Android开发艺术探索 - 第10章 Android的消息机制
1.概述
Handler的作用是将一个任务切换到指定的线程去执行。
UI操作只能在主线程进行，这个限制是在ViewRootImpl#checkThread中实现的：
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
Handler工作原理：
- Handler创建时，会采用当前线程的Looper来构建内部的消息循环，如果当前线程没有Looper，会抛出异常。
- 通过Handler的send发送一个msg，或者直接post一个Runnable，最终都会由Looper去执行handleMessage或Runnable，而Looper是在创建Handler所在的线程中运行的，所以实现了线程切换。
2.消息机制分析
- ThreadLocal的工作原理
ThreadLocal是一个线程内部的存储类，可以在指定的线程中存储数据，且这份数据仅供这个线程获取。其应用场景是，当某些数据是以线程为作用域并且不同线程具有不得数据副本。如Handler中的Looper，每个线程都有自己的Looper且互不相同。
另外一个使用场景，是在复杂逻辑下的对象传递，使该对象作为线程内的全局对象存在，在线程内部通过get方法就可以获得该对象。
ThreadLocal是个泛型类，创建一个ThreadLocal实例：private ThreadLocalmBooleanThreadLocal = new ThreadLocal();
当分别从主线程，子线程去获取该ThreadLocal的值时，他们分别会得到自己的线程中对应的ThreaLocal的值：
mBooleanThreadLocal.set(true);
Boolean value = mBooleanThreadLocal.get(); // true
new Thread(new Runnable() {
@Override
public void run() {
Boolean value = mBooleanThreadLocal.get(); // null
mBooleanThreadLocal.set(false);
value = mBooleanThreadLocal.get(); // false
}
}).start();
ThreadLocal#set，每个线程中都有一个ThreadLocalMap，用于存储
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
ThreadLocal#get，首先会尝试从ThreadLocalMap取与该ThreadLocal类型对应的其泛型的值，如果找不到，调用setInitialValue方法，通过initialValue方法得到初始值，然后如果ThreadLocalMap也不存在，则同时创建ThreadLocalMap，最后添加进map中：
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
initialValue方法的默认实现返回了null，可以依据具体ThreadLocal的泛型类型，重写该方法。
2. MessageQueue工作原理
enqueueMessage方法，MessageQueue内部通过单链表维护msg队列，入队的过程与队列类似，只是其中会根据msg的when字段，判断合适的入队位置，并不一定是队尾：
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;
}
next方法，会尝试获取下一个msg，如果获取到则从队列中删除并返回这个msg；如果没有msg则阻塞。新msg的到来nativePollOnce就会返回：
Message next() {
...
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;
}
...
}
}
- Looper工作原理
Looper的构造方法，创建了MessageQueue，同时将当前的线程对象保存起来：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
启动Looper的例子：
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}
在prepare方法中，创建了Looper对象，同时使用ThreadLocal存储了该Looper：
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));
}
除了prepare方法，Looper提供了prepareMainLooper方法，用于ActivityThread创建主线程Looper使用，其本质也是通过prepare来实现的。同时提供了getMainLooper方法，在其他线程中获取主线程的Looper。
Looper完成操作之后，需要调用quit或quitSafely退出，前者会直接退出，后者设置了一个标记，待所有的msg处理完毕再退出。如果子线程不调用quit，该线程会一直阻塞，直到调用quit时直接退出。Looper调用了quit之后，Handler发出的msg将失败，send方法会返回false。
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
loop方法，在该方法中真正处理了MessageQueue中的msg。首先通过ThreadLocal获得当前线程的Looper实例，然后开始取其MessageQueue中的msg，如果Looper调用了quit方法，这里取得的msg就是null，对应的Looper就会退出loop；如果取到了msg，就会调用msg.target.dispatchMessage(msg)执行Handler的dispatchMessage方法。MessageQueue中没有msg时，则阻塞：
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;
...
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
try {
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
...
}
- Handler的工作原理
Handler主要负责msg的发送和接收。post方法会将Runnable封装成msg，然后调用send发出msg：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
send的一系列方法，最终会将msg添加到MessageQueue中：
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
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);
}
MessageQueue#enqueueMessage方法在msg添加之后，会执行nativeWake方法，如果之前MessageQueue因为是空的而阻塞在next方法中，这时阻塞解除，开始获取msg。Looper得到MessageQueue的msg，调用Handler的dispatchMessage方法。
这里分几种情况：对于通过post方法传入的Runnable，则直接通过handleCallback方法去执行他；对于send方法传入的msg，首先会交由其mCallback的handleMessage方法去处理，该回调可以在创建Handler时指定，就不必重写Handler的handleMessage方法；如果该回调没有处理，即返回了false，则交由Handler自己的handleMessage处理：
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();
}
Handler的构造方法中，会判断当前线程的Looper是否存在，不存在则抛出异常。同时会获取Looper的MessageQueue以便后续添加msg：
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
3.主线程消息循环
Android的主线程就是ActivityThread，其main方法中，创建了主线程Looper，并启动了消息循环：
public static void main(String[] args) {
...
Looper.prepareMainLooper();
...
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
负责处理消息的Handler即ActivityThread.H，其中就包括了四大组件的启动和停止过程：
class H extends Handler {
public static final int BIND_APPLICATION = 110;
public static final int EXIT_APPLICATION = 111;
public static final int RECEIVER = 113;
public static final int CREATE_SERVICE = 114;
public static final int SERVICE_ARGS = 115;
public static final int STOP_SERVICE = 116;
主线程消息循环模型：
- ActivityThread提供ApplicationThread与AMS进行IPC
- AMS完成了ActivityThread的RPC之后，回调ApplicationThread中的远程方法
- ApplicationThread为Binder对象，其方法运行在Binder线程池中
- ApplicationThread通过H发送msg给ActivityThread的MessageQueue，Looper取出msg，在主线程中处理相应的msg