Handler源码解析 - 1835434698/1835434698.github.io GitHub Wiki
Handler源码解析
//调用方式有
handler.sendMessage(new Message());
handler.sendMessageDelayed(new Message(), 1000);
handler.sendEmptyMessage(0);
handler.sendEmptyMessageDelayed(1, 1000);
handler.sendEmptyMessageAtTime(2, 2000);
分为执行时间三类方式。
1、直接sendMessage，直接存入队列。
2、sendMessageDelayed，延迟几秒存入队列。
3、sendMessageAtTime,在什么时间存入队列。
另外还有sendEmptyMessage，post不再细讲。
这里选择sendMessageDelayed讲解。
//01
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);//延迟时间+当前时间戳，就是在指定时间开始执行。调用02
}
//02
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;//获取队列，给与当前对象。
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);//调用03
}
//03
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;//设置负责Handler
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {//锁定
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);//调用04
}
//04
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {//判断消息是否已经处理。
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {//异步锁定
if (mQuitting) {//线程已经死亡
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();//设置消息已经处理
msg.when = when;
Message p = mMessages;//取得目前队尾Message
boolean needWake;
//队列为null或者立即存入，或者存入时间小于队尾的存入时间
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;//当前消息的下一个消息指向队尾的Message
mMessages = msg;//当前消息设置为队尾消息
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 (;;) {//循环重新判断消息排序
prev = p;
p = p.next;
if (p == null || when < p.when) {//直到消息为空或者找到当前消息的位置（执行时间排序）。
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;//重新设置好队列
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {//是否需要唤醒
nativeWake(mPtr);
}
}
return true;
}
发送消息到队列完成。
接下来Hanler如何从消息队列拿到需要处理的消息。
//开始循环取消息。主线程系统调去。
Looper.loop();//调用 01
//01
public static void loop() {
final Looper me = myLooper();//得到当前Looper
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;//得到MessageQueue。
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
//确保此线程的标识是本地进程的标识，并跟踪该标识令牌的实际内容。
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);
boolean slowDeliveryDetected = false;
for (;;) {//无限循环
//只要不阻塞就一直取下去。取出一个消息
Message msg = queue.next(); // might block（调用005）
if (msg == null) {//为空则退出程序。
// 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;//打印日志
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;//观察者复制给当前观察者
//设置一些参数
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 {
//开始分发消息
msg.target.dispatchMessage(msg);//调用02
if (observer != null) {
observer.messageDispatched(token, msg);//通知观察者
}
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 (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
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();
}
}
//02
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {//Message是否设置了Runnable
handleCallback(msg);//交给Runnable处理。调用03
} else {
if (mCallback != null) {//是否创建了Callback，创建Handler时创建的。
if (mCallback.handleMessage(msg)) {//交给创建的Callback处理。
return;
}
}
handleMessage(msg);//使用默认的HandlerMessage方式处理。调用04
}
}
//03
private static void handleCallback(Message message) {
message.callback.run();
}
public Message setCallback(Runnable r) {
callback = r;
return this;
}
//04
Handler handler = new Handler(){
@Override
public void handleMessage(@NonNull Message msg) {
Logger.d(TAG, "handleMessage -> msg.whta = "+msg.what);
super.handleMessage(msg);
}
};
//005
@UnsupportedAppUsage
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();//flus缓存到bunder
}
nativePollOnce(ptr, nextPollTimeoutMillis);//等待（可能是等到优先级高的消息优先处理或者等待被唤醒）
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;
}
}