Unit Testing for the QuantumBits ObservableList

In using the ObservableList example from QuantumBits I came across a bit of a problem. This list ensures all event notifications happen on the UI thread by using the dispatcher. One change I added was to expose virtual methods in the ObservableList to "hook in" custom actions such as a flow-on effect for calculations as items in the list are changed. (OnItemAdded, OnItemRemoved, OnPropertyChanged etc. )I have extended functionality that interacts with these events to run calculations and I wanted to unit test this behaviour.

An example test:

[Test]
public void EnsureThatRelativeRateIsUpdatedWhenEarlierInterestRateChanges()
{
  var mortgageInterestList = new MortgageInterestList();
  var first = new MortgageInterest.Absolute(0.05m, DateTime.Today);
  var second = new MortgageInterest.Relative("1%", DateTime.Today.AddDays(2));
  mortgageInterestList.Add(first);
  mortgageInterestList.Add(second);

  first.UpdateInterestRate("+0.5%");
  Assert.AreEqual(0.055m, first.InterestRate, "Interest rate was not updated.");
  Assert.AreEqual(0.01m, second.InterestDelta, "Second's delta should not have been updated.");
  Assert.AreEqual(0.065m, second.InterestRate, "Second's interest rate should have been updated.");
}

An absolute interest rate is such as when a user says the interest rate was 5% as-of a specified date. A relative interest rate is one where the user specifies a +/- delta from any previous interest rate. The idea being that if an interest rate changes right before a relative rate, the relative's delta should be re-applied to adjust it's rate.

The problem is that while the MortgageInterestList (extending ObservableList) is wired up with the event to perform the calculation, running in a unit test the dispatcher doesn't actually run to set up the event hooks so that the edit triggers the flow-on effect.

The problem stems from this in the ObservableList:

public void Add(T item)
{
  this._list.Add(item);
  this._dispatcher.BeginInvoke(DispatcherPriority.Send,
    new AddCallback(AddFromDispatcherThread),
    item);
  OnItemAdded(item);
}

This method is perfectly correct, and in an application utilizing this functionality it works just great. However, unit tests don't like this kind of thing. The callback is added, but even with a Priority of "Send" (highest) the hookup doesn't actually happen during the test execution. The event doesn't fire, the rate isn't updated, the test fails.

After a lot of head scratching and digging around on the web I came across the solution on StackOverflow. Ironically while someone was having a similar problem to me with testing around Dispatcher-related tasks, this wasn't the accepted solution, but it worked a treat.
2nd Answer

public static class DispatcherUtil
{
  [SecurityPermissionAttribute(SecurityAction.Demand, Flags = SecurityPermissionFlag.UnmanagedCode)]
  public static void DoEvents()
  {
    DispatcherFrame frame = new DispatcherFrame();
    Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Background,
      new DispatcherOperationCallback(ExitFrame), frame);
    Dispatcher.PushFrame(frame);
  }

  private static object ExitFrame(object frame)
  {
    ((DispatcherFrame)frame).Continue = false;
    return null;
  }
}

Basically it's a mechanism that you can run inside a test to signal the dispatcher to go ahead and process anything currently pending. This way the events will be wired up before the test resumes. Damn simple and it works.

The test in this case changes to:

[Test]
public void EnsureThatRelativeRateIsUpdatedWhenEarlierInterestRateChanges()
{
   var mortgageInterestList = new MortgageInterestList();
   var first = new MortgageInterest.Absolute(0.05m, DateTime.Today);
   var second = new MortgageInterest.Relative("1%", DateTime.Today.AddDays(2));
   mortgageInterestList.Add(first);
   mortgageInterestList.Add(second);
   DispatcherUtil.DoEvents();

   first.UpdateInterestRate("+0.5%");
   Assert.AreEqual(0.055m, first.InterestRate, "Interest rate delta change was not based on the previous interest rate.");
   Assert.AreEqual(0.01m, second.InterestDelta, "Second's delta should not have been updated.");
   Assert.AreEqual(0.065m, second.InterestRate, "Second's interest rate should have been updated.");
}

SO if "jbe" should ever read this, thanks a bundle for the solution!

I've never been a fan of DoEvents coding in VB which this is pretty closely mirroring so this little gem will be located in my Unit Testing library where I have my base classes for uniform test fixture behaviour.

*Edit* I didn't much like the name of the class when I incorporated it into my test library. The updated signature is:

public static class DispatcherCycler
{
  [SecurityPermissionAttribute(SecurityAction.Demand, Flags = SecurityPermissionFlag.UnmanagedCode)]
  public static void Cycle()
  {
    DispatcherFrame frame = new DispatcherFrame();
    Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Background,
    new DispatcherOperationCallback((f) =>
      {
        ((DispatcherFrame)f).Continue = false;
        return null;
      })
      , frame);
    Dispatcher.PushFrame(frame);
  }
}