android开发的话现在handler应该都倒背如流了
handler发送message到message queue，looper 轮询 message queue，取到message后回调handleMessage方法。之前面试我也都是这么背的=。=
然后面试官会问到：handler是怎么发送message到message queue的呢，一个线程只对应一个looper吗，Looper是什么时候开始轮询的呢，子线程可以用hander吗，主线程调用Looper.loop()为什么不会卡死，知道同步障碍吗等。下面翻翻源码，从写代码的角度来讲先要new handler。
handler构造方法
//handler的成员变量
final Looper mLooper;
final MessageQueue mQueue;
final Callback mCallback;
final boolean mAsynchronous;
public interface Callback {
public boolean handleMessage(Message msg);
}
public Handler() {
this(null, false);
}
public Handler(Callback callback) {
this(callback, false);
}
public Handler(Looper looper) {
this(looper, null, false);
}
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
public Handler(boolean async) {
this(null, async);
}
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 " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
构造方法最多有三个参数，无论哪个构造方法最后都会给mLooper赋值。
- callback：消息回调。
- async：是否为异步消息。
- mQueue：赋值调用looper.mQueue()
结论：handler持有Looper和MessageQueue。
主线程中我习惯这样写
private val handler = Handler(Looper.getMainLooper())
调用的构造方法
public Handler(Looper looper) {
this(looper, null, false);
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
callback不传默认为null，async默认为false。
callback调用处
//Handler类方法，继承Handler重写的方法，接收消息回调。
public void handleMessage(Message msg) {}
//msg callback
private static void handleCallback(Message message) {
message.callback.run();
}
//首先判断Message的callback，不为空直接调用上面的handleCallback。
//为空再判断构造方法是否传了callback，传了调用callback回调，没传调用上面handleMessage()
//继承Handler的写法需要重写handleMessage()处理消息回调。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
handler会绑定到looper，如果构造方法没传looper呢？接下来看不传looper的构造方法。
public Handler() {
this(null, false);
}
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 " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
Looper.myLooper()
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
ThreadLocal具体实现这里就不展开了。简单来说，ThreadLocal会在不同线程保存不同的实例，那么这也解释了一个线程只对应一个looper。主线程中初始化Handler，不传looper的情况下，通过Looper.myLooper()取到ThreadLocal中对应的主线程looper实例。那么主线程的looper何时初始化存到ThreadLocal中呢。这里说结论，app初始化的时候ActivityThread.main()中调用Looper.prepareMainLooper()，初始化了主线程Looper。
//ActivityThread.main()
public static void main(String[] args) {
......
Looper.prepareMainLooper();
......
Looper.loop();
}
Looper.prepare()
public static void prepare() {
prepare(true);
}
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
//都会调用到此方法 quitAllowed是否可以取消、停止轮询。
//prepareMainLooper传的false，因为要保证app一直运行。
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
//一个线程只能创建一个looper
throw new RuntimeException("Only one Looper may be created per thread");
}
//初始化Looper
sThreadLocal.set(new Looper(quitAllowed));
}
Looper构造方法中初始化了MessageQueue，对应上了前面Handler构造方法中的looper.mQueue()
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
MessageQueue
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
void quit(boolean safe) {
if (!mQuitAllowed) {
//prepareMainLooper传的false，主线程调用quit()会抛出下面异常
//子线程调用quit()走下面removeMessages()相关方法
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
nativeWake(mPtr);
}
}
在子线程中创建handler手动调用Looper.prepare()，其实就是创建子线程对应的Looper，Looper构造方法又初始化了MessageQueue。Handler，Looper，MessageQueue都创建好了，接下来调用Looper.loop()开启轮询。主线程中无需调用也是因为ActivityThread中已经调用过了。
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
......
//MessageQueue.next()
for (;;) {
Message msg = queue.next();
if (msg == null) {
return;
}
//msg.target是Handler，调用Handler.dispatchMessage()完成消息回调。
msg.target.dispatchMessage(msg);
}
......
}
处理消息是在MessageQueue.next()方法。先解决上一个问题，handler怎么把message发送到MessageQueue中。
Handler.sendMessage()调用链
public final boolean sendMessage(Message msg) {
return sendMessageDelayed(msg, 0);
}
//delayMillis不传或小于0置为0，默认无延迟
public final boolean sendMessageDelayed(Message msg, long delayMillis){
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
//mQueue传递到enqueueMessage()
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
//无MessageQueue抛出异常
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MessageQueue.enqueueMessage()
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
//Handler构造方法中的参数async
if (mAsynchronous) {
//调用了Message的setAsynchronous方法
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
//Message.setAsynchronous()，操作Message的状态flags，位操作。
public void setAsynchronous(boolean async) {
if (async) {
flags |= FLAG_ASYNCHRONOUS;
} else {
flags &= ~FLAG_ASYNCHRONOUS;
}
}
MessageQueue.enqueueMessage()
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//判断Message的flags中是否有FLAG_IN_USE标记，可用状态。
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
//Thread dead状态抛异常
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
//Message.recycle()调用Message.recycleUnchecked()，回收message到消息池，重置message参数。
msg.recycle();
return false;
}
//flags增加FLAG_IN_USE状态
msg.markInUse();
msg.when = when;
Message p = mMessages;
//是否需要唤醒
boolean needWake;
if (p == null || when == 0 || when < p.when) {
//message是链表结构，mMessages为null说明是空链表，新message在头部。
//message.when == 0或者when < p.when，when值大小有优先级，小的放前面。
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
//mMessages不为空，将message插到上一条消息的后面。
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p;
prev.next = msg;
}
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
Message相关方法
void recycleUnchecked() {
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
boolean isInUse() {
return ((flags & FLAG_IN_USE) == FLAG_IN_USE);
}
void markInUse() {
flags |= FLAG_IN_USE;
}
handler.sendMessage()调用MessageQueue.enqueueMessage()将message插入message链表。链表中无消息放表头，有消息按when值大小，小的放前面。回到取消息的地方MessageQueue.next()
Message next() {
int pendingIdleHandlerCount = -1;
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
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 {
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
msg.markInUse();
//获取message返回给looper
return msg;
}
} else {
nextPollTimeoutMillis = -1;
}
}
}
}
前面handler.sendMessage()调用msg.setAsynchronous(true)
public void setAsynchronous(boolean async) {
if (async) {
flags |= FLAG_ASYNCHRONOUS;
} else {
flags &= ~FLAG_ASYNCHRONOUS;
}
}
//判断异步消息
public boolean isAsynchronous() {
return (flags & FLAG_ASYNCHRONOUS) != 0;
}
MessageQueue.next()对异步消息进行优先处理
MessageQueue.postSyncBarrier()
public int postSyncBarrier() {
return postSyncBarrier(SystemClock.uptimeMillis());
}
private int postSyncBarrier(long when) {
synchronized (this) {
final int token = mNextBarrierToken++;
final Message msg = Message.obtain();
msg.markInUse();
msg.when = when;
msg.arg1 = token;
Message prev = null;
Message p = mMessages;
if (when != 0) {
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
}
if (prev != null) {
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
return token;
}
}
开启同步障碍即调用MessageQueue.postSyncBarrier()发送异步消息，相当于开个后门，处理要紧事情。ViewRootImpl.scheduleTraversals()调用了MessageQueue.postSyncBarrier()发送异步消息开始绘制ui。Android系统为了保证ui绘制机制的优先度，防止画面卡顿，开启了同步障碍。
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
最后一个点，Looper.loop()为什么不会卡死。简单来说就是Linux pipe/epoll机制。无消息时阻塞在nativePollOnce()方法，释放资源，有消息时唤醒。
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