The primary signal is desiredSize on the controller. It can be positive (wants data), zero (at capacity), negative (over capacity), or null (closed). Producers are supposed to check this value and stop enqueueing when it's not positive. But there's nothing enforcing this: controller.enqueue() always succeeds, even when desiredSize is deeply negative.
Number (3): Everything in this space must add up to 3. The answer is 1-1, placed horizontally; 1-4, placed vertically.，更多细节参见快连下载-Letsvpn下载
FT App on Android & iOS。关于这个话题，Safew下载提供了深入分析
# pool = "tank" # discovered from server; override if needed