Write an Async method that will await a bool

In many situations like this what you need is a TaskCompletionSource.

You likely have a method that is able to generate the data at some point in time, but it doesn't use a task to do it. Perhaps there is a method that takes a callback which provides the result, or an event that is fired to indicate that there is a result, or simply code using a Thread or ThreadPool that you are not inclined to re-factor into using Task.Run.

public Task<SomeData> GetTheData()
{
    TaskCompletionSource<SomeData> tcs = new TaskCompletionSource<SomeData>();
    SomeObject worker = new SomeObject();
    worker.WorkCompleted += result => tcs.SetResult(result);
    worker.DoWork();
    return tcs.Task;
}

While you may need/want to provide the TaskCompletionSource to the worker, or some other class, or in some other way expose it to a broader scope, I've found it's often not needed, even though it's a very powerful option when it's appropriate.

It's also possible that you can use Task.FromAsync to create a task based on an asynchronous operation and then either return that task directly, or await it in your code.

This work for me:

while (IsLoading) await Task.Delay(100);

You could use a TaskCompletionSource as your signal, and await that:

TaskCompletionSource<bool> IsSomethingLoading = new TaskCompletionSource<bool>();
SomeData TheData;

public async Task<SomeData> GetTheData()
{
   await IsSomethingLoading.Task;
   return TheData;
}

And in your Prism event do:

IsSomethingLoading.SetResult(true);