How can I await until I receive a callback/notification without busy-waiting?

Question

I understand how I can await on library code to wait for a network request or other long-running action to complete, but how can I await on my own long-running action without busy waiting?

---

This is the busy-waiting solution. How can I make it event-driven?

public async Task<DataBlock> ClickSend() {
    // await until someone calls DataReceived()
    while (_Block == null) {
        await Task.Yield();
    }
    return _Block;
}

// Something external calls this function.
// Once called, ClickSend should complete with this data.
public void DataReceived(DataBlock data_block) {
    _Block = data_block;
}
DataBlock _Block = null;

(This question is completely different from How do I await events in C#? which is asking how to await on event handlers, but very similar to Promise equivalent in C#.)

Answer 1

Generally in concurrency a "future" is placeholder for a return value and it has an associated "promise" that is fulfilled to pass the final return value.

In C#, they have different names: the future is a Task and the promise is a TaskCompletionSource.

You can create a promise, await on it, and then fulfill it when you get your callback:

TaskCompletionSource<DataBlock> _Promise = new TaskCompletionSource<DataBlock>();

public async Task<DataBlock> ClickSend() {
    DataBlock block = await _Promise.Task;
    return block;
}

public void DataReceived(DataBlock data_block) {
    _Promise.TrySetResult(data_block);
}

Here's an executable example.