Question

I have an app that runs a function that could take a decent amount of time, so I need to add a callback method. How would I go about this?

Mainly, my question is what is the type that needs to be passed to the class constructor?

Answer 1

On C# (not only on WP7) you can call any function asynchronously by wrapping it in a delegate. On the delegate’s BeginInvoke call you’d pass a callback which will be invoked when the operation is completed. See the example below:

int MyLongOperation(int x, int y) {
   Thread.Sleep(10000);
   return x+y;
}

void CallingLongOperation(){
   int x = 4;
   int y = 5;
   Func<int, int, int> func = MyLongOperation;
   func.BeginInvoke(x, y, OperationCallback, func);
}

void OperationCallback(IAsyncResult asyncResult) {
   Func<int, int, int> func = (Func<int, int, int>) asyncResult.AsyncState;
   int result = func.EndInvoke(asyncResult);
   // do something with the result
}

If you need to pass some additional parameter in the asyncState / userState property, you can also use the AsyncDelegate property of the IAsyncResult parameter (which for delegate calls is always System.Runtime.Remoting.Messaging.AsyncResult) and retrieve the delegate from there as well, as shown below.

public int MyLongOperation(int x, int y)
{
    Thread.Sleep(10000);
    return x + y;
}
public void CallLongOperation()
{
    Func<int, int, int> func = MyLongOperation;
    func.BeginInvoke(5, 7, MyCallback, "Expected result: " + 12);
    Console.WriteLine("Called BeginInvoke");
    func.BeginInvoke(11, 22, MyCallback, "Expected result: " + 33);
    Console.WriteLine("Press ENTER to continue");
    Console.ReadLine();
}
void MyCallback(IAsyncResult asyncResult)
{
    Func<int, int, int> func = (Func<int, int, int>)((System.Runtime.Remoting.Messaging.AsyncResult)asyncResult).AsyncDelegate;
    string expectedResult = (string)asyncResult.AsyncState;
    int result = func.EndInvoke(asyncResult);
    Console.WriteLine("Result: {0} - {1}", result, expectedResult);
}