After

I will firstly present the modified animator logic here. In the next sections, I will then discuss why these are necessary and what problems it solves.

Code

The changes are marked as // CHANGE: ... below.

Animator::Animator()
    : layer_tree_pipeline_(std::make_shared<LayerTreePipeline>(2)) {}

void Animator::BeginFrame() {
  if (!producer_continuation_) {
    producer_continuation_ = layer_tree_pipeline_->Produce();

    if (!producer_continuation_) {
      TRACE_EVENT0("flutter", "PipelineFull");
      // CHANGE: do not return even if pipeline full
      // RequestFrame(); return;
    }
  }
  
  // CHANGE: remove this (since we do not early return if pipeline full)
  // FML_DCHECK(producer_continuation_);

  delegate_.OnAnimatorBeginFrame();
}

void Animator::Render(std::shared_ptr<flutter::LayerTree> layer_tree) {
  // CHANGE: add these several lines (if no continuation, trigger creating one if possible)
  if (!producer_continuation_) {
    producer_continuation_ = layer_tree_pipeline_->Produce();
  }
  
  // Commit the pending continuation.
  PipelineProduceResult result = producer_continuation_.Complete(layer_tree);

  if (!result.success) {
    FML_DLOG(INFO) << "No pending continuation to commit";
    return;
  }

  ... notify rasterizer ...
}

Summary

To summarize, there are two changes:

1. When Render, originally we early-return if there is no occupied seat (the continuation). However, we now produce one. (Notice the Produce may fail if there is really no room in pipeline.)
2. When BeginFrame, originally we early-return as long as the pipeline is full at that time. However, we are more optimistic and continue computing a frame even if it is full now.