用quit方法結束looper。這個後面再說
- 2.一個線程中只存在一個Looper對象,只存在一個MessageQueue對象,可以存在N個Handler對象,Handler對象內部關聯了本線程中唯一的Looper對象,Looper對象內部關聯著唯一的一個MessageQueue對象。
- 3.MessageQueue消息隊列不是通過列表保存消息(Message)列表的,而是通過Message對象的next屬性關聯下一個Message從而實現列表的功能,同時所有的消息都是按時間排序的。
其他介紹
01.關於博客匯總鏈接
- 1.技術博客匯總
- 2.開源項目匯總
- 3.生活博客匯總
- 4.喜馬拉雅音頻匯總
- 5.其他匯總
02.關於我的博客
- github:https://github.com/yangchong211
- 知乎:https://www.zhihu.com/people/yczbj/activities
- 簡書:http://www.jianshu.com/u/b7b2c6ed9284
- csdn:http://my.csdn.net/m0_37700275
- 喜馬拉雅聽書:http://www.ximalaya.com/zhubo/71989305/
- 開源中國:https://my.oschina.net/zbj1618/blog
- 泡在網上的日子:http://www.jcodecraeer.com/member/content_list.php?channelid=1
- 郵箱:[email protected]
- 阿裏雲博客:https://yq.aliyun.com/users/article?spm=5176.100- 239.headeruserinfo.3.dT4bcV
- segmentfault頭條:https://segmentfault.com/u/xiangjianyu/articles
- 掘金:https://juejin.im/user/5939433efe88c2006afa0c6e
Android異步消息機制