void BufferStateLayer::onLayerDisplayed(const sp<Fence>& releaseFence) {
    if (!releaseFence->isValid()) {
        return;
    }
    // The previous release fence notifies the client that SurfaceFlinger is done with the previous
    // buffer that was presented on this layer. The first transaction that came in this frame that
    // replaced the previous buffer on this layer needs this release fence, because the fence will
    // let the client know when that previous buffer is removed from the screen.
    //
    // Every other transaction on this layer does not need a release fence because no other
    // Transactions that were set on this layer this frame are going to have their preceeding buffer
    // removed from the display this frame.
    //
    // For example, if we have 3 transactions this frame. The first transaction doesn't contain a
    // buffer so it doesn't need a previous release fence because the layer still needs the previous
    // buffer. The second transaction contains a buffer so it needs a previous release fence because
    // the previous buffer will be released this frame. The third transaction also contains a
    // buffer. It replaces the buffer in the second transaction. The buffer in the second
    // transaction will now no longer be presented so it is released immediately and the third
    // transaction doesn't need a previous release fence.
    sp<CallbackHandle> ch;
    for (auto& handle : mDrawingState.callbackHandles) {
        if (handle->releasePreviousBuffer &&
            mDrawingState.releaseBufferEndpoint == handle->listener) {
            ch = handle;
            break;
        }
    }
    auto status = addReleaseFence(ch, releaseFence);
    if (status != OK) {
        ALOGE("Failed to add release fence for layer %s", getName().c_str());
    }

    mPreviousReleaseFence = releaseFence;

    // Prevent tracing the same release multiple times.
    if (mPreviousFrameNumber != mPreviousReleasedFrameNumber) {
        mPreviousReleasedFrameNumber = mPreviousFrameNumber;
    }
}

• 如果 releaseFence 无效，返回。
• mPreviousReleaseFence 用于通知 client，上一个 buffer 已经显示过了。
• 注释说明，只有当前帧的第一个要替换上一个已经显示的 buffer 的事务才需要 release fence，其他的事务不需要。
• 遍历 callbackHandles，查找 releasePreviousBuffer 是 true，且 releaseBufferEndPoint 和 listener 相同的 CallbackHandle。
  • releasePreviousBuffer 是在 BufferStateLayer::setBuffer() 中，当 mDrawingState 的 buffer 存在时设置 true 的。这也符合注释的说明。
  • releaseBufferEndpoint 也是在 BufferStateLayer::setBuffer() 中设置的。
• 调用 BufferStateLayer::addReleaseFence() 设置 previousReleaseFence。
• 同时设置 mPreviousReleaseFence，它是当前正在显示的 buffer 的 fence。
• 更新 mPreviousReleasedFrameNumber。