Question

I wrote the following code. It tries to create a storyboard that does the following:

1. fade in for 500ms
2. pause for 1000ms
3. fade out for 500ms

But at run time get an System.InvalidOperationException followed by the following output:

Additional information: Multiple animations in 
  the same containing Storyboard cannot target the
  same property on a single element.

This seems to suggest its trying to do all the animations at once rather than sequentially.

private Storyboard createStoryboard()
{
  Storyboard board = new Storyboard();

  addFadeToStoryboard(board, 0, 1, 500);
  addFadeToStoryboard(board, 1, 1, 1000);

  DoubleAnimation fadeOut = addFadeToStoryboard(board, 1, 0, 500);

  fadeOut.Completed += new EventHandler(onFadeCompleted);

  Storyboard.SetTarget(board, this);

  return board;
}

private DoubleAnimation addFadeToStoryboard(Storyboard board, 
  double fadeFrom, double fadeTo, double milliseconds)
{
  DoubleAnimation fade = new DoubleAnimation()
  {
    Duration = new Duration(TimeSpan.FromMilliseconds(milliseconds)),
    From = fadeFrom,
    To = fadeTo,
    RepeatBehavior = new RepeatBehavior(1)
  };

  Storyboard.SetTargetProperty(fade,
    new PropertyPath(UIElement.OpacityProperty));

  board.Children.Add(fade);

  return fade;
}

How can I make it sequential? Am I misinterpreting something fundamental about storyboards?

Thanks

Answer 1

I’ve solved this by creating my own sequencer class. It has a dependency on a LinkedList class, so you’ll have to swap that out with the standard one if you want to use this code:

using System;
using System.Windows.Media.Animation;

namespace YourNamespace
{
  // An object that contains multiple storyboards, and fires off 
  // subsequent ones as they finish.

  public class StoryboardSequencer
  {
    public StoryboardSequencer()
    {
    }

    public void add(Storyboard board)
    {
      m_boards.add(board);
      board.Completed += new EventHandler(onBoardCompleted);
    }

    // Starts the storyboard from the first.
    // If already started, this call is ignored.
    public void begin()
    {
      if (m_boards.size() > 0)
      {
        m_currentBoardIndex = 0;
        beginCurrentBoard();
      }
      else
      {
        onLastBoardCompleted();
      }
    }

    // Stops and rewinds.
    // Does not call completed handler.
    public void stop()
    {
      if (m_currentBoardIndex != -1)
      {
        Storyboard board = m_boards.get(m_currentBoardIndex);
        if (board != null)
        {
          board.Stop();
        }

        m_currentBoardIndex = -1;
      }
    }

    private void beginCurrentBoard()
    {
      Storyboard board = m_boards.get(m_currentBoardIndex);
      if (board == null)
      {
        onLastBoardCompleted();
      }
      else
      {
        board.Begin();
      }
    }

    // Triggered when the sequence completes.
    public event EventHandler Completed;

    private void onBoardCompleted(object sender, EventArgs e)
    {
      m_currentBoardIndex++;

      if (m_currentBoardIndex >= m_boards.size())
      {
        onLastBoardCompleted();
      }
      else
      {
        beginCurrentBoard();
      }
    }

    private void onLastBoardCompleted()
    {
      m_currentBoardIndex = -1;
      Completed.Invoke(this, null);
    }

    private LinkedList<Storyboard> m_boards = new LinkedList<Storyboard>();

    // The current storyboard playing, or -1 if none.
    private int m_currentBoardIndex = -1;
  }
}