Android消息机制分析：
Android的消息机制主要是指Handler的运行机制，Handler的运行需要由Loop,MessageQueuen,Message来支持。
作用：Android开发规范中，不能在子线程中更新UI，当我们做完耗时操作的时候需要更新UI，这时候就需要使用Handler机制。
大致流程：
Handler发送消息到MessageQueuen中，然后通过Looper.loop()启用死循环从MassageQueuen#next()取出消息，再分发给Messsage的目标Handler进行处理消息。
Handler:发送消息和处理消息分别对应Handler中的两个函数
Handler#enqueueMessage(Message msg) 压入到MessageQueuen
Handler#handleMessage(Message msg) 处理消息
Runable和Message被Hander"压入"到MessageQueuen中
Looper循环地去取出消息交给Hander处理
Handler的初始化
Handler发送消息的函数：
获取到Handler或它的子类的实例，
//构造方法
public Handler() {
this(null, false);
}
//
public Handler(@Nullable Callback callback, boolean async) {
//非静态成员造成内存泄漏提醒
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());
}
}
//通过静态方法获取到Looer实例
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
//获取到MessageQueuen
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;//异步调用
}
//不支持App调用，只提供给系统使用
@UnsupportedAppUsage
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
上面的代码块是Handler的可使用的构造方法中的其中两个，在构造方法中获取Looper的静态方法获取到Looper对象,如果Looper为null,就会抛出异常。接着会对MessageQueuen、CallBack、Asynchronous赋值.构造方法主要的任务就是初始化和赋值
Looper.java
/** 返回当前线程的Looper对象
* 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() {
//将会返回空，除非你调用了Looper.prepare()
return sThreadLocal.get();
}
通过Looper的静态方法myLooper方法，可以看到，如果sThreadLocal存在Looper实例就会返回Looper实例，否则就会返回null，并没有在这个函数中实例化Looper对象。在Handler中的构造方法中有Looper是否为空的检查，当myLooper为null就会抛出异常，所以要在初始化Handler之前实例化Looper对象，并保存在当前线程。
Looper.java
/** 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));
}
//quitAllowed：是否运行退出
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
从prepare函数可以得出每个线程只有一个Looper实例并且创建的Looper对象保存在当前线程的ThreadLocal中；在看Looper的构造方法，在它的构造方法中初始化了MessageQueue实例，而且获取到了当前线程。
总结：
-
初始化Handler对象之前，必须调用 Looper.prepare()确保Looper对象已经创建。
-
每一个线程最多有一个Looper对象，每个Looper对象有且只有一个MessageQueue.
消息发送
Handler消息发送相应功能的函数声明如下：
post系列
final boolean post(Runnable r);
final boolean postAtTime(Runable, long);
final boolean postDelayed(Runnable, long);
final boolean postAtFrontOfQueue( Runnable r)
send系列
final boolean sendEmptyMessage(int what);
final boolean sendEmptyMessageDelayed(int what, long delayMillis);
final boolean sendEmptyMessageAtTime(int what, long uptimeMillis);/*以上只发送包含what值的消息*/
final boolean sendMessage(Message msg);
final boolean sendMessageAtFrontOfQueue(Message msg);
final boolean sendMessageAtTime(Message msg, long uptimeMillis);
final boolean sendMessageAtFrontOfQueue(Message msg);
final boolean sendMessageDelayed(Message msg, long delayMillis);
现在将这两个系列的函数作为线索来分析Handler的消息发送的流程。
先从post系列入手：
Handler.java
/**
* Causes the Runnable r to be added to the message queue.
* The runnable will be run on the thread to which this handler is
* attached.
*
* @param r The Runnable that will be executed.
*
* @return Returns true if the Runnable was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean post(@NonNull Runnable r) {
//实际上post系列内部也是调用的send系列函数
return sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean postAtTime(@NonNull Runnable r, long uptimeMillis) {
return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}
public final boolean postDelayed(@NonNull Runnable r, long delayMillis) {
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
//将Runnable包装成Message
private static Message getPostMessage(Runnable r) {
//获取Message实例
Message m = Message.obtain();
m.callback = r;//将Runnabled对象设置为message的回调函数
return m;
}
分析post系列可以得出，它们将其他类型“零散”的信息转化成Message,然后在调用send系列的函数，所以接下来我们重点来分析send系列的函数。
Handler.java
/**
* Sends a Message containing only the what value.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis/*延迟该时间分发*/) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis/*在这个时间分发*/) {
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);
}
通过上面的代码块，我们可以得出结论：无论post系列还是send系列函数，最终都交给 sendMessageAtTime()函数进行放入队列的操作。咱们接着分析这个入队列的操作
Handler.java
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
//将当前Handler设置给Message的target
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();/**只给系统服务使用*/
//是否异步
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//将message放入到MessageQueuen
return queue.enqueueMessage(msg, uptimeMillis);
}
在Handler的enqueueMessage函数中将handler实例设置给Message的target，然后将入队列的操作交给MessageQueuen。好了，在Handler中的发送消息的流程已经结束了，现在我们去分析Message是怎样进入MessageQueuen的
MessageQueuen.java
boolean enqueueMessage(Message msg, long when) {
//首先判断，是否有处理该Message的handler
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//Message是否在已经使用
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();//将Message设置为已经使用
msg.when = when;
Message p = mMessages;//保存链表头部
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
//新节点，如果阻塞则唤醒事件队列
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 (;;) /*开始遍历链表*/{
prev = p;
p = p.next;
//当了尾部或者当前节点的时间大于要加入的时间，结束遍历
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
//将消息放入到链表
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;
}
分析以上代码，可以看出实际上MessageQueuen维护的是一个链表结构，enqueueMessage实际上将message放入到链表中，根据时间来决定就message放入到哪个位置。这一部分分析完Handler从发送消息到入队的过程。
处理消息
现在发送的消息已经保存到MessageQueuen中了，现在需要将MessageQueuen按规则取出来，我们已经知道Looper在消息机制中负责将MessageQueuen中的message取出并且分发出去，所以查看Looper中的方法，可以看到其中静态函数Loop就是完成相应功能的函数。
所以要想消息机制运行起来，得在创建完Handler实例后，调用Looper.Loop()函数
Looper.java
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
//获取到Looper
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//取出Looper所对应的MessageQueue
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();
// 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 (;;) {/*开启死循环*/
Message msg = queue.next(); // might block 可能会阻塞，调用Message.next方法取出Message
if (msg == null) {/*当没有消息时，表示消息队列正在退出*/
// 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);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
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 {
//开始分发给Message所属的Handler消息
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
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();
}
}
Looper开启一个死循环，不断地通过MessageQueuen.next取出消息,然后交给msg.target处理，让我们来看看MessageQueuen.next做了什么？
MessageQueuen.java
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;
}
//待处理的空闲处理程序计数
int pendingIdleHandlerCount = -1; // -1 only during first iteration
//下一个轮询超时 Millis
int nextPollTimeoutMillis = 0;
for (;;) {/*开启一个死循环*/
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
//jni
nativePollOnce(ptr, nextPollTimeoutMillis);
//开始检索下一条message，找到就返回
synchronized (this) {/**/
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();//获取现在的时间
Message prevMsg = null;
Message msg = mMessages;//保存链表头部
if (msg != null && msg.target == null) {/*taget为空*/
// 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.
//检索到的消息时间还没到，设置超时以在准备好时唤醒
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.获取以一个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;/*取出头部节点*/
}
} else {
// No more messages.
//没有message设置为-1
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
// 现在处理退出消息，所有挂起的消息都已处理完毕
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.
//阻塞
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;
}
}
首先，在MessageQueuen.next()中开启了一个死循环，当有message时且不大于当前时间返回当前头节点，如果message.when大于当前时间，就会设置一个超时然后唤醒循环。
现在重新回到Looper.loop函数，将取出的message交给当前它的Handler来分发处理
Handler.java
/*
* Handle system messages here.
* 开始分发
*/
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {/*如果callback不为空，也就是设置了runnable*/
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
到这里Handler的从发送Message到处理消息就处理完毕了！
网友评论