Android6.0 显示系统（二） SurfaceFlinger创建Surface

接上篇博客分析到SurfaceComposerClient中调用createSurface函数会最后调用到SurfaceFlinger的Client的createSurface函数。

我们来看下这个函数，其handle和gbp都是Binder对象，gbp就是图像缓冲区对象。这个handle对象的作用和前面介绍Activity中的token作用类似，也是利用Binder的唯一性为Surface生成一个唯一标识。

status_t Client::createSurface(
        const String8& name,
        uint32_t w, uint32_t h, PixelFormat format, uint32_t flags,
        sp* handle,
        sp* gbp)
{
    /*
     * createSurface must be called from the GL thread so that it can
     * have access to the GL context.
     */

    class MessageCreateLayer : public MessageBase {
        SurfaceFlinger* flinger;
        Client* client;
        sp* handle;
        sp* gbp;
        status_t result;
        const String8& name;
        uint32_t w, h;
        PixelFormat format;
        uint32_t flags;
    public:
        MessageCreateLayer(SurfaceFlinger* flinger,
                const String8& name, Client* client,
                uint32_t w, uint32_t h, PixelFormat format, uint32_t flags,
                sp* handle,
                sp* gbp)
            : flinger(flinger), client(client),
              handle(handle), gbp(gbp),
              name(name), w(w), h(h), format(format), flags(flags) {
        }
        status_t getResult() const { return result; }
        virtual bool handler() {
            result = flinger->createLayer(name, client, w, h, format, flags,
                    handle, gbp);
            return true;
        }
    };

    sp msg = new MessageCreateLayer(mFlinger.get(),
            name, this, w, h, format, flags, handle, gbp);
    mFlinger->postMessageSync(msg);
    return static_cast( msg.get() )->getResult();
}

这个函数中定义了一个消息类MessageCreateLayer，然后把它的对象通过postMessageSync方法发送出去，这个消息是以同步的方式发送，因此函数结束后可以直接返回结果。因此就直接到了handler函数，在这个函数中调用了SurfaceFlinger的createLayer函数。

一、创建Layer

这个函数会根据传进来的flags来创建普通的的Layer还是模糊的Layer。

status_t SurfaceFlinger::createLayer(
        const String8& name,
        const sp& client,
        uint32_t w, uint32_t h, PixelFormat format, uint32_t flags,
        sp* handle, sp* gbp)
{
    if (int32_t(w|h) < 0) {
        ALOGE("createLayer() failed, w or h is negative (w=%d, h=%d)",
                int(w), int(h));
        return BAD_VALUE;
    }

    status_t result = NO_ERROR;

    sp layer;

    switch (flags & ISurfaceComposerClient::eFXSurfaceMask) {
        case ISurfaceComposerClient::eFXSurfaceNormal:
            result = createNormalLayer(client,//普通的
                    name, w, h, flags, format,
                    handle, gbp, &layer);
            break;
        case ISurfaceComposerClient::eFXSurfaceDim://模糊的
            result = createDimLayer(client,
                    name, w, h, flags,
                    handle, gbp, &layer);
            break;
        default:
            result = BAD_VALUE;
            break;
    }

    if (result != NO_ERROR) {
        return result;
    }

    result = addClientLayer(client, *handle, *gbp, layer);
    if (result != NO_ERROR) {
        return result;
    }

    setTransactionFlags(eTransactionNeeded);
    return result;
}

我们先来看看创建普通的Layer，根据传入的format参数，选择不同格式。然后创建一个Layer，具体Layer我们后续介绍。然后获取handle和gbp。

status_t SurfaceFlinger::createNormalLayer(const sp& client,
        const String8& name, uint32_t w, uint32_t h, uint32_t flags, PixelFormat& format,
        sp* handle, sp* gbp, sp* outLayer)
{
    // initialize the surfaces
    switch (format) {
    case PIXEL_FORMAT_TRANSPARENT:
    case PIXEL_FORMAT_TRANSLUCENT:
        format = PIXEL_FORMAT_RGBA_8888;//透明
        break;
    case PIXEL_FORMAT_OPAQUE:
        format = PIXEL_FORMAT_RGBX_8888;//不透明
        break;
    }

    *outLayer = new Layer(this, client, name, w, h, flags);//创建一个Layer
    status_t err = (*outLayer)->setBuffers(w, h, format, flags);
    if (err == NO_ERROR) {
        *handle = (*outLayer)->getHandle();
        *gbp = (*outLayer)->getProducer();
    }

    ALOGE_IF(err, "createNormalLayer() failed (%s)", strerror(-err));
    return err;
}

下面我们再来看看模糊的Layer，这个比较简单，直接新建一个layerDim,然后获取handle和gbp。

status_t SurfaceFlinger::createDimLayer(const sp& client,
        const String8& name, uint32_t w, uint32_t h, uint32_t flags,
        sp* handle, sp* gbp, sp* outLayer)
{
    *outLayer = new LayerDim(this, client, name, w, h, flags);
    *handle = (*outLayer)->getHandle();
    *gbp = (*outLayer)->getProducer();
    return NO_ERROR;
}

二、获取handle

下面我们再来看handle的获取，只是新建一个Handle，而这个Handle只是一个Binder的实现，就是标识Surface的全局唯一性，没有什么实际的内容。

sp Layer::getHandle() {
    Mutex::Autolock _l(mLock);

    LOG_ALWAYS_FATAL_IF(mHasSurface,
            "Layer::getHandle() has already been called");

    mHasSurface = true;

    /*
     * The layer handle is just a BBinder object passed to the client
     * (remote process) -- we don't keep any reference on our side such that
     * the dtor is called when the remote side let go of its reference.
     *
     * LayerCleaner ensures that mFlinger->onLayerDestroyed() is called for
     * this layer when the handle is destroyed.
     */

    class Handle : public BBinder, public LayerCleaner {
        wp mOwner;
    public:
        Handle(const sp& flinger, const sp& layer)
            : LayerCleaner(flinger, layer), mOwner(layer) {
        }
    };

    return new Handle(mFlinger, this);
}

三、gbp图像缓冲区

那么我们再来看看gbp的获取,就是Layer中mProducer成员变量。

sp Layer::getProducer() const {
    return mProducer;
}

而mProducer是在Layer的onFirstRef中赋值的，我们注意MonitoredProducer构造函数的一个参数producer，事实上MonitoredProducer只是一个代理类，真正的实现在这个producer参数。它是在BufferQueue::createBufferQueue中创造的。

void Layer::onFirstRef() {
    // Creates a custom BufferQueue for SurfaceFlingerConsumer to use
    sp producer;
    sp consumer;
    BufferQueue::createBufferQueue(&producer, &consumer);
    mProducer = new MonitoredProducer(producer, mFlinger);
    ......

这个producer就是BufferQueueProducer对象。

void BufferQueue::createBufferQueue(sp* outProducer,
        sp* outConsumer,
        const sp& allocator) {

    sp core(new BufferQueueCore(allocator));
    sp producer(new BufferQueueProducer(core));
    sp consumer(new BufferQueueConsumer(core));

    *outProducer = producer;
    *outConsumer = consumer;
}

下篇博客我们就分析这个。