文章目录
一、Handler 构造函数
二、Handler 消息分发
三、MessageQueue 消息队列相关函数
一、Handler 构造函数
一般使用 Handler 时 , 调用 Handler 的普通 无参构造函数 ,
public class Handler { /** * 默认的构造函数 , 与当前线程相关联. * 如果该线程没有 Looper , 该 Handler 不能接受 Message 消息 , 并抛出异常 */ public Handler() { this(null, false); } }
上面的无参构造函数调用了下面的构造方法 ,
第一个参数 Callback callback 是一个回调 , mCallback = callback , 该回调直接设置给了 mCallback 成员变量 ,
在该方法中 , 调用 mLooper = Looper.myLooper() 获取线程本地变量 Looper ;
获取 Looper 中的消息队列 MessageQueue , mQueue = mLooper.mQueue ;
主线程的 Looper 是在 ActivityThread 中的 main 函数 中 , 使用 Looper.prepareMainLooper() 创建的 ,
在 ActivityThread 的 main 函数最后调用了 Looper.loop() , 无限循环获取主线程 Looper 中封装的 MessageQueue 消息队列中的消息 ;
参考 : 【Android 异步操作】Handler ( 主线程中的 Handler 与 Looper | Handler 原理简介 ) ,
public class Handler { /** * Use the {@link Looper} for the current thread with the specified callback interface * and set whether the handler should be asynchronous. * * Handlers are synchronous by default unless this constructor is used to make * one that is strictly asynchronous. * * Asynchronous messages represent interrupts or events that do not require global ordering * with respect to synchronous messages. Asynchronous messages are not subject to * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}. * * @param callback The callback interface in which to handle messages, or null. * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for * each {@link Message} that is sent to it or {@link Runnable} that is posted to it. * * @hide */ public Handler(@Nullable 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; } }
Handler 中的 Callack 回调 接口 ;
public class Handler { /** * Callback interface you can use when instantiating a Handler to avoid * having to implement your own subclass of Handler. */ public interface Callback { /** * @param msg A {@link android.os.Message Message} object * @return True if no further handling is desired */ boolean handleMessage(@NonNull Message msg); } }
二、Handler 消息分发
Handler 中的消息分发 , 在 Looper 的 loop 方法中 , 调用该消息 dispatchMessage 分发消息的方法 ,
在该分发消息方法中 , 首先会查看 消息 Message 中 是否有 Callback 回调 ,
如果有执行该回调 , 就是构造函数中赋值的 mCallback ,
如果没有就调用 Handler 中的 handleMessage 方法 ;
public class Handler { /** * 在这里处理 Message 消息. */ public void dispatchMessage(@NonNull Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } }
使用 Handler 发送消息时 , 会 调用各种发送消息的方法 , 如
public final boolean sendMessage(@NonNull Message msg)
public final boolean sendEmptyMessage(int what)
public final boolean sendEmptyMessageDelayed
public final boolean sendEmptyMessageAtTime
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis)
等方法 , 所有的发送消息的方法 , 最终都会调用 public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) 方法 ,
在该方法中 , 调用 MessageQueue queue = mQueue , 获取 消息队列 MessageQueue ,
然后调用 enqueueMessage(queue, msg, uptimeMillis) 方法 , 将消息加入到 消息队列 MessageQueue 中 ;
public class Handler { /** * Enqueue a message into the message queue after all pending messages * before the absolute time (in milliseconds) <var>uptimeMillis</var>. * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b> * Time spent in deep sleep will add an additional delay to execution. * You will receive it in {@link #handleMessage}, in the thread attached * to this handler. * * @param uptimeMillis The absolute time at which the message should be * delivered, using the * {@link android.os.SystemClock#uptimeMillis} time-base. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. Note that a * result of true does not mean the message will be processed -- if * the looper is quit before the delivery time of the message * occurs then the message will be dropped. */ public boolean sendMessageAtTime(@NonNull 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); } }
三、MessageQueue 消息队列相关函数
下面的代码是将消息存储到消息队列中的 enqueueMessage 方法 ;
public final class MessageQueue { 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; // 如果当前消息为空 , 时间小于当前该消息的发送时间 , 需要马上将该消息发送出去 // 将表头设置成发送进来的消息 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 { // 如果该消息不急着发送 , 那么将该消息放在消息队列列表尾部 // 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; } }
从链表中取出数据 , 调用的是 消息队列 MessageQueue 的 next 方法 , 获取消息时 , 需要获取当前的时间 , 用于判定是否有需要延迟发送的消息 ;
public final class MessageQueue { @UnsupportedAppUsage 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) { // 获取当前的时间 , 需要判定是否有需要延迟发送的消息 // 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; } // 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(TAG, "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; } } }
