android multimedia的代码边读边记
system/core/include/system/window.h中的ANativeWindow其实是一个底层的API接口定义，要求实现者去具体实现其中定义的接口，比如dequeueBuffer，queueBuffer，cancelBuffer，query，perform。而在目前的实现中，Surface就是这么一个具体去实现这些接口的类，定义在frameworks/native/libs/gui/Surface.cpp这个文件中。构造函数里第一步就是定义这些接口：
42 Surface::Surface(
43 const sp
44 bool controlledByApp)
45 : mGraphicBufferProducer(bufferProducer),
46 mCrop(Rect::EMPTY_RECT),
47 mGenerationNumber(0),
48 mSharedBufferMode(false),
49 mAutoRefresh(false),
50 mSharedBufferSlot(BufferItem::INVALID_BUFFER_SLOT),
51 mSharedBufferHasBeenQueued(false)
52 {
53 // Initialize the ANativeWindow function pointers.
54 ANativeWindow::setSwapInterval = hook_setSwapInterval;
55 ANativeWindow::dequeueBuffer = hook_dequeueBuffer;
56 ANativeWindow::cancelBuffer = hook_cancelBuffer;
57 ANativeWindow::queueBuffer = hook_queueBuffer;
58 ANativeWindow::query = hook_query;
59 ANativeWindow::perform = hook_perform;
......
然而很多实现最终是交给了它的成员变量去完成具体工作，比如：
mGraphicBufferProducer->queueBuffer;
mGraphicBufferProducer的定义如下：
sp
这个又是个binder的东东，它的native侧的实现是：
frameworks/native/libs/gui/BufferQueueProducer.cpp
如果是研究video播放，到这里可以停止了，这里已经是graphics里面的producer的概念了，video只要从这里拿buffer然后把解码完的数据放进去然后再queueBuffer就行了。
关于OMX这边的一些关键代码点：
frameworks/av/media/libstagefright/omx/OMXNodeInstance.cpp文件中
171 OMX_CALLBACKTYPE OMXNodeInstance::kCallbacks = {
172 &OnEvent, &OnEmptyBufferDone, &OnFillBufferDone
173 };
这是对于三个回调函数的定义，三个函数的注册在：
frameworks/av/media/libstagefright/omx/OMX.cpp文件中
261 OMX_ERRORTYPE err = mMaster->makeComponentInstance(
262 name, &OMXNodeInstance::kCallbacks,
263 instance, &handle);
看来ACodec那边收到的消息是从这里发出去的：
690 OMX_ERRORTYPE OMX::OnEmptyBufferDone(
691 node_id node, buffer_id buffer, OMX_IN OMX_BUFFERHEADERTYPE *pBuffer, int fenceFd) {
692 ALOGV("OnEmptyBufferDone buffer=%p", pBuffer);
693
694 omx_message msg;
695 msg.type = omx_message::EMPTY_BUFFER_DONE;
696 msg.node = node;
697 msg.fenceFd = fenceFd;
698 msg.u.buffer_data.buffer = buffer;
699
700 findDispatcher(node)->post(msg);
701
702 return OMX_ErrorNone;
703 }
接下来就是同一个文件中的另一个类了：
OMX::CallbackDispatcher
看看调用顺序：
OMX创建了OMXMaster：
169 OMX::OMX()
170 : mMaster(new OMXMaster),
171 mNodeCounter(0) {
172 }
OMX::allocateNode创建OMXNodeInstance并调用OMXMaster::makeComponentInstance：
243 status_t OMX::allocateNode(
244 const char *name, const sp
245 sp
246 Mutex::Autolock autoLock(mLock);
247
248 *node = 0;
249 if (nodeBinder != NULL) {
250 *nodeBinder = NULL;
251 }
252
253 if (mNodeIDToInstance.size() == kMaxNodeInstances) {
254 // all possible node IDs are in use
255 return NO_MEMORY;
256 }
257
258 OMXNodeInstance *instance = new OMXNodeInstance(this, observer, name);
259
260 OMX_COMPONENTTYPE *handle;
261 OMX_ERRORTYPE err = mMaster->makeComponentInstance(
262 name, &OMXNodeInstance::kCallbacks,
263 instance, &handle);
264
265 if (err != OMX_ErrorNone) {
266 ALOGE("FAILED to allocate omx component '%s' err=%s(%#x)", name, asString(err), err);
267
268 instance->onGetHandleFailed();
269
270 return StatusFromOMXError(err);
271 }
272
273 *node = makeNodeID_l(instance);
274 mDispatchers.add(*node, new CallbackDispatcher(instance));
275
276 instance->setHandle(*node, handle);
277
278 mLiveNodes.add(IInterface::asBinder(observer), instance);
279 IInterface::asBinder(observer)->linkToDeath(this);
280
281 return OK;
282 }
OMXMaster::makeComponentInstance调用plugin的makeComponentInstance并把callbacks传进去，这个plugin就是各芯片厂商自己的实现，是在OMXMaster的构造函数里调用addVendorPlugin->addPlugin(其实就是dlopen了libstagefrighthw.so)加载进来的：
145 OMX_ERRORTYPE OMXMaster::makeComponentInstance(
146 const char *name,
147 const OMX_CALLBACKTYPE *callbacks,
148 OMX_PTR appData,
149 OMX_COMPONENTTYPE **component) {
150 ALOGI("makeComponentInstance(%s) in %s process", name, mProcessName);
151 Mutex::Autolock autoLock(mLock);
152
153 *component = NULL;
154
155 ssize_t index = mPluginByComponentName.indexOfKey(String8(name));
156
157 if (index < 0) {
158 return OMX_ErrorInvalidComponentName;
159 }
160
161 OMXPluginBase *plugin = mPluginByComponentName.valueAt(index);
162 OMX_ERRORTYPE err =
163 plugin->makeComponentInstance(name, callbacks, appData, component);
接下来的代码就不方便帖出来了，大概就是会在某个时候调用传进去的三个callback，一路就会调到OMXNodeInstance的三个函数，比如OMXNodeInstance::OnEmptyBufferDone，进而通过下面这句调到OMX::OnEmptyBufferDone
1720 return instance->owner()->OnEmptyBufferDone(instance->nodeID(),
1721 instance->findBufferID(pBuffer), pBuffer, fenceFd);
在这里就和前面说的连接上了，OMX::OnEmptyBufferDone会去封个消息然后post出去，然后OMX::CallbackDispatcher会在loop中把消息从队列中取出来然后dispatch出去：
130 void OMX::CallbackDispatcher::dispatch(std::list
131 if (mOwner == NULL) {
132 ALOGV("Would have dispatched a message to a node that's already gone.");
133 return;
134 }
135 mOwner->onMessages(messages);
136 }
这个mOwner就是OMXNodeInstance，于是调到：
1610 void OMXNodeInstance::onMessages(std::list
1611 for (std::list
1612 if (handleMessage(*it)) {
1613 messages.erase(it++);
1614 } else {
1615 ++it;
1616 }
1617 }
1618
1619 if (!messages.empty()) {
1620 mObserver->onMessages(messages);
1621 }
1622 }
这个mObserver就回到ACodec了：
在ACodec的onAllocateComponent函数中，有下面这段代码段：
6381 sp
6382 IOMX::node_id node = 0;
6383
6384 status_t err = NAME_NOT_FOUND;
6385 for (size_t matchIndex = 0; matchIndex < matchingCodecs.size();
6386 ++matchIndex) {
6387 componentName = matchingCodecs[matchIndex];
6388 quirks = MediaCodecList::getQuirksFor(componentName.c_str());
6389
6390 pid_t tid = gettid();
6391 int prevPriority = androidGetThreadPriority(tid);
6392 androidSetThreadPriority(tid, ANDROID_PRIORITY_FOREGROUND);
6393 err = omx->allocateNode(componentName.c_str(), observer, &mCodec->mNodeBinder, &node);
6394 androidSetThreadPriority(tid, prevPriority);
这里的omx就是OMX.cpp里的OMX，所以传进去的就是CodecObserver，这个CodecObserver再通过OMX的allocateNode调用new OMXNodeInstance(this, observer, name)的时候传给了OMXNodeInstance。所以前面的onMessages最后是被CodecObserver处理了：