Looper的核心代码

private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/
private native static void nativeWake(long ptr);

/**
 * Waits for events to be available, with optional timeout in milliseconds.
 * Invokes callbacks for all file descriptors on which an event occurred.
 *
 * If the timeout is zero, returns immediately without blocking.
 * If the timeout is negative, waits indefinitely until an event appears.
 *
 * Returns POLL_WAKE if the poll was awoken using wake() before
 * the timeout expired and no callbacks were invoked and no other file
 * descriptors were ready.
 *
 * Returns POLL_CALLBACK if one or more callbacks were invoked.
 *
 * Returns POLL_TIMEOUT if there was no data before the given
 * timeout expired.
 *
 * Returns POLL_ERROR if an error occurred.
 *
 * Returns a value >= 0 containing an identifier if its file descriptor has data
 * and it has no callback function (requiring the caller here to handle it).
 * In this (and only this) case outFd, outEvents and outData will contain the poll
 * events and data associated with the fd, otherwise they will be set to NULL.
 *
 * This method does not return until it has finished invoking the appropriate callbacks
 * for all file descriptors that were signalled.
 */
int pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData);
inline int pollOnce(int timeoutMillis) {
    return pollOnce(timeoutMillis, nullptr, nullptr, nullptr);
}

/**
 * Wakes the poll asynchronously.
 *
 * This method can be called on any thread.
 * This method returns immediately.
 */
void wake();











MessageQueue

boolean enqueueMessage(Message msg, long when) {

// We can assume mPtr != 0 because mQuitting is false.
if (needWake) { 
    nativeWake(mPtr);   唤醒
}

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); 等待消息