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How to trigger

By normal function call. The psuedo simplified core code looks like:

// you know, the workhorse in layout phase
void performLayout() {
  if (shouldTriggerPreempt()) preemptRender();
  ...original code...
}

bool shouldTriggerPreempt() => /* will discuss in next sections */
void preemptRender() => /* will discuss in next sections */

Implementation details​

As for implementation details, I create PreemptPoint widgets which does this "should trigger preempt" check at build and layout time. An alternative solution, if Flutter merges my PR, is to inject this if-clause into RenderObject.performLayout, so users do not need to manually do anything.

A small comparsion​

If you are familiar with the prior approaches (discussed in literature review), this may need a bit of time to grasp. In prior approaches, early return is utilized, such as the following pseudo code:

void performLayout() {
  if (nearDeadline()) return;
  somehow_continue_from_last_early_return;
  ...original code...
}

In those approaches, the main thread finally has zero call stack depth. However, to the contrary, this approach only calls a normal function.