android 子執行緒和UI執行緒的互動主要使用Handler的方法進行通訊。本文分析Handler機制
Handler 如何使用？
Handler的使用比較簡單
public class MainActivity extends Activity{
private Handler handler = new Handler() {
public void handleMessage(Message msg) {
switch (msg.what) {
case 0x01:
//do somethings
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
handler = new Handler();
new Thread(new Runnable(){
@Override
public void run(){
Message message = new Message();
message.what = 0x01;
handler.sendMessage(message);
}
}).start();
}
}複製程式碼如程式碼就是一個簡單的Handler的使用Demo,有如下幾個問題
- Handler 是否可以在子執行緒中初始化。可以，但是如下程式碼執行的話會丟擲該錯誤"Can't create handler inside thread that has not called Looper.prepare() ".說
是不能再沒有呼叫Looper.prepare()的執行緒中建立Handler。因此如果需要線上程中建立Handler首先呼叫一下Looper.prepare
new Thread(new Runnable() {
@Override
public void run() {
Handler handler = new Handler();
}
}).start();複製程式碼這樣呼叫將不會丟擲異常。
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
Handler handler = new Handler();
Looper.loop();
}
}).start();複製程式碼Looper和Handler的聯絡是什麼樣的呢？
我們看一下Handler初始化的程式碼
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 that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}複製程式碼可以看到如果mLooper是通過Looper.myLooper獲得一個Looper物件，如果Looper物件為空，則丟擲上述異常。那Looper.myLooper是如何定義的呢？
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}複製程式碼該方法很簡單就是從sThreadLocal物件中獲得Looper物件。如果sThreadLocal中存在就返回Looper，如果沒有就返回null。那Looper是如何存放在sThreadLocal中，
不錯就是Looper.prepare。
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));
}複製程式碼可以看到，首先判斷sThreadLocal中是否存在Looper物件，如果已經存在，那麼如果還有prepare Looper則丟擲異常；否則就新建一個Looper存放到sThreadLocal中。
該程式碼同時說明每個執行緒最多一個Looper物件。
Looper採用ThreadLocal來維護各個執行緒的Looper物件。ThreadLocal是什麼呢？官方定義是：ThreadLocal實現了執行緒本地儲存。所有執行緒共享同一個ThreadLocal物件，但不同執行緒僅能訪問與其執行緒相關聯的值，一個執行緒修改ThreadLocal物件對其他執行緒沒有影響。
我們可以將ThreadLocal理解為一塊儲存區，將這一大塊儲存區分割為多塊小的儲存區，每一個執行緒擁有一塊屬於自己的儲存區，那麼對自己的儲存區操作就不會影響其他執行緒。對於ThreadLocal，則每一小塊儲存區中就儲存了與特定執行緒關聯的Looper。
主執行緒中使用Handler時為什麼沒有執行Looper.prepare()也可以使用Handler呢？其實在程式啟動的時候我們已經建立了主執行緒也依賴的Looper，程式碼在ActivityThread中main方法中。
public static void main(String[] args) {
....
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}複製程式碼可以看到該方法執行的是Looper.prepareMainLooper方法,可看到歸根到底還是執行prepare方法
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}複製程式碼Message又是如何引入到Handler機制的？
眾所周知，我們都知道Handler，Message，Looper是Handler機制不可或缺的要素。那麼Message都是如何引入到Handler.我們看一下上述例子Message是通過handler.sendMessage(message)引入到Handler中。
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}複製程式碼可以看到sendMessage最終呼叫MessageQueue中enqueueMessage
boolean enqueueMessage(Message msg, long when) {
....
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;
}複製程式碼MessageQueue並沒有使用一個集合把資訊儲存，它只是通過使用mMessage物件表示當前需要處理訊息，然後根據時間把msg進行排序。具體方法是根據時間順序呼叫msg.next。從而為每一個訊息指定它
的下一個訊息是什麼。如果需要將msg作為隊頭插入到MessageQueue中可以呼叫sendMessageAtFrontOfQueue實現。
這樣訊息就進入到MessageQueue中，那如何從MessageQueue中將訊息取出來呢？
大家有沒有注意到Loop.prepare一般和Looper.loop對應使用。其實Looper.loop就是用來從MessageQueue中取出message。
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
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.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
....
}
}複製程式碼上文我們知道每個Thread都有一個Looper，其實每個Looper都對應一個MessageQueue。loop方法我們獲得對應looper中的MessageQueue不斷取出msg，並傳入到dispatchMessage.
dispatchMessage方法將取出的msg傳遞到定義Handler時重寫的handleMessage方法。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}複製程式碼Handler,Message,MessageQueue,Looper流程示意圖如下：
