thread_loop与run方法

以一个例子分析。

    class DoCaptureThread : public Thread {
        V4L2CameraDevice*    mV4l2CameraDevice;
        bool                mRequestExit;
    public:
        DoCaptureThread(V4L2CameraDevice* dev) :
            Thread(false),
            mV4l2CameraDevice(dev),
            mRequestExit(false) {
        }
        void startThread() {
            run("CameraCaptureThread", PRIORITY_URGENT_DISPLAY);
        }
        void stopThread() {
            mRequestExit = true;
        }
        virtual bool threadLoop() {
            if (mRequestExit) {
                return false;
            }
            return mV4l2CameraDevice->captureThread();
        }
    };

这个用法其实挺简单的，就是创建一个captureThread。

封装run函数，也就是thread name以及priority。

重写threadLoop，添加了一个退出变量。不懂为啥不用requestExit函数。。。

那么使用的时候，

1，先创建

    mCaptureThread = new DoCaptureThread(this);

2，mCaptureThread->startThread(); //开启线程 run函数会调用，_threadloop，_threadloop会循环调用threadLoop.

3，mCaptureThread->stopThread();// 让threadLoop返回false，退出线程循环。

看一下Thread基类的定义。
http://androidxref.com/6.0.0_r1/xref/system/core/include/utils/Thread.h
http://androidxref.com/6.0.0_r1/xref/system/core/libutils/Threads.cpp

class Thread : virtual public RefBase
{
public:
    // Create a Thread object, but doesn't create or start the associated
    // thread. See the run() method.
                        Thread(bool canCallJava = true);
    virtual             ~Thread();

    // Start the thread in threadLoop() which needs to be implemented.
    virtual status_t    run(    const char* name = 0,
                                int32_t priority = PRIORITY_DEFAULT,
                                size_t stack = 0);

    // Ask this object's thread to exit. This function is asynchronous, when the
    // function returns the thread might still be running. Of course, this
    // function can be called from a different thread.
    virtual void        requestExit();

    // Good place to do one-time initializations
    virtual status_t    readyToRun();

    // Call requestExit() and wait until this object's thread exits.
    // BE VERY CAREFUL of deadlocks. In particular, it would be silly to call
    // this function from this object's thread. Will return WOULD_BLOCK in
    // that case.
            status_t    requestExitAndWait();

    // Wait until this object's thread exits. Returns immediately if not yet runnin
    // Do not call from this object's thread; will return WOULD_BLOCK in that case.
            status_t    join();

    // Indicates whether this thread is running or not.
            bool        isRunning() const;

#ifdef HAVE_ANDROID_OS
    // Return the thread's kernel ID, same as the thread itself calling gettid(),
    // or -1 if the thread is not running.
            pid_t       getTid() const;
#endif

protected:
    // exitPending() returns true if requestExit() has been called.
            bool        exitPending() const;

private:
    // Derived class must implement threadLoop(). The thread starts its life
    // here. There are two ways of using the Thread object:
    // 1) loop: if threadLoop() returns true, it will be called again if
    //          requestExit() wasn't called.
    // 2) once: if threadLoop() returns false, the thread will exit upon return.
    virtual bool        threadLoop() = 0;

private:
    Thread& operator=(const Thread&);
    static  int             _threadLoop(void* user);
    const   bool            mCanCallJava;
    // always hold mLock when reading or writing
            thread_id_t     mThread;
    mutable Mutex           mLock;
            Condition       mThreadExitedCondition;
            status_t        mStatus;
    // note that all accesses of mExitPending and mRunning need to hold mLock
    volatile bool           mExitPending;
   volatile bool           mRunning;
            sp      mHoldSelf;
#ifdef HAVE_ANDROID_OS
    // legacy for debugging, not used by getTid() as it is set by the child thread
    // and so is not initialized until the child reaches that point
            pid_t           mTid;
#endif
};

/*
 * This is our thread object!
 */

Thread::Thread(bool canCallJava)
    :   mCanCallJava(canCallJava),
        mThread(thread_id_t(-1)),
        mLock("Thread::mLock"),
        mStatus(NO_ERROR),
        mExitPending(false), mRunning(false)
#ifdef HAVE_ANDROID_OS
        , mTid(-1)
#endif
{
}

Thread::~Thread()
{
}

status_t Thread::readyToRun()
{
    return NO_ERROR;
}

status_t Thread::run(const char* name, int32_t priority, size_t stack)
{
    Mutex::Autolock _l(mLock);

    if (mRunning) {
        // thread already started
        return INVALID_OPERATION;
    }

    // reset status and exitPending to their default value, so we can
    // try again after an error happened (either below, or in readyToRun())
    mStatus = NO_ERROR;
    mExitPending = false;
    mThread = thread_id_t(-1);

    // hold a strong reference on ourself
    mHoldSelf = this;

    mRunning = true;

    bool res;
    if (mCanCallJava) {
        res = createThreadEtc(_threadLoop,
                this, name, priority, stack, &mThread);
    } else {
        res = androidCreateRawThreadEtc(_threadLoop,
                this, name, priority, stack, &mThread);
    }

    if (res == false) {
        mStatus = UNKNOWN_ERROR;   // something happened!
        mRunning = false;
        mThread = thread_id_t(-1);
        mHoldSelf.clear();  // "this" may have gone away after this.

        return UNKNOWN_ERROR;
    }

    // Do not refer to mStatus here: The thread is already running (may, in fact
    // already have exited with a valid mStatus result). The NO_ERROR indication
    // here merely indicates successfully starting the thread and does not
    // imply successful termination/execution.
    return NO_ERROR;

    // Exiting scope of mLock is a memory barrier and allows new thread to run
}

int Thread::_threadLoop(void* user)
{
    Thread* const self = static_cast(user);

    sp strong(self->mHoldSelf);
    wp weak(strong);
    self->mHoldSelf.clear();

#ifdef HAVE_ANDROID_OS
    // this is very useful for debugging with gdb
    self->mTid = gettid();
#endif

    bool first = true;

    do {
        bool result;
        if (first) {
            first = false;
            self->mStatus = self->readyToRun();
            result = (self->mStatus == NO_ERROR);

            if (result && !self->exitPending()) {
                // Binder threads (and maybe others) rely on threadLoop
                // running at least once after a successful ::readyToRun()
                // (unless, of course, the thread has already been asked to exit
                // at that point).
                // This is because threads are essentially used like this:
                //   (new ThreadSubclass())->run();
                // The caller therefore does not retain a strong reference to
                // the thread and the thread would simply disappear after the
                // successful ::readyToRun() call instead of entering the
                // threadLoop at least once.
                result = self->threadLoop();
            }
        } else {
            result = self->threadLoop();
        }

        // establish a scope for mLock
        {
        Mutex::Autolock _l(self->mLock);
        if (result == false || self->mExitPending) {
            self->mExitPending = true;
            self->mRunning = false;
            // clear thread ID so that requestExitAndWait() does not exit if
            // called by a new thread using the same thread ID as this one.
            self->mThread = thread_id_t(-1);
            // note that interested observers blocked in requestExitAndWait are
            // awoken by broadcast, but blocked on mLock until break exits scope
            self->mThreadExitedCondition.broadcast();
            break;
        }
        }

        // Release our strong reference, to let a chance to the thread
        // to die a peaceful death.
        strong.clear();
        // And immediately, re-acquire a strong reference for the next loop
        strong = weak.promote();
    } while(strong != 0);

    return 0;
}

void Thread::requestExit()
{
    Mutex::Autolock _l(mLock);
    mExitPending = true;
}

status_t Thread::requestExitAndWait()
{
    Mutex::Autolock _l(mLock);
    if (mThread == getThreadId()) {
        ALOGW(
        "Thread (this=%p): don't call waitForExit() from this "
        "Thread object's thread. It's a guaranteed deadlock!",
        this);

        return WOULD_BLOCK;
    }

    mExitPending = true;

    while (mRunning == true) {
        mThreadExitedCondition.wait(mLock);
    }
    // This next line is probably not needed any more, but is being left for
    // historical reference. Note that each interested party will clear flag.
    mExitPending = false;

    return mStatus;
}

status_t Thread::join()
{
    Mutex::Autolock _l(mLock);
    if (mThread == getThreadId()) {
        ALOGW(
        "Thread (this=%p): don't call join() from this "
        "Thread object's thread. It's a guaranteed deadlock!",
        this);

        return WOULD_BLOCK;
    }

    while (mRunning == true) {
        mThreadExitedCondition.wait(mLock);
    }

    return mStatus;
}

bool Thread::isRunning() const {
    Mutex::Autolock _l(mLock);
    return mRunning;
}

#ifdef HAVE_ANDROID_OS
pid_t Thread::getTid() const
{
    // mTid is not defined until the child initializes it, and the caller may need it earlier
    Mutex::Autolock _l(mLock);
    pid_t tid;
    if (mRunning) {
        pthread_t pthread = android_thread_id_t_to_pthread(mThread);
        tid = pthread_gettid_np(pthread);
    } else {
        ALOGW("Thread (this=%p): getTid() is undefined before run()", this);
        tid = -1;
    }
    return tid;
}
#endif

bool Thread::exitPending() const
{
    Mutex::Autolock _l(mLock);
    return mExitPending;
}