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一、Handler的使用场景
Android系统中更新UI只能在主线程中进行,如果在子线程中访问UI程序会抛出CalledFromWrongThreadException的异常。而且Android又不建议在主线程中进行耗时操作,不然有可能会出现ANR问题。因此对于像网络请求、数据库查询这样的耗时操作一般是放在子线程中执行的。当子线程获取数据成功以后需要将数据更新到UI界面,由于UI更新不能在子线程中进行,因此在这样的场景下就用到了Handler机制。我们可以通过Handler将子线程中的数据发送出去,并在主线程中接受,从而实现在主线程中更新UI的操作。对于Handler的作用我们可以归结为一句话:Handler是为了解决Android中子线程无法访问UI的问题。
到这里我们或许有个疑问。Android系统中为什么不允许在子线程中访问UI呢?这是因为Android的UI控件是线程不安全的。如果在多线程中并发访问可能会导致UI控件处于不可预期的状态。那为什么不给UI空间加锁呢?缺点有两个:首先加锁会让UI访问变得复杂;其次加锁会降低UI访问的效率,因为锁机制为阻塞某些线程的执行。
二、Android消息机制概述
Handler的运行需要MessageQueue和Looper的支撑。
Handler在创建时会采用当前线程的Looper来构建内部消息循环系统。Android系统主线程是默认已经创建了Looper,因此我们在Activity中是可以直接使用Handler的,但是如果想要在子线程中使用Handler,我们需要在子线程中手动创建Looper,否则程序会报异常。 Looper的开启也比较简单可直接在子线程中调用Looper.prepare()和Loop.loop()两个方法。代码如下:
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();
}
}
MessageQueue即为消息队列,它存储的是Handler发送的一组消息。MessageQueue以队列的形式对外提供插入和删除的操作。虽然叫消息队列,但是他的内部存储采用的是单链表的数据结构。单链表在插入和删除数据时效率较高,相对于队列有明显优势。
Looper在消息机制中扮演着消息循环的角色,它会不停的从MessageQueue中抽取消息,如果MessageQueue中没有消息了,那么Looper的循环就会一直被阻塞。
Handler通过post方法将一个Runnable发送到Handler内部的Looper中去处理,也可以通过send方法发送一个消息,这个消息同样会在Looper中处理。其实post方法最终也是通过send方法来完成的。那么接下来看下send方法的工作过程。当Handler的send方法被调用时它会调用MessageQueue的enqueueMessage方法将这个消息放入消息队列中。接下来Looper通过循环会拿到这条消息来处理。最终消息中的Runnable或者Handler的handleMessage方法就会被调用。因为Looper是运行在创建Handler所在的线程中的。这样以来Handler中的业务逻辑就被切换到了创建Handler所在的线程中执行了。这个过程如下图所示:
三、MessageQueue的工作原理
上节中已经对MessageQueue有了一个大概的了解。本节内容将详细的介绍MessageQueue的工作原理。MessageQueue主要包括两个操作:插入和读取。读取操作本身会伴随着删除操作。插入和读取对应的方法分别是enqueueMessage和next。其中enqueueMessage方法是向MessageQueue中插入一条消息,而next是从MessageQueue中读取一条消息并将其移除。尽管MessageQueue叫消息队列,但它的内部是由一个单链表的数据结构来维护消息列表。接下来我们可以通过源码来看下 enqueueMessage和next两个方法是如何实现的。
首先enqueueMessage的源码如下:
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;
}
从上面代码中可以看出来enqueueMessage方法其实就是将一个msg插入到链表的过程。
接下来看next方法的实现:
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;
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;
}
// 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;
}
}
next方法是一个无限循环方法,如果消息队列中没有消息,那么next方法会一直阻塞。当有新的消息时next方法会返回这条消息并将其从单链表中移除。
四、Looper的工作原理
Looper在消息机制中扮演消息循环的角色。它会不断的查看MessageQueue中是否有新的消息。如果有消息就会立刻处理。否则就会一直阻塞。Looper中最重要的一个方法是loop方法。只有调用loop方法后消息循环系统才会真正运作起来。下面结合loop方法的源码来看。
/**
* 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 traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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();
}
}
从上面代码中可以看出loop方法是一个死循环。只有当MessageQueue.next()返回null时才会跳出循环。由于next是一个阻塞操作当MessageQueue中没有消息是next方法会一直被阻塞,因此也致使loop方法被阻塞。如果MessageQueue中添加了一条消息,则会在loop方法中调用MessageQueue.next方法取出消息并处理。通过调用msg.target.dispatchMessage(msg)处理消息。而msg.target是什么呢?我们不妨去Message的源码中看一番。代码如下:
public final class Message implements Parcelable {
...
/*package*/ Handler target;
...
}
没错,msg.target就是Handler,也就是通过loop方法最终调用了Handler去处理了消息。而msg.target是在什么时候赋值的的呢?现在我们不妨去看一下Handler的源码吧,首先可以从Handler调用sendMessage()或者postDelay()着手。代码如下:
mHandler.postDelayed(this, interval);
跟进postDelayed到Handler中:
public final boolean postDelayed(Runnable r, long delayMillis)
{
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
继续跟进sendMessageDelayed()方法:
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
再接着跟进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);
}
然后继续跟进enqueueMessage方法:
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
终于在这里找到了答案:在MessageQueue中插入消息时顺带调用msg.target=this,即将当前Handler对象赋值给了target。
了解了这个流程以后我们对我们已经将Android消息机制串联起来了。接下来我们就只需要看dispatchMessage方法做了那些操作了!
五、Handler的工作原理
上节内容我们已经大致了解了Handler通过sendMessage()或者postDelay()向MessageQueue插入消息的一个流程。那么当loop方法获取消息之后调用dispatchMessage(msg)后做了什么操作呢?接下来继续分析dispatchMessage方法的源码:
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
看到这里想必大家应该就都明白了,因为handleMessage(msg)方法其实正是我们实例化Handler时重写的方法!上述代码首先检查了Message的callback是否为null,不为null就通过handleCallback来处理消息。Message的callback是一个Runnable对象,实际上就是Handler的post方法传递的Runnable参数。handleCallback方法代码如下:
private static void handleCallback(Message message) {
message.callback.run();
}
其次检查mCallback是否为null,不为null就调用mCallback的handleMessage方法处理消息。否则的话调用了Handler中的handleMessage(msg)方法。这不正对应了我们实例化Handler时的操作嘛!因此现在再来看下面的代码会不会豁然开朗呢?
// 实例化Handler对象 传入Callback参数并重写handleMessage(Message msg)
static Handler mHandler1 = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
return false;
}
});
// 实例化Handler对象 并重写Handler中的handleMessage(Message msg)
handleMessage(Message msg)
static Handler mHandler = new Handler(){
@Override
public boolean handleMessage(Message msg) {
return false;
}
};
到这里关于Android的消息机制的分析就告一段落了,看完这篇文章相信大家对Android消息机制及Handler原理又有了更深的理解!
参考书籍 《Android开发艺术探索》任玉刚