什么Handler
在Android中我们通常在子线程中进行一些耗时操作，然后将执行的结果告诉UI线程，所以Handler是跨线程之间的一种数据传输机制。
Handler正确使用姿势
通常我们在UI线程中，直接new一个Handler，然后在子线程中创建消息，在发送消息。
private Handler mHandler = new Handler(){
@Override
public void handleMessage(@NonNull Message msg) {
super.handleMessage(msg);
System.out.println(msg.obj);
}
};
/**
* 发送消息
*/
public void sendMessage(View view) {
Thread thread = new Thread(new Runnable() {
@Override
public void run() {
SystemClock.sleep(3000);
Message message = Message.obtain();
message.obj = "I am Main Message";
mHandler.sendMessage(message);
}
});
thread.start();
}
但是这种姿势会给我们误导，使我们更加难理解Handler的原理。回归本质，Handler是一种线程之间的数据传输机制，并不只是主线程和子线程。Android系统在启动我们的App的时候，帮助了我们初始化一下东西，导致我们在主线程创建Handler的时候，会省略一下步骤，现在我们通过两个普通子线程进行演示。
public class HandlerActivity extends AppCompatActivity {
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handler);
}
Handler mHandleMessageHandler;
/**
* 发送消息
*/
public void sendMessage(View view) {
//创建线程handleMessage，接受处理message
Thread handleMessageThread = new Thread(new Runnable() {
@Override
public void run() {
//1.Looper初始化准备
Looper.prepare();
//2.创建Handler
mHandleMessageHandler = new Handler(Looper.myLooper()){
@Override
public void handleMessage(@NonNull Message msg) {
super.handleMessage(msg);
System.out.println("I am handleMessage start handler Message");
System.out.println(msg.obj);
System.out.println("I am "+mHandleMessageHandler.getLooper().toString());
}
};
//3.Looper循环
Looper.loop();
}
});
//创建线程sendMessageThread，发送message
Thread sendMessageThread = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("I am sendMessageThread send Message");
SystemClock.sleep(3000);
//4.创建Message
Message message = Message.obtain();
message.obj = "I am sendMessageThread Message";
//5.发送消息
mHandleMessageHandler.sendMessage(message);
}
});
handleMessageThread.start();
sendMessageThread.start();
}
}
//打印日志
I/System.out: I am sendMessageThread send Message
I/System.out: I am handleMessage start handler Message
I/System.out: I am sendMessageThread Message
I/System.out: I am Looper (Thread-4, tid 28079) {eab3d45}
上面的一段代码创建了两个线程handleMessageThread和sendMessageThread，handleMessageThread线程处理接收到的线程，sendMessageThread线程睡眠3000秒发送消息。
通过上面的一个案例发现，发送一个消息有5步：Looper初始化、创建Handler、Looper循环、创建Message和发送消息。
Handler核心原理
我们将从上面五个步骤进行Handler的源码分析：
Looper的初始化
Looper是一直从消息队列中轮询获取消息，将取得得消息分配对应的Hander处理。
通过源码分析Looper的构造方法被私有化，只能通过prepare()方法获取。通过构造方法发现Looper创建的时候，同时也创建了消息队列MessageQueue，这个消息队列用于后面接受消息。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
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));
}
消息的发送
通常我们通过sendMessage()、sendEmptyMessage()等方法发送一条消息，这些方法的最后都会走到sendMessageDelayed(@NonNull Message msg, long delayMillis)方法。通过延时时间和当前时间，算出消息要执行的时刻，在调用enqueueMessage方法，记录次消息执行的Handler和消息的workSourceUid，通过消息队列queue调用enqueueMessage(Message msg, long when)方法，根据时间when 插入到消息队列中的对应的位置，下面是对应的源码，可以参考源码观看。
public final boolean sendMessage(@NonNull Message msg) {
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull 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(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
//target是一个Handler对象，将消息指派到相应的handler处理
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
//根据when时间将消息插入都消息队列的相应的位置
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;
}
消息的取出
从上面流程我们已经知道message已经放到消息队列MessageQueue中，那如何从消息队列中取出呢？
之前我们说过Looper一直轮询从消息队列MessageQueue从取消息，那为什么这么说呢？刚开始演示的案例中说过创建消息有5步，那我们看一下第三部Looper循环。
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();
// 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 (;;) {
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);
}
// 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 {
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();
}
}
loop()方法虽然很多代码，但是我们真正关心的也就几句核心代码，精简如下：
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;
.....
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
.....
try {
msg.target.dispatchMessage(msg);
.....
}
.....
//重置消息状态
msg.recycleUnchecked();
}
}
首先获取当前Looper对象，判断looper是否存在，在获取当前Looper对应的消息队列MessageQueue，死循环这个队列，取出队列中的每一个消息，如果消息msg不为空，通过消息的target(实际就是Hander)，找到需要处理消息的Handler。
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
根据Hander中的dispatchMessage(@NonNull Message msg) 源码可知，如果msg没有设置回调callback，会走else逻辑；这里判断mCallback是否为空，如果不为空，会走到mCallback的handleMessage()方法，否则走handleMessage(Message msg)。mCallback为Handler的Callback类型的成员变量，它是在Handler构造方法赋值。
- 创建Handler的时候，添加了CallBack参数，最后会走到CallBack的handleMessage方法中。
private Handler mHandler = new Handler(new Handler.Callback() {
//实现Callback中的handleMessage方法
@Override
public boolean handleMessage(@NonNull Message msg) {
return false;
}
});
- 如果没有设置CallBack(),最后我们会走到Handler的handleMessage方法总，那我们只需要重写handleMessage方法即可。
private Handler mHandler = new Handler(){
//重写Handler中的handleMessage方法
@Override
public void handleMessage(@NonNull Message msg) {
super.handleMessage(msg);
}
};
以上分析就是Handler的原理，它主要由Handler、Message、Looper、MessageQueue四个类来完成消息的传递过程。Handler主要负责处理Message，Looper主要负责管理消息队列MessageQueue和分发消息，MessageQueue主要按照消息执行的时间点来排列和管理消息。