Chapter 36 Key Takeaways

TThread Essentials

• Subclass TThread, override Execute
• Create(True) = suspended; call Start to begin
• Terminate sets Terminated := True — a polite request, not a kill
• Check Terminated periodically in your Execute loop
• WaitFor blocks until the thread finishes
• FreeOnTerminate := True = auto-freed (do not access the object after Start)

The Golden Rule

If two threads can access the same data, and at least one writes, you MUST synchronize access. No exceptions.

GUI Updates

• Never access GUI controls from a worker thread
• Synchronize(@Method) — blocks worker until main thread executes
• Queue(@Method) — non-blocking, main thread executes later (preferred)

TCriticalSection

Lock.Enter;
try
  { Protected code — only one thread at a time }
finally
  Lock.Leave;  { ALWAYS in a finally block }
end;

• One thread at a time inside the Enter/Leave block
• Other threads block at Enter until the first calls Leave
• Always use try..finally — exceptions must not leave the lock held

Thread-Safe Data Structures

• Encapsulate a TCriticalSection inside the data structure
• Lock every method that reads or writes shared fields
• Use the snapshot pattern for iteration: lock, copy, unlock, iterate copy

Thread Pool Pattern

• Fixed set of worker threads + shared work queue
• Avoids thread creation overhead for many tasks
• Workers loop: dequeue, process, repeat until terminated

Common Bugs

PennyWise Background Sync

• TBackgroundSync thread runs continuously, syncing every 30 seconds
• Pending expenses queued via QueueExpense (protected by critical section)
• Status updates via Queue (non-blocking GUI update)
• Failed syncs re-queue the expense for retry
• Cooperative termination via Terminated check