即使是工具人，也要成为个有思想，有深度的工具人
首先看看常见的几个问题：
- 请说说handler机制
- post runnable 和 message有什么区别
- postDelay的处理过程
- 队列中的消息是线程同步还是异步的，如果要异步，怎么处理。
- IdleHandler是什么，有什么作用
- 如何统计一个消息处理花费的时间
<p>sd</p>
大家都知道Handler机制大概设计到 Handler、Message、Looper、MessageQueue。我们先看看Message
从源码可以看出，Message是一种单向链表结构、带有target、callback、when等参数。关键还有一个缓存池，用来缓存Message对象，源码看最大是50个，常用的函数是obtain，默认是从缓存池获取message,并标志为正在使用。同时也有回收功能，并把message存入缓存池。所以Android中是推荐使用obtain去实例化Message对象，这样效率和性能更高，不推荐直接new
/*package*/ int flags;
/*package*/ long when;
/*package*/ Bundle data;
/*package*/ Handler target;
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
/*package*/ Message next;
// 获取message对象，先从缓存池获取，否则自己创建
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag 清除flags标志
sPoolSize--;
return m;
}
}
return new Message();
}
// 缓存池信息
private static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
private static boolean gCheckRecycle = true;
回收过程
public void recycle() {
// 判断是否在用
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
// 回收处理
recycleUnchecked();
}
/**
* Recycles a Message that may be in-use.
* Used internally by the MessageQueue and Looper when disposing of queued Messages.
*/
void recycleUnchecked() {
// 重置信息，并且吧flags设置为FLAG_IN_USE
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
// 加入缓存池
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
那什么时候会变回收呢，猜想就是message被处理之后就应该被回收，所以看看消息处理的地方。消息处理是在Looper.java中处理，我们直接看看loop函数
public static void loop() {
...
for (;;) {
....
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
// 1. 这里处理消息
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);
}
// 2. 最后开始调用回收，加入缓存池
msg.recycleUnchecked();
}
}
这里就看到消息处理之后就回收。
接着看看Message源码，发现有这个
/**
* Returns true if the message is asynchronous, meaning that it is not
* subject to {@link Looper} synchronization barriers.
*
* @return True if the message is asynchronous.
*
* @see #setAsynchronous(boolean)
*/
public boolean isAsynchronous() {
return (flags & FLAG_ASYNCHRONOUS) != 0;
}
/**
* Sets whether the message is asynchronous, meaning that it is not
* subject to {@link Looper} synchronization barriers.
* <p>
* Certain operations, such as view invalidation, may introduce synchronization
* barriers into the {@link Looper}'s message queue to prevent subsequent messages
* from being delivered until some condition is met. In the case of view invalidation,
* messages which are posted after a call to {@link android.view.View#invalidate}
* are suspended by means of a synchronization barrier until the next frame is
* ready to be drawn. The synchronization barrier ensures that the invalidation
* request is completely handled before resuming.
* </p><p>
* Asynchronous messages are exempt from synchronization barriers. They typically
* represent interrupts, input events, and other signals that must be handled independently
* even while other work has been suspended.
* </p><p>
* Note that asynchronous messages may be delivered out of order with respect to
* synchronous messages although they are always delivered in order among themselves.
* If the relative order of these messages matters then they probably should not be
* asynchronous in the first place. Use with caution.
* </p>
*
* @param async True if the message is asynchronous.
*
* @see #isAsynchronous()
*/
public void setAsynchronous(boolean async) {
if (async) {
flags |= FLAG_ASYNCHRONOUS;
} else {
flags &= ~FLAG_ASYNCHRONOUS;
}
}
由上面注释可以发现，这里就是设置同步消息或者异步消息的入口，但是这个怎么使用，我们得去其他地方看，这里先注意一下。什么时候调用setAsynchronous函数，可以在Handler中查询到，这里先mark一下，现在在看看MessageQueue
MessageQueue是Message的消息队列，由looper去分发，然后消息不是直接添加进队列，而是通过handler关联的looper获取队列添加进的。简单看看MessageQueue有什么关键的定义
public final class MessageQueue {
// True if the message queue can be quit.
private final boolean mQuitAllowed;
@SuppressWarnings("unused")
private long mPtr; // used by native code
Message mMessages;
private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
private SparseArray<FileDescriptorRecord> mFileDescriptorRecords;
private IdleHandler[] mPendingIdleHandlers;
private boolean mQuitting;
// Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.
private boolean mBlocked;
// The next barrier token.
// Barriers are indicated by messages with a null target whose arg1 field carries the token.
private int mNextBarrierToken;
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/
private native static void nativeWake(long ptr);
private native static boolean nativeIsPolling(long ptr);
private native static void nativeSetFileDescriptorEvents(long ptr, int fd, int events);
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
}
这里注意到有个mMessages，由于Message是链表结构，所以这里用来存储message，然后有个IdleHandler，还有quit，先看看IdleHandler是个什么东西，
/**
* Callback interface for discovering when a thread is going to block
* waiting for more messages.
*/
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();
}
大概就是message队列为空或者需要等待的时候，会调用，然后reture ture意味着只监听一次，否则会多次监听。看看哪里调用
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;
}
// 1. 初始化为-1
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;
// 2. 这里判断当前message的target是否为空，开始查询下一个异步消息，这里就是上面提到的异步处理，优先处理，这里啥时候target会是空呢,通过handler发送的消息都是有target的，看看下面代码
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;
}
}
上面过程是在处理消息，如果有异步消息，就优先处理，如果没有就处理普通消息，如果没消息处理，就通知idlehandler回调，这里我们就知道IdleHandler就是用来通知队列没消息，或者消息延时还没到的时候，做出通知，可以用来对消息队列的一些状态进行处理，再接着看看
public int postSyncBarrier() {
return postSyncBarrier(SystemClock.uptimeMillis());
}
private int postSyncBarrier(long when) {
// Enqueue a new sync barrier token.
// We don't need to wake the queue because the purpose of a barrier is to stall it.
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) { // invariant: p == prev.next
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
return token;
}
}
这两个函数，就是用来插入target== null 的messge消息，有个
public void removeSyncBarrier(int token)
与之对应。这里可以看出，通过初始化Handler 发送消息，然后由MessageQueue的postSyncBarrier来触发对异步消息的优先处理，就是不知道这个的场景会用在什么地方。
接着分析messagequeue，这个的主要就两个函数
Message next() // 处理消息 一般由looper调用
boolean enqueueMessage(Message msg, long when) // 添加消息,一般有handler调用
Looper.java
看看这个，消息循环处理，MessageQueue是在Looper中创建的，Handler通过looper获取的messagequeue进行的发送消息。每一个线程只有一个Looper，这个是怎么做到的呢，这里面是通过ThreadLocal来实现
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
这里主要是通过当前线程做为key去保存looper的实例对象，通过这种方式实现对不同线程保存实例。
主线程在启动的时候已经调用了prepare，所以在使用looper之前，一定 要先调用prepare，才可以，而且只能调用一次。
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);
}
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));
}
looper中的loop循环
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the 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 (;;) {
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 {
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();
}
}
这是个死循环，停止的使用要调用quit，这里通过queue.next()获取要处理的消息，然后通过printer打印日志
然后message.target.dispatchmessage处理。实现消息在当前线程去处理。
这里的Printer,可以通过下面方式来跟踪，实现对消息处理耗时的追踪
/**
* Control logging of messages as they are processed by this Looper. If
* enabled, a log message will be written to <var>printer</var>
* at the beginning and ending of each message dispatch, identifying the
* target Handler and message contents.
*
* @param printer A Printer object that will receive log messages, or
* null to disable message logging.
*/
public void setMessageLogging(@Nullable Printer printer) {
mLogging = printer;
}
/** {@hide} */
public void setTraceTag(long traceTag) {
mTraceTag = traceTag;
}
最后来看看Handler。Handler创建可以传入Looper，来指定对应的线程looper，如果没有指定，默认是通过当前线程
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 that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
是由Looper.myLooper获取，通过sThreadLocal实现获取当前线程
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
简单看看ThreadLocal
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
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();
}
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
这里是通过一个ThreadLocalMap，由thread的ThreadLocalMap获取到ThreadLocal，在获取
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean postDelayed(Runnable r, long delayMillis)
{
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
可以看出post最后都是调用postDelay,其实是一样的，只是延时是0， 然后Runnable最后包装成Message，只是通过callback赋值到message上，看看消息处理,优先处理runnable，这下就明白了吧。最后都是处理消息，只是提供了几种方式。非常方便
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
大概也就整理完了，上面的问题也就可以一一解答，这里就不总结了。有什么问题可以评论交流。如果有什么分析不对的，欢迎指正
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