簡單講，handler就是兩個功能
插入訊息，enqueuemessage，msg，when
從訊息佇列中遍歷所有訊息，比對msg.when和當前的when，找到合適的位置插入
處理訊息，looper.loop會從messagequeue中呼叫next，取訊息，如果訊息還沒到時間該執行，就會比對時間，下次輪詢就通過binder寫入，native函式休眠，到時間喚醒執行，
handler記憶體泄漏
GCRoot 一般是靜態變數或者常量可以作為GCROOT
GCROOT 是ThreadLocal，存在于Looper中，Looper被加載就存在，
handler持有activity或者fragment，handler又被message持有，message的target屬性，message被messagequeue持有，messagequeue被looper中的threadlocal持有
java中匿名內部類會默認持有外部類的參考
打斷持有鏈
- handler.removemesage handler.removecallbacks
- handler使用static修飾，
主執行緒的Looper不允許退出
處理訊息，looper取出來后，呼叫message.tager.dispatchemesage后面呼叫了handler的handlemessage方法，
還有個callback物件，如果有callback，dispatch會先執行callback的處理，calllback回傳true，后面就不處理了，callback回傳false就給handler的handlemessage處理了
Meesage物件創建
message創建用的obtain，池化，頻繁的創建銷毀會導致記憶體不穩定，抖動，造成卡頓 oom等問題
message pool的最大快取50
阻塞和休眠,阻塞是被動的，休眠是主動的，阻塞不會讓出cpu，休眠會，thread.yield會讓出cpu，
子執行緒主執行緒通信
handler，livedata，eventbus，flow，rxjava，broadcast，觀察者模式不能跨執行緒
最終都是handler完成的，
Handler監聽卡頓
原理是在looper內完成的，looper處理訊息的時候，會列印內容，就是Printer，looper可以設定它，
Message訊息的分類
同步訊息，普通的訊息都是同步訊息
異步訊息，創建handler的時候設定async為true即可
同步屏障，需要通過反射呼叫，app層無法直接呼叫，是messagequeue提供的posSyncBarrier方法實作的，回傳一個token，時msg的arg1值，用它來取消同步屏障，和普通訊息的區別是，msg，target屬性為null，
重繪UI的訊息是異步訊息，發送前先插入了一個同步屏障訊息，異步訊息處理完成后，要將同步屏障訊息移除佇列
訊息入隊
handler訊息加入佇列，有一系列方法，如下：
// 發送空訊息
public final boolean sendEmptyMessage(int what){}
public final boolean sendEmptyMessageDelayed(int what,long delay){}
public final boolean sendEmptyMessageAtTime(int what,long when){}
// 發送訊息
public final boolean sendMessage(@NonNull Message msg){}
public final boolean sendMessageDelayed(@NonNull Message msg,long time){}
public final boolean sendMessageAtTime(@NonNull Message msg,long when){}
public final boolean sendMessageAtFrontOfQueue(Message msg) {}
// post發送
public final boolean post(@NonNull Runnable r) {}
public final boolean postAtTime(@NonNull Runnable r, long uptimeMillis) {}
public final boolean postAtTime(
@NonNull Runnable r, @Nullable Object token, long uptimeMillis) {}
public final boolean postDelayed(@NonNull Runnable r, long delayMillis) {}
public final boolean postDelayed(Runnable r, int what, long delayMillis) {}
public final boolean postDelayed(
@NonNull Runnable r, @Nullable Object token, long delayMillis) {}
public final boolean postAtFrontOfQueue(@NonNull Runnable r) {}
// enqueue
public final boolean executeOrSendMessage(@NonNull Message msg) {}
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {}
最終都是掉用的enqueueMessage加入佇列
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this; // 系結訊息處理物件
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) { // 根據handler創建是否是異步的，來將訊息標記為異步訊息
msg.setAsynchronous(true);
}
// 呼叫messagequeue的方法，加入佇列，
return queue.enqueueMessage(msg, uptimeMillis);
}
下面看下訊息如何入隊的
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
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;
// 當前佇列的頭
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
// 訊息佇列為null，或者訊息是立刻執行的，或者要執行的時間先于頭訊息的時間，將當前訊息作為新的佇列的頭
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();
// 遍歷佇列，找到合適的位置，將當前訊息插入進去
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;
}
// 如果需要的話，喚醒佇列，開始處理訊息，就是MessageQueue的next方法開始執行
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
取訊息以及訊息處理
取訊息1
取訊息入口是Looper的loop方法處理的
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
if (me.mInLoop) {
Slog.w(TAG, "Loop again would have the queued messages be executed"
+ " before this one completed.");
}
me.mInLoop = true;
// 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();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start' final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
me.mSlowDeliveryDetected = false;
// 無限回圈，通過loopOnce取訊息
for (;;) {
if (!loopOnce(me, ident, thresholdOverride)) {
return;
}
}
}
處理訊息
private static boolean loopOnce(final Looper me,
final long ident, final int thresholdOverride) {
// 取訊息
Message msg = me.mQueue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return false;
}
// 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);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
Object token = null;
if (observer != null) {
token = observer.messageDispatchStarting();
}
long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
try {
// 這里取出msg的target屬性，就是handler物件，進行訊息的處理，注意：屏障訊息是沒有target的，
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (me.mSlowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
me.mSlowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
me.mSlowDeliveryDetected = true;
}
}
}
// 這里會列印那些執行慢的訊息
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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();
return true;
}
取訊息2
再看下實際的取訊息的方法，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();
}
// 這個方法會在沒有訊息的時候阻塞
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
// 訊息頭保存
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
// 這里處理同步屏障訊息，如果當前訊息是異步訊息，就跳出回圈，否則繼續回圈
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
// 回圈的作用，找出佇列里的異步訊息，存盤在msg里，或者將同步屏障訊息存盤在msg中，prevMsg存盤的是同步訊息
}
if (msg != null) {
if (now < msg.when) {
// 下次的喚醒時間
// 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();
// 將取出的訊息交給handler處理，
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;
}
}
handler的訊息處理
是從dispatchMessage方法開始的，里面進行訊息處理
public void dispatchMessage(@NonNull Message msg) {
// 檢查message是否設定了callback物件（runnable型別的）
if (msg.callback != null) {
// 執行其run方法
handleCallback(msg);
} else {
// 檢查handler是否設定了callback物件
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
// 如果回傳true，后續就不執行了，
return;
}
}
// 自定義handler的時候，實作該方法處理訊息
handleMessage(msg);
}
}
IdleHandler是什么？干什么？繼續看MessageQueue類，
IdleHandler是MessageQueue的靜態內部介面，如下
public static interface IdleHandler {
/**
* Called when the message queue has run out of messages and will now * wait for more. Return true to keep your idle handler active, false * to have it removed. This may be called if there are still messages * pending in the queue, but they are all scheduled to be dispatched * after the current time. */
boolean queueIdle();
}
當前執行緒的訊息對立內當前沒有訊息要處理時，會取出idlehander執行任務，因為執行在主執行緒，禁止執行耗時操作，回傳true表示執行完并不會移除該物件，false執行完一次就移除，
而且，執行時機是不確定的，
執行的地方在next方法內部，
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;
}
// 執行到這里，說明沒有找到message物件需要執行了，且執行緒沒有退出，
// 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;
}
// 取出idlehandlers
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 {
// 執行idlehandler的方法
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
// 需要移除的話，在此處進行移除
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;
}
}
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