Handler：Android 消息机制，我有必要再讲一次！

super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main3)
// 请求网络
subThread.start()
}
override fun onDestroy() {
subThread.interrupt()
super.onDestroy()
}
private val handler by lazy(LazyThreadSafetyMode.NONE) { MyHandler() }
private val subThread by lazy(LazyThreadSafetyMode.NONE) { SubThread(handler) }
private class MyHandler : Handler() {
override fun handleMessage(msg: Message) {
super.handleMessage(msg)
// 主线程处理逻辑，一般这里需要使用弱引用持有 Activity 实例，以免内存泄漏
}
}
private class SubThread(val handler: Handler) : Thread() {
override fun run() {
super.run()
// 耗时操作 比如做网络请求
// 网络请求完毕，咱们就得哗哗哗通知 UI 刷新了，直接直接考虑 Handler 处理，其他方案暂时不做考虑
// 第一种方法，一般这个 data 是请求结果解析的内容
handler.obtainMessage(1,data).sendToTarget()
// 第二种方法
val message = Message.obtain() // 尽量使用 Message.obtain() 初始化
message.what = 1
message.obj = data // 一般这个 data 是请求结果解析的内容
handler.sendMessage(message)
// 第三种方法
handler.post(object : Thread() {
override fun run() {
super.run()
// 处理更新操作
}
})
}
}

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);
}
</pre>

上面的代码出现了一个 MessageQueue，并且最终调用了 MessageQueue#enqueueMessage 方法进行消息的入队，我们不得不简单说一下 MessageQueue 的基本情况。

# **MessageQueue**

顾名思义，MessageQueue 就是消息队列，即存放多条消息 Message 的容器，它采用的是单向链表数据结构，而非队列。它的 next() 指向链表的下一个 Message 元素。

<pre class="ql-align-justify" style="margin: 0px; padding: 15px; vertical-align: baseline; border: 0px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-weight: 400; font-stretch: inherit; line-height: inherit; font-family: monospace; font-size: 18px; white-space: pre-line; word-break: break-all; background-color: rgb(244, 245, 246); color: rgb(34, 34, 34); letter-spacing: normal; orphans: 2; text-align: justify; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">boolean enqueueMessage(Message msg, long when) {
// ... 省略一些检查代码
synchronized (this) {
// ... 省略一些检查代码
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;
}

Message next() {
// ...
int nextPollTimeoutMillis = 0;
for (;;) {
// ...
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;
}
//...
}
//...
// 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;
}
}

public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
// ...
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
//...
try {
// 分发消息给 handler 处理
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
// ...
}
// ...
}
}

public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}

public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

ThreadLocal<Boolean> local = new ThreadLocal<>();
// 设置初始值为true.
local.set(true);
Boolean bool = local.get();
Logger.i("MainThread读取的值为：" + bool);
new Thread() {
@Override
public void run() {
Boolean bool = local.get();
Logger.i("SubThread读取的值为：" + bool);
// 设置值为false.
local.set(false);
}
}.start():
// 主线程睡1秒，确保上方子线程执行完毕再执行下面的代码。
Thread.sleep(1000);
Boolean newBool = local.get();
Logger.i("MainThread读取的新值为：" + newBool);

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

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));
}

public static void main(String[] args) {
// ...
Looper.prepareMainLooper();
// ...
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
//...
Looper.loop();
// ...
}

public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}