Android轮子哥 的博客地址：
https://www.jianshu.com/u/f7bb67d86765
Handler 组成部分
Message：消息对象
MessageQueue：消息队列
Looper：消息轮询器
Handler 工作原理
Message：用于记录消息携带的信息
MessageQueue：存取 Message 的队列集合
Looper：不断获取是否有新的 Message 需要执行
Message 对象介绍
获取 Message 对象的两种方式：
有什么不一样？接下来查看一下 Message.obtain 这个静态方法做了什么操作。
先翻译一下 obtain 的方法的注释文档。
Return a new Message instance from the global pool.
Allows us to avoid allocating new objects in many cases.
从全局池返回一个新的消息实例。允许我们在许多情况下避免分配新对象。
看到这里大家心里应该有底了，就是在复用之前用过的 Message 对象，这里实际上是用到了一种享元设计模式，这种设计模式最大的特点就是复用对象，避免重复创建导致的内存浪费。
再介绍一下 Message 对象的一些特殊的属性，待会我们会用得到。
Handler.sendMessage 解析
public final boolean sendMessage(Message msg) {
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessage(int what) {
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
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);
}
public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
简单过一遍，发现一个问题，sendXXX 这些方式最终还是会调用到 enqueueMessage 这个方法上来，所以让我们重点看一下这个方法。
就在刚刚给大家看了一下 Handler 的特殊属性，target 其实就是一个 Handler 类型的对象，现在给它赋值为当前的 Handler 对象，其实这样我们已经不难断定，它最后肯定会这样回调 Handler 的 handleMessage 的方法了。
msg.target.handleMessage(msg);
MessageQueue.enqueueMessage 解析
这里只是设想，接下来继续看 queue.enqueueMessage 的方法，发现这里标红点不进去，我们可以直接点击 MessageQueue 对象进去，由于 enqueueMessage 代码太长，没法放截图，就直接放代码了。
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;
}
这里我们先讲一个细节的问题，MessageQueue 类中的几乎所有的方法里面都有 synchronized 关键字，证明这个类已经处理过线程安全的问题了。
刚刚的源码你只需要简单过一遍，接下来我们挑重点的讲，如果对链表不熟悉的先去百度了解一下（简单点的来说就是对象自己嵌套自己），这里用的是单向链表，我已经把注释打上去了，要集中精力看
// 标记这个 Message 已经被使用
msg.markInUse();
msg.when = when;
// mMessages 是一个 Message 对象
Message p = mMessages;
boolean needWake;
// 如果这个是第一个消息，如果这个消息需要马上执行，如果这个消息执行的时间要比之前的消息要提前的话
if (p == null || when == 0 || when < p.when) {
// 把这个 Message 对象放置在链表第一个位置
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
needWake = mBlocked && p.target == null && msg.isAsynchronous();
// 这块比较难理解了，要注意集中精力，不要脑子被转晕了
// 记录跳出循环前最后的一个 Message 对象
Message prev;
// 不断循环，根据执行时间进行对链表进行排序
for (;;) {
// 你没有看错，这个对象就只是记录而已，循环里面没有用到
prev = p;
// 获取链表的下一个
p = p.next;
// 如果这个是链表的最后一个，如果这个消息执行时间要比链表的下一个要提前的话
if (p == null || when < p.when) {
// 跳出循环
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
// 将刚刚符合要求的对象 p 排在 msg 后面
msg.next = p;
// 再将 msg 排在 prev 的后面（温馨提醒：prev 和 p 是不一样的，p 其实等于 prev.next，不信你回去看源码）
prev.next = msg;
// 排序前：prev ---> p
// 排序后：prev ---> msg ---> p
}
Message（消息） 对象已经在 MessageQueue（消息队列）中排序好了，那么问题来了，MessageQueue.enqueueMessage 方法压根没调用 Handler.handleMessage 方法？你让我情何以堪？
纠正一个刚刚的设想
Handler.handleMessage 到底被谁调用了？请看下图：
handleMessage 原来是被 Handler.dispatchMessage 回调的，那么我们之前那种设想还不太对。
// 刚刚的设想
msg.target.handleMessage(msg); // 错误
// 现在的设想
msg.target.dispatchMessage(msg); // 正确
Handler 和 Looper 的关系
让我们先来看一下 Handler 构造函数。
public class Handler {
public Handler() {
this(null, false);
}
public Handler(Callback callback) {
this(callback, false);
}
public Handler(Looper looper) {
this(looper, null, false);
}
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
public Handler(boolean async) {
this(null, async);
}
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 " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
}
我们先来看一下两句重点代码：
mLooper = looper;
mQueue = looper.mQueue
你会发现，Handler 和 Looper 有很大关系，就连 MessageQueue 也是 Looper 里面的对象，看来还真的不简单。
Looper.loop
既然如此，我上去一顿搜索，Looper 类中只有一个地方调用了 Handler.dispatchMessage 方法。
由于这个方法太长，我们把这个方法源码单独拎出来，简单过一遍就好。
/**
* 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();
// 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);
}
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;
try {
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
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();
}
}
我们先翻译一下这个 Looper.loop 方法的注释。
Run the message queue in this thread.
Be sure to call {@link #quit()} to end the loop.
在这个线程中运行消息队列。确保调用{@link #quit()}来结束循环。
看完这个翻译你是不是顿悟了，原来 MessageQueue 消息队列最后是在这个方法执行的，最后我们分析一下里面比较重点的源码。
// 不断循环
for (;;) {
// 取 MessageQueue 中的 Message 对象，具体方法就不带大家看了
Message msg = queue.next();
if (msg == null) {
// 直到消息队列没有 Message 对象了就跳出循环和退出方法
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;
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;
try {
// msg.target 之前说过了，在 sendMessage 的时候已经赋值自身给这个字段了
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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