android 子线程和UI线程的交互主要使用Handler的方法进行通信。本文分析Handler机制
Handler 如何使用？
Handler的使用比较简单
public class MainActivity extends Activity{
private Handler handler = new Handler() {
public void handleMessage(Message msg) {
switch (msg.what) {
case 0x01:
//do somethings
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
handler = new Handler();
new Thread(new Runnable(){
@Override
public void run(){
Message message = new Message();
message.what = 0x01;
handler.sendMessage(message);
}
}).start();
}
}复制代码如代码就是一个简单的Handler的使用Demo,有如下几个问题
- Handler 是否可以在子线程中初始化。可以，但是如下代码执行的话会抛出该错误"Can't create handler inside thread that has not called Looper.prepare() ".说
是不能再没有调用Looper.prepare()的线程中创建Handler。因此如果需要在线程中创建Handler首先调用一下Looper.prepare
new Thread(new Runnable() {
@Override
public void run() {
Handler handler = new Handler();
}
}).start();复制代码这样调用将不会抛出异常。
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
Handler handler = new Handler();
Looper.loop();
}
}).start();复制代码Looper和Handler的联系是什么样的呢？
我们看一下Handler初始化的代码
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;
}复制代码可以看到如果mLooper是通过Looper.myLooper获得一个Looper对象，如果Looper对象为空，则抛出上述异常。那Looper.myLooper是如何定义的呢？
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}复制代码该方法很简单就是从sThreadLocal对象中获得Looper对象。如果sThreadLocal中存在就返回Looper，如果没有就返回null。那Looper是如何存放在sThreadLocal中，
不错就是Looper.prepare。
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));
}复制代码可以看到，首先判断sThreadLocal中是否存在Looper对象，如果已经存在，那么如果还有prepare Looper则抛出异常；否则就新建一个Looper存放到sThreadLocal中。
该代码同时说明每个线程最多一个Looper对象。
Looper采用ThreadLocal来维护各个线程的Looper对象。ThreadLocal是什么呢？官方定义是：ThreadLocal实现了线程本地存储。所有线程共享同一个ThreadLocal对象，但不同线程仅能访问与其线程相关联的值，一个线程修改ThreadLocal对象对其他线程没有影响。
我们可以将ThreadLocal理解为一块存储区，将这一大块存储区分割为多块小的存储区，每一个线程拥有一块属于自己的存储区，那么对自己的存储区操作就不会影响其他线程。对于ThreadLocal，则每一小块存储区中就保存了与特定线程关联的Looper。
主线程中使用Handler时为什么没有执行Looper.prepare()也可以使用Handler呢？其实在进程启动的时候我们已经创建了主线程也依赖的Looper，代码在ActivityThread中main方法中。
public static void main(String[] args) {
....
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}复制代码可以看到该方法执行的是Looper.prepareMainLooper方法,可看到归根到底还是执行prepare方法
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}复制代码Message又是如何引入到Handler机制的？
众所周知，我们都知道Handler，Message，Looper是Handler机制不可或缺的要素。那么Message都是如何引入到Handler.我们看一下上述例子Message是通过handler.sendMessage(message)引入到Handler中。
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}复制代码可以看到sendMessage最终调用MessageQueue中enqueueMessage
boolean enqueueMessage(Message msg, long when) {
....
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;
}复制代码MessageQueue并没有使用一个集合把信息保存，它只是通过使用mMessage对象表示当前需要处理消息，然后根据时间把msg进行排序。具体方法是根据时间顺序调用msg.next。从而为每一个消息指定它
的下一个消息是什么。如果需要将msg作为队头插入到MessageQueue中可以调用sendMessageAtFrontOfQueue实现。
这样消息就进入到MessageQueue中，那如何从MessageQueue中将消息取出来呢？
大家有没有注意到Loop.prepare一般和Looper.loop对应使用。其实Looper.loop就是用来从MessageQueue中取出message。
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);
}
}
....
}
}复制代码上文我们知道每个Thread都有一个Looper，其实每个Looper都对应一个MessageQueue。loop方法我们获得对应looper中的MessageQueue不断取出msg，并传入到dispatchMessage.
dispatchMessage方法将取出的msg传递到定义Handler时重写的handleMessage方法。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}复制代码Handler,Message,MessageQueue,Looper流程示意图如下：
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