Asynchronous Trigger

AsyncTrigger is the most general implementation of synchronization primitive compatible with asynchronous code. It can be used as a core for broad set of synchronization cases. The magic hides in ability to supply external state and predicate describing condition for resuming suspended callers. Each caller can supply its own condition for resuming and wait for signal. Signaling caller must supply the state that matches to one or more suspended callers. This concept is represented by the following methods:

• Signal method and its overloads can be used to signal about changes in external state which is also should be supplied as a parameter
• WaitAsync method and its overloads can be used to suspend the caller and specify predicate that describes condition for resuming.
• SignalAndWaitAsync method is a combination of Signal and WaitAsync that works atomically. Useful in situations when you need to notify about state change and wait for another state.

As a result, this synchronization primitive can be used for building complex asynchronous state machines.

Signaling method behaves in two ways:

• Resume all suspended callers in unordered fashion. For instance, if you have the queue with configiration A1-A2-A3 and A1, A2 return true then they will be removed from the queue and A2 remains untouched.
• Fair dequeuing when suspended callers resumed in strict order. In this case only A1 will be removed. A2 and A3 remain untouched because A2 returns false.

Fairness policy for dequeuing can be passed as bool parameter to Signal and SignalAndWaitAsync methods.

You can check this class as an example of complex asynchronous lock constructed on top of AsyncTrigger. The implementation is used by persistent Write-Ahead Log to allow multiple writers co-exist without blocking in some specific cirtumstances.