看了網(wǎng)上的一些handler原理分析，是從源碼級別直接看的。但是總覺得沒有從實戰(zhàn)上分析嗎，感覺有點蹩腳。這篇文章算是自己分析的，也參考了《android開發(fā)藝術探索》，還有網(wǎng)上的一些其他資料，如有侵權請私信
先來舉個栗子吧
一般在使用handler的時候，用其來更新UI，也就是說在主線程進行更新界面操作，當時子線程請求網(wǎng)絡數(shù)據(jù)，如此handler剛好派上用場。先看實例吧
public class MainActivity extends Activity {
private TextView text;
private Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case 1:
text.setText("使用Handler更新了界面");
break;
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
text = (TextView) findViewById(R.id.id_text);
new Thread("#Thread1") {
@Override
public void run() {
//...你的業(yè)務邏輯；
Message message = new Message();//發(fā)送一個消息，該消息用于在handleMessage中區(qū)分是誰發(fā)過來的消息；
message.what = 1;
handler.sendMessage(message);
}
}.start();
}
}
以上的例子呢，應該算是handler使用的最普遍的，當然還有其它用法，handler創(chuàng)建在主線程中，也就是Activitythread，在"#Thread1"中執(zhí)行耗時操作，在主線程中更新UI，創(chuàng)建handler。
在創(chuàng)建handler之前需要注意：
在創(chuàng)建handler之前，必須先在相同線程內(nèi)創(chuàng)建一個Looper，每個線程中最多只能有一個 Looper 對象，由 Looper 來管理此線程里的 MessageQueue (消息隊列)。
如果不在主線程創(chuàng)建handler的話，應該這么寫代碼:
new Thread("Thread#2"){
@override
public void run(){
Looper.prepare();
Handler handler = new Handler();
Looper.loop();
}
}
首先利用Looper.prepare()創(chuàng)建looper，然后創(chuàng)建handler，然后調用Looper.loop()開啟消息循環(huán)。這是在非UI線程，但是我們在之前的handler使用例子中并未簡單looper的創(chuàng)建啊，其實looper 已經(jīng)在activityThread當中給你創(chuàng)建完了?？梢詠砜纯丛创a
public static void main(String[] args) {
、、、、、此處省略部分代碼
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
AsyncTask.init();
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
可以看出activitythread執(zhí)行了兩個looper方法
Looper.prepareMainLooper();
Looper.loop();
所以我們只需要創(chuàng)建handler就可以了。那我們接著看我們文章開頭那個實例，重寫了handler的handlemessage方法，然后新建線程，利用handler發(fā)送消息。然后主線程的handler就會接收到消息，并在handlemessage中處理消息。上面整體思路沒問題吧，這也是最初在使用handler的時候，大家的通常思維，那handler到底是怎么切換線程的，從thread1切換到了主線程？？？
先上一張整體的流程圖
這張圖是我自己畫的，可能畫的不太好，也看了一些大佬的流程圖，畢竟我基礎差，總覺得他們的圖理解起來不是很容易。
MessaggeQueue類詳解
根據(jù)我們的操作先來看啊，主線程已經(jīng)準備就緒，handler創(chuàng)建完了，然后調用sendmessage方法，最后調用的事enqueuemessage()方法，然后就到了messgaeQueue類，這個類又是從哪來的呢？？？？這個類其實是從Looper里面蹦出來的，在創(chuàng)建Looper的時候，就會生成MessageQueue,來看一下源碼
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));
}
/**
* 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();
}
}
我們知道在主線程的main函數(shù)中，已經(jīng)調用了prepareMainLooper()方法，而后prepareMainLooper()方法又調用了 prepare()方法,在這個方法里面，我們調用looper的構造函數(shù)，并將新建的looper存入到了ThreadLocal里面（ThreadLocal后邊講），然后我們來看這個looper的構造函數(shù)
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
可以清晰的看到Looper在構造函數(shù)中創(chuàng)建了MessageQueue,知道了MessageQueue的由來，我們簡單來說一下MessageQueue這個類啊
enqueueMessage()：這個是通過維護一個單鏈表，handler添加消息以后，MessageQueue會將消息添加到最后。
next():用于取出消息，并將消息傳遞給Looper 。注意這個next方法很重要，是個死循環(huán)，當沒有消息的時候會一直阻塞在這里，直到有消息會將消息傳遞出去,代碼如下，不用看的很詳細。
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類詳解
我們這時候來看看looper，looper是在主線程的，剛剛說過了，looper在handler創(chuàng)建之前就已經(jīng)創(chuàng)建完成了，順帶把MessageQueue和looper.loop()方法都執(zhí)行了。這時候我們來看看這個loop方法是干嘛地
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();
}
}
和上面next方法一樣，這塊也是個死循環(huán)，一直在等待messagequeue的消息。等它拿到消息以后，就會調用msg.target.dispatchMessage(msg);這行代碼，這行代碼其實就是調用了handler的dispatchMessage方法，后邊經(jīng)過一些列的判斷會調用handler的handleMessage()方法。這是整體過程。
總結
下面從別的文章上抄來的圖，簡單粗暴。很明顯就能看出如何切換線程的，首先主線程就相當于A線程，B線程就相當于#Thread1
POST就相當于在thread1里面使用handler發(fā)送消息，然后把消息放在messageQueue當中，主線程里面的Looper就回調用loop方法，一直從messagequeue類的next方法中獲取消息，然后再次調用handler消化這個消息。
引用的技術文章
https://blog.csdn.net/AdobeSolo/article/details/75195394
https://mp.weixin.qq.com/s/D1v7b5CUT-3JMhxR6iCpcA
https://blog.csdn.net/CHENYUFENG1991/article/details/46910675
《android開發(fā)藝術探索》
