只需要解釋一點:為什么handler可以用于子線程更新UI
(1)當(dāng)UI線程創(chuàng)建的時候會執(zhí)行ActivityThread的main方法:
Looper.prepareMainLooper()
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
//每個線程只允許有一個Looper,所以當(dāng)該線程的Looper存在時,報異常
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
我們看看prepare()方法做了什么
private static void prepare(boolean quitAllowed) {
//sThreadLocal 是一個ThreadLocal類型的實例,這個類的作用簡單說就是為每個線程提供某個變量的副本來保證線程安全,以后會詳細(xì)說
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
以上兩步做一個總結(jié),UI線程在初始化的時候會實例化一個Looper對象和一個MessageQueue對象,注意:一個線程只有一個looper實例,只有一個MessageQueue實例.
(2)ActivityThread中的另外一個方法:loop()
//不需要每行都看
public static void loop() {
//myLooper()返回的就是當(dāng)前線程的looper對象
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//獲取當(dāng)前線程的MessageQueue對象
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();
//開啟無線循環(huán)去取消息
for (;;) {
//這里可能發(fā)生阻塞,為什么會阻塞?因為queue有可能取不出消息,因此就會等待消息
Message msg = queue.next();
//當(dāng)取出的消息為空,退出循環(huán).什么時候為空,需要看next源碼,到這里我們看看next()方法做了什么
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.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
//這里是關(guān)鍵點,取出消息來以后,把消息交給它的target對象去處理,target是誰,一會揭曉答案
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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();
...
msg.recycleUnchecked();
}
}
我們看看MessageQueue的next()方法做了什么
//只需要看中文注釋的地方
Message next() {
//這個mPtr決定了是否返回null,返回null的話則looper的loop()方法就會退出,也就是不會再處理消息
//used by native code 這個是這個字段的解釋,也就是說這個字段由native方法控制
//當(dāng)我們調(diào)用looper的quit()或者quitSafely()方法的時候,這個字段的值就會被賦值為0
//quit()指的是立即退出消息循環(huán),不管MessageQueue里面還有沒有待處理的消息
//quitSafely()是等MessageQueue里面的消息處理完成后再退出
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
//開啟無限循環(huán)去取消息,不用深究
for (;;) {
...
}
(3)通過looper的loop()方法,我們知道最終處理消息的是msg的target對象,下面我們來看看這個target是什么
我們在使用handler的時候,會使用這個方法:
handler.sendMessage(msg);
我們看看這個方法做了什么:
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
接著看:
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
接著看:
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
//這個消息隊列,就是當(dāng)前線程的唯一消息隊列,就是在創(chuàng)建looper的時候創(chuàng)建的那個唯一的MessageQueue,handler在哪個線程創(chuàng)建,這個MessageQueue就是哪個線程的
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);
}
接著看:
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//這個就是關(guān)鍵點!target對象就是handler本身,也就是說這個msg是誰發(fā)出的,最終由誰的dispatchMessage()方法處理!!!
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//這個方法就是把消息加入到MessageQueue的消息隊列里面,本著先進先出的原則,先加入到隊列的消息優(yōu)先處理
return queue.enqueueMessage(msg, uptimeMillis);
}
(4)從上面看出方法的最終執(zhí)行者是我們創(chuàng)建的handler的dispatchMessage()方法:
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
//當(dāng)使用handler.post(Runnable r)這個方法的時候會執(zhí)行這個方法,這個方法說到底就是執(zhí)行r中的run()方法
handleCallback(msg);
} else {
//看到handleMessage()方法可能會更加熟悉,因為我們在創(chuàng)建handler的時候會重寫這個方法,也就是最終調(diào)用的是我們重寫的方法
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
(5)handler不僅可以用來更新ui,我們還可以在子線程創(chuàng)建handler,然后在其他線程使用這個handl實例發(fā)送消息,在子線程處理相應(yīng)的邏輯,注意我們需要手動Looper.prepare().
new Thread(){
@Override
public void run() {
Looper.prepare();
handler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
super.run();
}
}.start();
