bool BufferStateLayer::setBuffer(const std::shared_ptr<renderengine::ExternalTexture>& buffer,
                                 const sp<Fence>& acquireFence, nsecs_t postTime,
                                 nsecs_t desiredPresentTime, bool isAutoTimestamp,
                                 const client_cache_t& clientCacheId, uint64_t frameNumber,
                                 std::optional<nsecs_t> dequeueTime, const FrameTimelineInfo& info,
                                 const sp<ITransactionCompletedListener>& releaseBufferListener,
                                 const sp<IBinder>& releaseBufferEndpoint) {
    if (mDrawingState.buffer) {
        mReleasePreviousBuffer = true;
        if (mDrawingState.buffer != mBufferInfo.mBuffer ||
            mDrawingState.frameNumber != mBufferInfo.mFrameNumber) {
            // If mDrawingState has a buffer, and we are about to update again
            // before swapping to drawing state, then the first buffer will be
            // dropped and we should decrement the pending buffer count and
            // call any release buffer callbacks if set.
            callReleaseBufferCallback(mDrawingState.releaseBufferListener,
                                      mDrawingState.buffer->getBuffer(), mDrawingState.frameNumber,
                                      mDrawingState.acquireFence, mTransformHint,
                                      mFlinger->getMaxAcquiredBufferCountForCurrentRefreshRate(
                                              mOwnerUid));
            decrementPendingBufferCount();
            if (mDrawingState.bufferSurfaceFrameTX != nullptr &&
                mDrawingState.bufferSurfaceFrameTX->getPresentState() != PresentState::Presented) {
              addSurfaceFrameDroppedForBuffer(mDrawingState.bufferSurfaceFrameTX);
              mDrawingState.bufferSurfaceFrameTX.reset();
            }
        }
    }

• 如果当前 mDrawingState.buffer 不空，执行下面操作。
• mReleasePreviousBuffer 设置为 true，在 BufferStateLayer::setTransactionCompletedListeners() 中会把该值设置给 CallbackHandle 的成员 releasePreviousBuffer。
• 如果 mDrawingState.buffer 和 mBufferInfo.mBuffer 不相同，或 frameNumber 不相同，说明当前 mBufferInfo 还未显示 mDrawingState.buffer 已经更新了，所以要调用 callReleaseBufferCallback() 通知 client，释放了 mDrawingState 中的 buffer。正常情况下不会执行该分支。

    mDrawingState.frameNumber = frameNumber;
    mDrawingState.releaseBufferListener = releaseBufferListener;
    mDrawingState.buffer = buffer;
    mDrawingState.clientCacheId = clientCacheId;
    mDrawingState.modified = true;
    setTransactionFlags(eTransactionNeeded);

• 设置 mDrawingState 中与 buffer 相关的成员。
• 调用 SurfaceFlinger::setTransactionFlags()。

    ...
    mDrawingState.width = mDrawingState.buffer->getBuffer()->getWidth();
    mDrawingState.height = mDrawingState.buffer->getBuffer()->getHeight();
    mDrawingState.releaseBufferEndpoint = releaseBufferEndpoint;

    return true;
}

• 略过了 frame event 和 frame trace 相关的代码。
• 设置 width 和 height。
• 设置 releaseBufferEndpoint。
• 其实 releaseBufferEndpoint 和 releaseBufferListener 是同一个对象 binder，只是类型不同，前者是 sp<IBinder>，后者是 sp<ITransactionCompletedListener>。
• 返回 true，该函数只返回 true。