文章简介:本文通过阅读android源码来搞懂handler运行原理,好了废话不多说,马上开始装逼:
几个定义
Looper:轮询从消息队列中取消息的对象
MessageQueue:用来保存所有消息的消息队列
Message:消息对象,可以保存数据
Handler:用来发送消息和处理消息的对象
Message创建的三种方式:
new Message();
Message.obtain();
Handler.obtainMessage();
注:Handler.obtainMessage()其实最后也是调用Message.obtain(),用Message.obtain()的原因是这种方法可以复用消息池中的消息
Handler初始化
new Handler(){
handleMessage(){
}
}
//sdk中handler初始化源代码
public Handler(Callback callback, boolean async) {
//给自己的mLooper对象赋值
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//将looper中的消息队列赋值给mQueue变量
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
Looper中myLooper()方法的代码如下
public static @Nullable Looper myLooper() {
//从当前线程中获取looper对象
return sThreadLocal.get();
}
注:threadLocal是用来保证多线程并发时数据一致性的一个对象,类似于一个map对象,可以存取数据,使不同线程的数据不受其他线程的干扰
因为handler大多是在主线程创建的,因此这个looper是从主线程()也就是ActivityThread中取出来的.那么ActivityThread是什么时候将looper对象Put进LocalThread的呢?看下方代码
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
SamplingProfilerIntegration.start();
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
EventLogger.setReporter(new EventLoggingReporter());
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
//准备looper对象
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);
//开始轮询消息队列
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
可以看到上述代码中出现了Looper.prepareMainLooper();这个方法,我们跳到Looper.java中看看这个方法中做了什么
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//往当前线程中设置Looper对象
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
// 创建消息队列
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
上面的代码最终调用到了prepare方法,在这个方法中创建了一个Looper对象并set到了sThreadLocal对象中.至此,Handler创建时的Looper来源已经找到了,是ActivityThread线程main方法设置到主线程ThreadLocal对象中的一个looper.因此可以得出一个结论==在同一个线程中创建的handler共享同一个looper==.那么问题来了,looper取出消息后如何知道将消息交给谁处理?再往后看
handler发送消息
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);
}
//将消息加入到消息队列
return queue.enqueueMessage(msg, uptimeMillis);
}
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;
//消息需要立即处理,放在队头
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.
//判断是否应该唤醒
if (needWake) {
//拿写描述符写数据,会唤醒主线程
nativeWake(mPtr);
}
}
return true;
}
Looper.loop()
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 (;;) {
//拿读描述符读东西,可能阻塞
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 {
//调用msg中保存的target变量,即发消息的handler的dispatchMessage方法
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();
}
}
阻塞机制,采用的是linux中的一种进程间通信方式,管道(pipe),有一个特殊的文件,有两个描述符,一个读,一个写.一个进程拿读描述符读数据,另一个进程拿写描述符写数据.如果读数据进程读不到数据就会阻塞,写数据进程写入数据后就会唤醒读数据进程读数据
messageQueue 调用next()会去读消息,读不到就会阻塞, 代码如下
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;//阻塞的时间
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
//就是这个native方法会产生阻塞
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) {
//根据时间判断当前消息是否需要处理
if (now < msg.when) {
//当前消息没到时间不需要处理,根据当前时间计算新的睡眠时间
// 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 {
//当前消息需要处理,返回该消息
// 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;
}
}
}
}
Looper.loop()方法中调用了msg.target.dispatchMessage(msg)方法,那么这个target是什么呢?我们回到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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//此处将当前的handler对象赋值给msg.target
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//将消息加入到消息队列
return queue.enqueueMessage(msg, uptimeMillis);
}
从上面的代码可以看出,msg.target.dispatchMessage(msg)中的target就是发送该消息的handler,下面跟进dispatchMessage方法
public void dispatchMessage(Message msg) {
//此处的callback就是postdelay中传入Runnable对象
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//如果没有callback就调用用户重写的handleMessage()方法
handleMessage(msg);
}
}
好了,逼装完了,领盒饭了.
urpig.jpg
网友评论