As you may know, there is no double-click event for an Image in WPF. However, there are definitely cases where we might want one. But the question is, how do we pull this off while upholding a pure MVVM implementation?
Some considerations:
- Consideration 1: We should use Commands instead of Events.
- Consideration 2: In order to get 2 clicks, we will have to look at the MouseDown event, and look at it’s ClickCount since that is something an Image can handle.
- Consideration 3: Since this is an Image, there is no inherent Command property. In this case, we will need to use a library that provides one.
To start, let’s create a separate class that implements ICommand. In this example all we are doing is firing a MessageBox on successful completion of the event. In a pure implementation a MessageBox would not be considered kosher, but we are just going to use it for our example so we know if it worked:
using System;
using System.Windows;
using System.Windows.Input;
namespace DoubleClickImageTest
{
/// <summary>
/// A test command.
/// </summary>
public class TestCommand : ICommand
{
public virtual bool CanExecute(object parameter)
{
return true;
}
public virtual void Execute(object parameter)
{
// We shouldn't put message boxes in commands normally, but we will for this test.
MessageBox.Show("Hey, we just double-clicked our Image!",
"Double-Click Test", MessageBoxButton.OK);
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
}
}
Next, because this is MVVM we should create a View Model that will hold our command as a property:
using System;
namespace DoubleClickImageTest
{
public class TestViewModel
{
#region Properties
/// <summary>
/// Gets or sets my command.
/// </summary>
/// <value>
/// My command.
/// </value>
public TestCommand MyCommand { get; set; }
#endregion
#region Constructors
/// <summary>
/// Initializes a new instance of the <see cref="TestViewModel"/> class.
/// </summary>
public TestViewModel()
{
MyCommand = new TestCommand();
}
#endregion
}
}
To finish setting up the View Model, we will add it to the code-behind of our View and set the Data Context of the View to the View Model instance.
using System;
using System.Windows;
namespace DoubleClickImageTest
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
#region Members
private TestViewModel _vm;
#endregion
#region Constructors
/// <summary>
/// Initializes a new instance of the <see cref="MainWindow"/> class.
/// </summary>
public MainWindow()
{
// Get and set VM
_vm = new TestViewModel();
this.DataContext = _vm;
// Initialize UI
InitializeComponent();
}
#endregion
}
}
We have now satisfied the conditions of Consideration 1.
In order to satisfy Consideration 2, we will need to do the following:
- Write a class called CustomImage that extends System.Windows.Control.Image.
- In CustomImage create a RoutedEvent and RoutedEventHandler to facilitate the double-click event.
- In CustomImage override OnMouseLeftButtonDown to evaluate the click count.
First, we create the RoutedEvent:
/// <summary>
/// The mouse left button double click event
/// </summary>
public static readonly RoutedEvent MouseLeftButtonDoubleClick =
EventManager.RegisterRoutedEvent(
"MouseLeftButtonDoubleClick",
RoutingStrategy.Bubble,
typeof(RoutedEventHandler),
typeof(CustomImage));
Next we will create the RoutedEventHandler to execute MouseLeftButtonDoubleClick :
/// <summary>
/// Occurs when [mouse left button double click event handler].
/// </summary>
public event RoutedEventHandler MouseLeftButtonDoubleClickEvent
{
add
{
AddHandler(MouseLeftButtonDoubleClick, value);
}
remove
{
RemoveHandler(MouseLeftButtonDoubleClick, value);
}
}
Lastly, we need to override the OnMouseLeftButtonDown event.
So, why are we doing this? We know images have OnMouseLeftButtonDown, so we can use that to observe the click-count and call our event. In this case, we observe the ClickCount of our MouseButtonEventArgs. If we observe 2 clicks, we raise the MouseLeftButtonDoubleClick event:
/// <summary>
/// Invoked when an unhandled <see cref="E:System.Windows.UIElement.MouseLeftButtonDown" />
/// routed event is raised on this element. Implement this method to add class handling for
/// this event.
/// </summary>
/// <param name="e">The <see cref="T:System.Windows.Input.MouseButtonEventArgs" /> that
/// contains the event data. The event data reports that the left mouse button was pressed.
/// </param>
protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
{
// Double-click
if (e.ClickCount == 2)
{
RaiseEvent(new MouseLeftButtonDoubleClickEventArgs(
MouseLeftButtonDoubleClick, this));
}
base.OnMouseLeftButtonDown(e);
}
/// <summary>
/// MouseLeftButtonDoubleClick EventArgs.
/// </summary>
public class MouseLeftButtonDoubleClickEventArgs : RoutedEventArgs
{
/// <summary>
/// Initializes a new instance of the <see cref="MouseLeftButtonDoubleClickEventArgs"/>
/// class.
/// </summary>
/// <param name="routedEvent">The routed event identifier for this instance of the
/// <see cref="T:System.Windows.RoutedEventArgs" /> class.</param>
/// <param name="source">An alternate source that will be reported when the event is
/// handled. This pre-populates the
/// <see cref="P:System.Windows.RoutedEventArgs.Source" /> property.</param>
public MouseLeftButtonDoubleClickEventArgs(RoutedEvent routedEvent, object source)
: base(routedEvent, source)
{
}
}
So, in it’s entirety, the CustomImage class looks like this:
using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
namespace DoubleClickImageTest
{
/// <summary>
/// Custom image.
/// </summary>
public class CustomImage : Image
{
/// <summary>
/// The mouse left button double click event
/// </summary>
public static readonly RoutedEvent MouseLeftButtonDoubleClick =
EventManager.RegisterRoutedEvent(
"MouseLeftButtonDoubleClick",
RoutingStrategy.Bubble,
typeof(RoutedEventHandler),
typeof(CustomImage));
/// <summary>
/// Occurs when [mouse left button double click event handler].
/// </summary>
public event RoutedEventHandler MouseLeftButtonDoubleClickEvent
{
add
{
AddHandler(MouseLeftButtonDoubleClick, value);
}
remove
{
RemoveHandler(MouseLeftButtonDoubleClick, value);
}
}
/// <summary>
/// Invoked when an unhandled <see cref="E:System.Windows.UIElement.MouseLeftButtonDown" />
/// routed event is raised on this element. Implement this method to add class handling for
/// this event.
/// </summary>
/// <param name="e">The <see cref="T:System.Windows.Input.MouseButtonEventArgs" /> that
/// contains the event data. The event data reports that the left mouse button was pressed.
/// </param>
protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
{
// Double-click
if (e.ClickCount == 2)
{
RaiseEvent(new MouseLeftButtonDoubleClickEventArgs(
MouseLeftButtonDoubleClick, this));
}
base.OnMouseLeftButtonDown(e);
}
/// <summary>
/// MouseLeftButtonDoubleClick EventArgs.
/// </summary>
public class MouseLeftButtonDoubleClickEventArgs : RoutedEventArgs
{
/// <summary>
/// Initializes a new instance of the <see cref="MouseLeftButtonDoubleClickEventArgs"/>
/// class.
/// </summary>
/// <param name="routedEvent">The routed event identifier for this instance of the
/// <see cref="T:System.Windows.RoutedEventArgs" /> class.</param>
/// <param name="source">An alternate source that will be reported when the event is
/// handled. This pre-populates the
/// <see cref="P:System.Windows.RoutedEventArgs.Source" /> property.</param>
public MouseLeftButtonDoubleClickEventArgs(RoutedEvent routedEvent, object source)
: base(routedEvent, source)
{
}
}
}
}
For Consideration 3 we need to extend our image to allow Commanding, and to do that we will use the Interactivity Library from Expression Blend.
Now, we will build our XAML. Let’s start by adding our CustomImage to a window:
<Window x:Class="DoubleClickImageTest.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:DoubleClickImageTest"
Title="MainWindow" Height="350" Width="525">
<Grid>
<local:CustomImage
Width="200"
Height="200"
Source="/DoubleClickImageTest;component/cat_popcorn.jpg"
HorizontalAlignment="Center"
VerticalAlignment="Center">
</local:CustomImage>
</Grid>
</Window>
It looks like this at runtime:
So far, there is nothing overly significant about this implementation. Our CustomImage is behaving just like a normal image and it’s center-mass in our window.
Now, let’s add our new MouseLeftButtonDoubleClick event.
<Window x:Class="DoubleClickImageTest.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:DoubleClickImageTest"
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
Title="MainWindow" Height="350" Width="525">
<Grid>
<local:CustomImage
Width="200"
Height="200"
Source="/DoubleClickImageTest;component/cat_popcorn.jpg"
HorizontalAlignment="Center"
VerticalAlignment="Center">
<i:Interaction.Triggers>
<i:EventTrigger EventName="MouseLeftButtonDoubleClickEvent">
<i:InvokeCommandAction Command="{Binding Path=MyCommand}" />
</i:EventTrigger>
</i:Interaction.Triggers>
</local:CustomImage>
</Grid>
</Window>
You will notice we included the Interactivity Library in the header:
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
Also, this new block inside CustomImage:
<i:Interaction.Triggers>
<i:EventTrigger EventName="MouseLeftButtonDoubleClickEvent">
<i:InvokeCommandAction Command="{Binding Path=MyCommand}" />
</i:EventTrigger>
</i:Interaction.Triggers>
This says: “whenever MouseLeftButtonDoubleClick is fired, invoke MyCommand“.
Note: you will notice this works exactly the same as setting a command on a Button.
So, now when we double-click our image, we see this:
And just like that, we created our double-clickable image.
Until next time…