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Recognize a tap gesture

  • Artikkeli

A .NET Multi-platform App UI (.NET MAUI) tap gesture recognizer is used for tap detection and is implemented with the TapGestureRecognizer class. This class defines the following properties:

  • Buttons, of type ButtonsMask, which defines whether the primary or secondary mouse button, or both, triggers the gesture on Android, Mac Catalyst, and Windows. For more information, see Define the button masks.
  • Command, of type ICommand, which is executed when a tap is recognized.
  • CommandParameter, of type object, which is the parameter that's passed to the Command.
  • NumberOfTapsRequired, of type int, which represents the number of taps required to recognize a tap gesture. The default value of this property is 1.

These properties are backed by BindableProperty objects, which means that they can be targets of data bindings, and styled.

The TapGestureRecognizer class also defines a Tapped event that's raised when a tap is recognized. The TappedEventArgs object that accompanies the Tapped event defines a Parameter property of type object that indicates the value passed by the CommandParameter property, if defined. The TappedEventArgs object also defines a Buttons property, and a GetPosition method. The Buttons property is of type ButtonsMask, and can be used to determine whether the primary or secondary mouse button triggered the gesture recognizer on Android, Mac Catalyst, and Windows. The GetPosition method returns a Point? object that represents the position at which the tap gesture was detected. For more information about button masks, see Define the button mask. For more information about the GetPosition method, see Get the gesture position.

Warning

A TapGestureRecognizer can't recognize more than a double tap on Windows.

Create a TapGestureRecognizer

To make a View recognize a tap gesture, create a TapGestureRecognizer object, handle the Tapped event, and add the new gesture recognizer to the GestureRecognizers collection on the view. The following code example shows a TapGestureRecognizer attached to an Image:

<Image Source="dotnet_bot.png">
    <Image.GestureRecognizers>
        <TapGestureRecognizer Tapped="OnTapGestureRecognizerTapped"
                              NumberOfTapsRequired="2" />
  </Image.GestureRecognizers>
</Image>

The code for the OnTapGestureRecognizerTapped event handler should be added to the code-behind file:

void OnTapGestureRecognizerTapped(object sender, TappedEventArgs args)
{
    // Handle the tap
}

The equivalent C# code is:

TapGestureRecognizer tapGestureRecognizer = new TapGestureRecognizer();
tapGestureRecognizer.Tapped += (s, e) =>
{
    // Handle the tap
};
Image image = new Image();
image.GestureRecognizers.Add(tapGestureRecognizer);

By default the Image will respond to single taps. When the NumberOfTapsRequired property is set to greater than one, the event handler will only be executed if the taps occur within a set period of time. If the second (or subsequent) taps don't occur within that period, they're effectively ignored.

Define the button mask

A TapGestureRecognizer object has a Buttons property, of type ButtonsMask, that defines whether the primary or secondary mouse button, or both, triggers the gesture on Android, Mac Catalyst, and Windows. The ButtonsMask enumeration defines the following members:

  • Primary represents the primary mouse button, which is typically the left mouse button.
  • Secondary represents the secondary mouse button, which is typically the right mouse button.

The following example shows a TapGestureRecognizer that detects taps with the secondary mouse button:

<Image Source="dotnet_bot.png">
    <Image.GestureRecognizers>
        <TapGestureRecognizer Tapped="OnTapGestureRecognizerTapped"
                              Buttons="Secondary" />
  </Image.GestureRecognizers>
</Image>

The event handler for the Tapped event can determine which button triggered the gesture:

void OnTapGestureRecognizerTapped(object sender, TappedEventArgs args)
{
    // Handle the tap
    if (args.Buttons == ButtonsMask.Secondary)
    {
        // Do something
    }
}

The equivalent C# code is:

TapGestureRecognizer tapGestureRecognizer = new TapGestureRecognizer
{
    Buttons = ButtonsMask.Secondary
};
tapGestureRecognizer.Tapped += (s, e) =>
{
    // Handle the tap
    if (args.Buttons == ButtonsMask.Secondary)
    {
        // Do something
    }
};
Image image = new Image();
image.GestureRecognizers.Add(tapGestureRecognizer);

Warning

On Windows, a TapGestureRecognizer that sets the Buttons property to Secondary doesn't respect the NumberOfTapsRequired property when it's greater than one.

In addition, a TapGestureRecognizer can be defined so that either the primary or secondary mouse button triggers the gesture:

<TapGestureRecognizer Tapped="OnTapGestureRecognizerTapped"
                      Buttons="Primary,Secondary" />

The equivalent C# code is:

TapGestureRecognizer tapGestureRecognizer = new TapGestureRecognizer
{
    Buttons = ButtonsMask.Primary | ButtonsMask.Secondary
};

Get the gesture position

The position at which a tap gesture occurred can be obtained by calling the GetPosition method on a TappedEventArgs object. The GetPosition method accepts an Element? argument, and returns a position as a Point? object:

void OnTapGestureRecognizerTapped(object sender, TappedEventArgs e)
{
    // Position inside window
    Point? windowPosition = e.GetPosition(null);

    // Position relative to an Image
    Point? relativeToImagePosition = e.GetPosition(image);

    // Position relative to the container view
    Point? relativeToContainerPosition = e.GetPosition((View)sender);
}

The Element? argument defines the element the position should be obtained relative to. Supplying a null value as this argument means that the GetPosition method returns a Point? object that defines the position of the tap gesture inside the window.