void SurfaceFlinger::commitTransactionLocked() {
    if (!mLayersPendingRemoval.isEmpty()) {
        // Notify removed layers now that they can't be drawn from
        for (const auto& l : mLayersPendingRemoval) {
            recordBufferingStats(l->getName(), l->getOccupancyHistory(true));

            // Ensure any buffers set to display on any children are released.
            if (l->isRemovedFromCurrentState()) {
                l->latchAndReleaseBuffer();
            }

            // If the layer has been removed and has no parent, then it will not be reachable
            // when traversing layers on screen. Add the layer to the offscreenLayers set to
            // ensure we can copy its current to drawing state.
            if (!l->getParent()) {
                mOffscreenLayers.emplace(l.get());
            }
        }
        mLayersPendingRemoval.clear();
    }

• 遍历 mLayersPendingRemoval。
  • 如果该 layer 的 isRemovedFromCurrentState() 返回 true，调用 BufferLayer::latchAndReleaseBuffer() 获取 buffer 并释放。
  • 如果该 layer 不存在父，则加入到 mOffscreenLayers。
• 清空 mLayersPendingRemoval。

void SurfaceFlinger::commitTransactionLocked() {
    ...
    // If this transaction is part of a window animation then the next frame
    // we composite should be considered an animation as well.
    mAnimCompositionPending = mAnimTransactionPending;

    mDrawingState = mCurrentState;
    // clear the "changed" flags in current state
    mCurrentState.colorMatrixChanged = false;

    if (mVisibleRegionsDirty) {
        for (const auto& rootLayer : mDrawingState.layersSortedByZ) {
            rootLayer->commitChildList();
        }
    }

    commitOffscreenLayers();
    if (mNumClones > 0) {
        mDrawingState.traverse([&](Layer* layer) { layer->updateMirrorInfo(); });
    }
}

• mAnimTransactionPending 是在 SurfaceFlinger::applyTransactionState() 中设置的，根据 eAnimation flag。如果是动画事务，设置 mAnimCompositionPending 为 true，在 SurfaceFlinger::handleTransaction() 中会动画帧跟踪。
• 拷贝 mCurrentState 到 mDrawingState。
• 如果有可见脏区域，遍历 mDrawingState 中的 layer，调用 Layer::commitChildList()，它会遍历所有 mCurrentChildren，拷贝 mCurrentChildren 到 mDrawingChildren，拷贝 mCurrentParent 到 mDrawingParent。
• 调用 commitOffscreenLayers()，它遍历 mOffscreenLayers 先调用 doTransaction() 再调用 Layer::commitChildList()。