#606 – Reusing an Existing Routed Event in Your Class

July 19, 2012 1 Comment

When you are defining a new CLR event in a class that will wrap a routed event, you can either register your own new routed event or you can reuse a routed event that already exists in the WPF framework.

To reuse an existing routed event, you call the AddOwner method on the existing event.

    public class MyButton : Button
    {
        public static readonly RoutedEvent MyTextChangedEvent;

        static MyButton()
        {
            MyTextChangedEvent = TextBoxBase.TextChangedEvent.AddOwner(typeof(MyButton));
        }

        public event TextChangedEventHandler MyTextChanged
        {
            add { AddHandler(MyTextChangedEvent, value); }
            remove { RemoveHandler(MyTextChangedEvent, value); }
        }

        protected virtual void OnMyTextChanged()
        {
            TextChangedEventArgs evargs = new TextChangedEventArgs(MyTextChangedEvent, UndoAction.None);
            RaiseEvent(evargs);
        }

        public MyButton()
        {
            this.Click += new RoutedEventHandler(MyButton_Click);
        }

        void MyButton_Click(object sender, RoutedEventArgs e)
        {
            this.Content = this.Content + ".";
            OnMyTextChanged();
        }
    }

Filed under Events Tagged with , , ,

#605 – Using Subclasses of RoutedEventArgs

July 18, 2012 1 Comment

When you define your own routed event in a class, you raise the event using the UIElement.RaiseEvent method.  The RaiseEvent method accepts an instance of a RoutedEventArgs object.  Notice that this is same class type passed to event handlers that are declared using the RoutedEventHandler delegate type.

public delegate void RoutedEventHandler(Object sender, RoutedEventArgs e);

When firing a routed event, you can choose to use one of the subclasses of RoutedEventArgs, if you have additional information to pass back.

For example, you might declare your event to be of type MouseEventHandler and then pass back an instance of MouseEventArgs when raising the event.

        public event MouseEventHandler RightDrag
        {
            add { AddHandler(RightDragEvent, value); }
            remove { RemoveHandler(RightDragEvent, value); }
        }

        protected virtual void OnRightDrag(MouseEventArgs e)
        {
            MouseEventArgs evargs = new MouseEventArgs(e.MouseDevice, 0);
            evargs.RoutedEvent = RightDragEvent;
            RaiseEvent(evargs);
        }

Filed under Events Tagged with , ,

#604 – Defining a New Routed Event

July 17, 2012 1 Comment

You can create a new routed event in your own class, typically in a control.  Here, we define a new event named RightDrag in the MyButton class, which derives from Button.

Define a static object of type RoutedEvent.

public static readonly RoutedEvent RightDragEvent;

In the static constructor, register the event and set the RoutedEvent object.

        static MyButton()
        {
            RightDragEvent = EventManager.RegisterRoutedEvent("RightDrag", RoutingStrategy.Bubble, typeof(RoutedEventHandler), typeof(MyButton));
        }

Define a CLR event of type RoutedEventHandler or one of its subtypes.  Call the AddHandler/RemoveHandler methods.

        public event RoutedEventHandler RightDrag
        {
            add { AddHandler(RightDragEvent, value); }
            remove { RemoveHandler(RightDragEvent, value); }
        }

Add a helper method to fire the event.

        protected virtual void OnRightDrag()
        {
            RoutedEventArgs evargs = new RoutedEventArgs(RightDragEvent, this);
            RaiseEvent(evargs);
        }

Finally, fire your event when appropriate.

        void MyButton_MouseMove(object sender, System.Windows.Input.MouseEventArgs e)
        {
            if (e.RightButton == MouseButtonState.Pressed)
                OnRightDrag();
        }

Filed under Events Tagged with , , ,

#601 – The RoutedEventHandler Delegate Type

July 12, 2012 Leave a comment

In the example showing the implementation of the ButtonBase.Click routed event, you’ll notice that the Click event is declared as a standard CLR event whose type is the RoutedEventHandler delegate type.

If we look at RoutedEventHandler, we see that it has this signature:

public delegate void RoutedEventHandler(object sender, RoutedEventArgs e);

In WPF, many routed events have this signature and others declare a new delegate type that passes back a subclass of RoutedEventArgs.  For example, UIElement.KeyDown has a delegate type of KeyEventHandler, which has the following signature:

public delegate void KeyEventHandler(Object sender, KeyEventArgs e)

The KeyEventArgs type inherits from RoutedEventArgs (indirectly), adding some properties that make sense for key press events.

So event handlers for predefined routed events in WPF will normally be passed either an instance of RoutedEventArgs or an instance of a type that inherits from RoutedEventArgs.

Filed under Events Tagged with , , , ,

#600 – Registering a Routed Event

July 11, 2012 Leave a comment

When you implement a routed event in a class, you end up creating an instance of the RoutedEvent type.

Instead of explicitly creating the RoutedEvent instance, you create it indirectly by calling EventManager.RegisterRoutedEvent.  This method accepts some information about the routed event that you want to create and returns an instance of a RoutedEvent, which you typically store in a static field.  You typically register the event in your static constructor, or at the time that it is declared, so you end up having only one instance of the RoutedEvent, no matter how many instances of your class get created.

        // Define/create the routed event object
        public static readonly RoutedEvent ClickEvent = EventManager.RegisterRoutedEvent("Click", RoutingStrategy.Bubble, typeof(RoutedEventHandler), typeof(ButtonBase));

RegisterRoutedEvent takes the following parameters:

  • name – the name of the event
  • routingStrategy – the routing strategy–tunneling, bubbling or direct
  • handlerType – the delegate type for event handlers
  • ownerType – the class type for the event’s owner

Filed under Events Tagged with , , ,

#599 – A Complete Example of a Routed Event

July 11, 2012 1 Comment

To help you understand how routed events in WPF work, it’s helpful to look at how they are implemented.  Here is all of the relevant code from the ButtonBase class for the Click event.

    public abstract class ButtonBase : ContentControl, ICommandSource
    {
        // Define/create the routed event object
        public static readonly RoutedEvent ClickEvent = EventManager.RegisterRoutedEvent("Click", RoutingStrategy.Bubble, typeof(RoutedEventHandler), typeof(ButtonBase));

        // CLR event wrapper, adds/removes handlers
        public event RoutedEventHandler Click { add { AddHandler(ClickEvent, value); } remove { RemoveHandler(ClickEvent, value); } }

        // Method used internally to fire the Click event
        protected virtual void OnClick()
        {
            RoutedEventArgs newEvent = new RoutedEventArgs(ButtonBase.ClickEvent, this);
            RaiseEvent(newEvent);
        }
    }

Both the AddHandler and RaiseEvent methods are defined in UIElement.

Filed under Events Tagged with , ,

#598 – Three Flavors of Routed Events

July 10, 2012 1 Comment

In WPF, every routed event will adopt one of three different routing strategies:

  • Bubbling – the event propagates up the hierarchy of elements in the user interface
  • Tunneling – the event propagates down the hierarchy of elements
  • Direct – the event fires on the source element, but does not propagate

Routed events that are part of the WPF class libraries will have one of these three routing strategies.  For example:

  • ButtonBase.Click is a bubbling event
  • UIElement.PreviewKeyDown is a tunneling event
  • UIElement.MouseEnter is a direct event

When you define your own routed events, you can also specify one of these routing strategies for the event.

Filed under Events Tagged with , , , , ,

#594 – Routed Events Under the Covers

July 4, 2012 Leave a comment

Traditional CLR events are basically a wrapper around a private instance of a multicast delegate type.  When you add an event handler using the += operator, your handler gets added to the delegate’s invocation list.

// CLR event under the covers
private EventHandler barked;
public event EventHandler Barked
{
    add { barked += value; }
    remove { barked -= value; }
}

Routed events in WPF look like standard CLR events because they are exposed via a CLR event.  But they are implemented differently.

When you use += for a routed event, the UIElement.AddHandler method is called.

    // From ButtonBase.cs
    public static readonly RoutedEvent ClickEvent;
    public event RoutedEventHandler Click { add { AddHandler(ClickEvent, value); } remove { RemoveHandler(ClickEvent, value); } }

The AddHandler method adds information about both the event and the handler to list of event handlers that is stored within the UIElement, used when the event is raised.

Filed under Events Tagged with , , , ,

#593 – AddHandler Method Can Add Handler for Any Event

July 3, 2012 1 Comment

If you’re adding an event handler from code, rather than specifying the handler in XAML, you can just use the += notation for an event that is defined for the control in question.  For example, the Button control defines a Click control, so you can do the following:

myButton.Click += new RoutedEventHandler(Button_Click);

But let’s say that you want to add a handler for the Click event to a StackPanel control, which does not define the Click event, and you want to do it from code.  You can then use the AddHandler syntax:

myStackPanel.AddHandler(ButtonBase.ClickEvent, (RoutedEventHandler)HandleTheClick);

Notice that the AddHandler method accepts any routed event type as its first parameter.  This means that any UIElement instance can call its AddHandler method to indicate that it wants to handle any routed event.  Even though StackPanel doesn’t define a Click event, it can handle the Button’s Click event.

Filed under Events Tagged with , , ,

#592 – Adding an Event Handler in Code

July 2, 2012 1 Comment

You typically specify handlers for routed events in XAML:

        <Button Content="Like" Click="Button_Click"/>

You then add the code for the handler in your code-behind:

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            MessageBox.Show("Click");
        }

You can also add the handler from code. Adding a handler to the Button control for the  Click event requires that you give the Button control a name that you can then reference from your code-behind.

        <Button Name="myButton" Content="Like"/>

You can then add a handler by using the += operator on the event for which you want a handler.

myButton.Click += new RoutedEventHandler(Button_Click);

This is equivalent to calling the UIElement.AddHandler method.

            myButton.AddHandler(ButtonBase.ClickEvent, (RoutedEventHandler)Button_Click);

Filed under Events Tagged with , , ,

← Older posts

Follow

Get every new post delivered to your Inbox.

Join 238 other followers