1. 创建AudioTrack对象

在创建AudioTrack对象时，会调用到set()函数：

在set()方法中，调用了createTrack_l, 创建IAudioTrack

在createTrack_l（）中，获取到AudioFlinger服务，然后通过AudioFlinger来创建对应的track

接下来会调用到本地服务层AudioFlinger中，相对比较复杂，会在后续文章中说明

2. 开始播放

调用audioTrack.play();最终调用到框架层AudioTrack.cpp的start();

static void
android_media_AudioTrack_start(JNIEnv *env, jobject thiz)
{
    sp<AudioTrack> lpTrack = getAudioTrack(env, thiz);
    if (lpTrack == NULL) {
        jniThrowException(env, "java/lang/IllegalStateException",
            "Unable to retrieve AudioTrack pointer for start()");
        return;
    }

    lpTrack->start();
}

调用流程如下：

3. 写数据

audiotrack.write()最终会调用到AudioTrack::write(),把数据拷贝到共享buffer中，后续章节会讲解。

template <typename T>
static jint writeToTrack(const sp<AudioTrack>& track, jint audioFormat, const T *data,
                         jint offsetInSamples, jint sizeInSamples, bool blocking) {
    // give the data to the native AudioTrack object (the data starts at the offset)
    ssize_t written = 0;
    // regular write() or copy the data to the AudioTrack's shared memory?
    size_t sizeInBytes = sizeInSamples * sizeof(T);
    if (track->sharedBuffer() == 0) {
        written = track->write(data + offsetInSamples, sizeInBytes, blocking);
        // for compatibility with earlier behavior of write(), return 0 in this case
        if (written == (ssize_t) WOULD_BLOCK) {
            written = 0;
        }
    } else {
        // writing to shared memory, check for capacity
        if ((size_t)sizeInBytes > track->sharedBuffer()->size()) {
            sizeInBytes = track->sharedBuffer()->size();
        }
        memcpy(track->sharedBuffer()->unsecurePointer(), data + offsetInSamples, sizeInBytes);
        written = sizeInBytes;
    }
    if (written >= 0) {
        return written / sizeof(T);
    }
    return interpretWriteSizeError(written);
}

ssize_t AudioTrack::write(const void* buffer, size_t userSize, bool blocking)
{
    if (mTransfer != TRANSFER_SYNC && mTransfer != TRANSFER_SYNC_NOTIF_CALLBACK) {
        return INVALID_OPERATION;
    }

    if (isDirect()) {
        AutoMutex lock(mLock);
        int32_t flags = android_atomic_and(
                            ~(CBLK_UNDERRUN | CBLK_LOOP_CYCLE | CBLK_LOOP_FINAL | CBLK_BUFFER_END),
                            &mCblk->mFlags);
        if (flags & CBLK_INVALID) {
            return DEAD_OBJECT;
        }
    }

    if (ssize_t(userSize) < 0 || (buffer == NULL && userSize != 0)) {
        // Sanity-check: user is most-likely passing an error code, and it would
        // make the return value ambiguous (actualSize vs error).
        ALOGE("%s(%d): AudioTrack::write(buffer=%p, size=%zu (%zd)",
                __func__, mPortId, buffer, userSize, userSize);
        return BAD_VALUE;
    }

    size_t written = 0;
    Buffer audioBuffer;

    while (userSize >= mFrameSize) {
        audioBuffer.frameCount = userSize / mFrameSize;

        status_t err = obtainBuffer(&audioBuffer,
                blocking ? &ClientProxy::kForever : &ClientProxy::kNonBlocking);
        if (err < 0) {
            if (written > 0) {
                break;
            }
            if (err == TIMED_OUT || err == -EINTR) {
                err = WOULD_BLOCK;
            }
            return ssize_t(err);
        }

        size_t toWrite = audioBuffer.size;
        memcpy(audioBuffer.i8, buffer, toWrite);
        buffer = ((const char *) buffer) + toWrite;
        userSize -= toWrite;
        written += toWrite;

        releaseBuffer(&audioBuffer);
    }

    if (written > 0) {
        mFramesWritten += written / mFrameSize;

        if (mTransfer == TRANSFER_SYNC_NOTIF_CALLBACK) {
            const sp<AudioTrackThread> t = mAudioTrackThread;
            if (t != 0) {
                // causes wake up of the playback thread, that will callback the client for
                // more data (with EVENT_CAN_WRITE_MORE_DATA) in processAudioBuffer()
                t->wake();
            }
        }
    }

    return written;
}

调用流程如下：

4.停止播放

audiotrack.stop()，最终会调用到AudioTrack::stop()。

static void
android_media_AudioTrack_stop(JNIEnv *env, jobject thiz)
{
    sp<AudioTrack> lpTrack = getAudioTrack(env, thiz);
    if (lpTrack == NULL) {
        jniThrowException(env, "java/lang/IllegalStateException",
            "Unable to retrieve AudioTrack pointer for stop()");
        return;
    }

    lpTrack->stop();
}

5.释放资源

在完全停止播放后，必须调用该方法释放资源。audiotrack.release();,

define CALLBACK_COND_WAIT_TIMEOUT_MS 1000
static void android_media_AudioTrack_release(JNIEnv *env,  jobject thiz) {
    sp<AudioTrack> lpTrack = setAudioTrack(env, thiz, 0);
    if (lpTrack == NULL) {
        return;
    }
    //ALOGV("deleting lpTrack: %x\n", (int)lpTrack);

    // delete the JNI data
    AudioTrackJniStorage* pJniStorage = (AudioTrackJniStorage *)env->GetLongField(
        thiz, javaAudioTrackFields.jniData);
    // reset the native resources in the Java object so any attempt to access
    // them after a call to release fails.
    env->SetLongField(thiz, javaAudioTrackFields.jniData, 0);

    if (pJniStorage) {
        Mutex::Autolock l(sLock);
        audiotrack_callback_cookie *lpCookie = &pJniStorage->mCallbackData;
        //ALOGV("deleting pJniStorage: %x\n", (int)pJniStorage);
        while (lpCookie->busy) {
            if (lpCookie->cond.waitRelative(sLock,
                                            milliseconds(CALLBACK_COND_WAIT_TIMEOUT_MS)) !=
                                                    NO_ERROR) {
                break;
            }
        }
        sAudioTrackCallBackCookies.remove(lpCookie);
        // delete global refs created in native_setup
        env->DeleteGlobalRef(lpCookie->audioTrack_class);
        env->DeleteGlobalRef(lpCookie->audioTrack_ref);
        delete pJniStorage;
    }
}