Handler 是什么？
Handler 是一个可以实现多线程间切换的类，通过 Handler 可以轻松地将一个任务切换到 Handler 所在的线程中去执行。我们最常用的使用的场景就是更新 UI 了，比如我们在子线程中访问网络，拿到数据后我们 UI 要做一些改变，如果此时我们直接访问 UI 控件，就会触发异常了。这个时候我们往往会通过 Handler 将更新 UI 的操作切换到主线程中。
Handler 的基本使用
用法一：通过 send 方法
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";
private MyHandler mMyHandler = new MyHandler();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new Thread(new Runnable() {
@Override
public void run() {
Message message = Message.obtain(mMyHandler,0,"通过 send 方法");
mMyHandler.sendMessage(message);
}
}).start();
}
private static class MyHandler extends Handler{
@Override
public void handleMessage(Message msg) {
switch (msg.what){
case 0:
Toast.makeText(MainActivity.this,msg.obj.toString(),Toast.LENGTH_SHORT).show();
break;
}
}
}
}
用法二：通过 post 方法
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";
private Handler mMyHandler = new Handler();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new Thread(new Runnable() {
@Override
public void run() {
mMyHandler.post(new Runnable() {
@Override
public void run() {
Toast.makeText(MainActivity.this,"通过post方法",Toast.LENGTH_SHORT).show();
}
});
}
}).start();
}
}
其实，通过 post 方法最后通过 send 方法来完成的。这个我们稍后会分析。讲到 Handler，我们不得不提起 MessageQueue 类 和 Looper 类。 Handler 通过 send 方法 发送一个消息，会调用 MessageQueue 的 enqueueMessage 方法 将这个消息插入到 MessageQueue 中，然后 Looper 发现有消息来临时，通过一系列的方法调用后，Handler 如果是通过 post 方法就会执行 post 方法里面的 Runnable ，如果是通过 send 方法就会执行 Handler 的 handleMessage 。这么说感觉有点云里雾里的，让我们仔细的来看下 Handler 类、MessageQueue 类和 Looper 类。
Handler 类
我们先来看下 Handler 类的结构
Handler 类结构.png
Handler 的工作主要包括消息的发送和接收过程。一般来说，消息的发送和消息的接收是位于不同的线程。我们首先来看 post 方法。
/**
* 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(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
这里调用了 sendMessageDelayed 方法
/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @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. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
而 sendMessageDelayed 又调用了 sendMessageAtTime（） 方法
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @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. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
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);
}
千呼万唤始出来，在 sendMessageAtTime 这个方法我们终于看到了 MessageQueue 类，这里的逻辑主要向 MessageQueue 中插入了一条消息（Message）。咦？我们不是通过 post 方法传进来的 Runnable 么？什么时候变成 Message 了？其实刚才我们忽略了一个方法。
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
没错，就是 getPostMessage 方法
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
从这里看到，系统通过调用 Message.obtain() 创建一个 Message，并把我们通过 post 方法传进来的 Runnable 赋值给 Message 的 callback。这里的 callback 需要留意，这个在我们之后的分析会用到。接下里我们看 Handler 的 send 方法。
/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @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 sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
是不是很熟悉？post 方法也是调用这个 sendMessageDelayed 方法，这也是为什么我们之前说 post 方法 也是通过 send 方法来执行的。到此为止，我们已经弄懂 Handler 的消息发送过程。总结的来说，通过 post 方法系统会把 我们传进来的 Runnable 转变成 Message，然后就和 send 方法一样，通过一系列的方法调用之后把 Message 插入到 MessageQueue 当中。至于 Handler 的消息接收过程，我们暂且放一下，先来看 MessageQueue 类。
MessageQueue 类
前面说到，Handler 发送消息的过程就是往 MessageQueue 中插入 一个 Message，即调用 MessageQueue 的 enqueueMessage 方法。首先，我们来看下 MessageQueue 的类结构
MessageQueue类结构.png
我们看到 MessageQueue 是比较简单的。其实，MessageQueue 主要包含两个操作：插入和读取。
插入方法:enqueueMessage
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("MessageQueue", 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;
}
读取方法:next
需要注意的是：读取操作本身会伴随着删除操作
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 (false) Log.v("MessageQueue", "Returning message: " + msg);
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("MessageQueue", "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;
}
}
Looper 类
首先，我们也来看下 Looper 的类结构
Looper类结构.png
关于 Looper ，我们首先要明确一点，Looper 是线程相关的，即每个线程的 Looper 是不一样的，但是线程默认是没有 Looper 的。可能会有点绕，要理清这里面的逻辑的关系，我们首先要了解 ThreadLocal，关于 ThreadLocal 网上的资料挺多的。简单地来说，ThreadLocal 是一个线程内部的数据存储类，比如有有一个 int 类型的 x，在线程 A 的值是 1，在线程 B 的值可以是 0,1，2,..,在线程 C 的值可以是 0,1,2... 我们来看下 Looper 相关的源码
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<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));
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();
}
我们为什么要明确 Looper 是线程相关的呢？因为 Handler 创建的时候会采用当前线程的 Looper 来构造消息循环系统的。Handler 创建的时候要先创建 Looper，这时候疑问就来了？我们平常创建 Handler 的时候直接就创建了啊，没有创建什么 Looper 啊。这是因为我们通常是在主线程 ActivityThread 中创建 Handler。我们看到 Loop 类中有个 prepareMainLooper 方法。
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
主线程在创建时，就会调用这个方法创建 Looper。但是如果我们在子线程（如下代码）直接创建 Handler 就会抛出异常
new Thread(new Runnable() {
@Override
public void run() {
//Looper.prepare();
Handler handler = new Handler();
// Looper.loop();
}
}).start();
这时只要我们把注释去掉就不会报异常了。通过源码我们知道 Looper.prepare() 主要是为当前线程一个 Looper 对象。
/** 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));
}
那么，Looper.loop（）方法是干什么的呢？其实，Looper 最重要的一个方法就是 loop 方法了。只有调用 loop 后，消息系统才会真正地起作用。我们来看 loop 方法
/**
* 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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(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();
}
}
我们可以看到 loop 方法是一个死循环，在这个死循环方法里面会调用 MessageQueue 的 next 方法来获取新消息。但是如果 next 方法返回了 null，loop 就退出循环。这种情况发生在 Loop 的 quit 方法被调用时，Looper 会 调用 MessageQueue 的 quit 方法来通知消息队列退出，当消息队列被标记退出状态时，它的 next 方法就会返回 null。由于 next 是一个阻塞方法，所以 loop 也会一直阻塞在那里，如果有消息到来， msg.target.dispatchMessage(msg)。这个 msg.target 就是发送这个消息的 Handler 对象啦。这样 Handler 发送的消息最终又交给自己的 dispatchMessage 方法来处理了。因为 Handler 的 dispatchMessage 方法是创建 Handler 时使用的 Looper 中执行的，这样就成功地完成线程切换了。
Handler 的消息接收过程
经过跋山涉水，通过 Handler 发送的消息最终又会回到自己的 diapatchMessage 中来，那就让我们来看下 diapatchMessage 方法。
/**
* 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);
}
}
首先，检查 Messgae 的 callback 是否为 null，不为 null 就调用 handleCallback 方法，这个 Message 的 callback 就是我们之前post的。其次，检查 mCallback 是否为 null ，不为 null 就调用 mCallback 的 handleMessage 方法来处理消息。如果我们是通过继承 Handler 来实现逻辑的话，此时的mCallback 是为空的，即会调用 handleMessage(msg)，也就是我们重写的 handleMessage 方法。至此，完成了完美的闭环。
有的同学可能会疑问 mCallback 是什么？什么时候会为空？
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public interface Callback {
public boolean handleMessage(Message msg);
}
/**
* Constructor associates this handler with the {@link Looper} for the
* current thread and takes a callback interface in which you can handle
* messages.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Callback callback) {
this(callback, false);
}
通过源码可以看出，我们也可以采用 Handler handler = new Handler(callback) 来创建 Handler，这时dispatchMessage 里面就会走 mCallback 不为空的逻辑。
参考资料
[1] 任玉刚. Android 开发艺术探索[M]. 北京 : 电子工业出版社 , 2015.9.
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