Recognize a pointer gesture

Cikk
05/14/2024

A .NET Multi-platform App UI (.NET MAUI) pointer gesture recognizer detects when the pointer enters, exits, and moves within a view and is implemented with the PointerGestureRecognizer class. This class defines the following properties:

PointerEnteredCommand, of type ICommand, which is the command to invoke when the pointer enters the bounding area of the view.
PointerEnteredCommandParameter, of type object, which is the parameter that's passed to PointerEnteredCommand.
PointerExitedCommand, of type ICommand, which is the command to invoke when the pointer that's in the view's bounding area leaves that bounding area.
PointerExitedCommandParameter, of type object, which is the parameter that's passed to PointerExitedCommand.
PointerMovedCommand, of type ICommand, which is the command to invoke when the pointer moves while remaining within the bounding area of the view.
PointerMovedCommandParameter, of type object, which is the parameter that's passed to PointerMovedCommand.
PointerPressedCommand, of type ICommand, which is the command to invoke when the pointer initiates a press within the view.
PointerPressedCommandParameter, of type object, which is the parameter that's passed to the PointerPressedCommand.
PointerReleasedCommand, of type ICommand, which is the command to invoke when the pointer that has previously initiated a press is released, while within the view.
PointerReleasedCommandParameter, of type object, which is the parameter that's passed to the PointerReleasedCommand.

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

The PointerGestureRecognizer class also defines the following events:

PointerEntered, that's raised when the pointer enters the bounding area of the view.
PointerExited, that's raised when the pointer that's in the view's bounding area leaves that bounding area.
PointerMoved, that's raised when the pointer moves while remaining within the bounding area of the view.
PointerPressed, that's raised when the pointer initiates a press within the view.
PointerReleased, that's raised when the pointer that has previously initiated a press is released, while within the view.

A PointerEventArgs object accompanies the events, and defines a PlatformArgs property of type PlatformPointerEventArgs that provides access to platform-specific arguments for the event.

On Android, the PlatformPointerEventArgs class defines the following properties:

Sender, of type View, represents the native view attached to the event.
MotionEvent, of type MotionEvent, indicates the native event or handler attached to the view.

In addition, the PointerEventArgs object defines a GetPosition method that returns a Point? object that represents the position of the pointer when the gesture was detected. For more information about the GetPosition method, see Get the gesture position.

Important

Pointer gesture recognition is supported on Android, iPadOS, Mac Catalyst, and Windows.

.NET MAUI also defines a PointerOver visual state. This state can change the visual appearance of a view when it has a mouse cursor hovering over it, but isn't pressed. For more information, see Visual states.

Create a PointerGestureRecognizer

To make a View recognize pointer gestures, create a PointerGestureRecognizer object, handle the required events, and add the gesture recognizer to the GestureRecognizers collection on the view. Alternatively, create a PointerGestureRecognizer object, and bind the required commands to ICommand implementations, and add the gesture recognizer to the GestureRecognizers collection on the view.

The following code example shows a PointerGestureRecognizer attached to an Image. The PointerGestureRecognizer uses events to respond to the detection of pointer gestures:

<Image Source="dotnet_bot.png">
    <Image.GestureRecognizers>
        <PointerGestureRecognizer PointerEntered="OnPointerEntered"
                                  PointerExited="OnPointerExited"
                                  PointerMoved="OnPointerMoved" />
  </Image.GestureRecognizers>
</Image>

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

void OnPointerEntered(object sender, PointerEventArgs e)
{
    // Handle the pointer entered event
}

void OnPointerExited(object sender, PointerEventArgs e)
{
    // Handle the pointer exited event
}

void OnPointerMoved(object sender, PointerEventArgs e)
{
    // Handle the pointer moved event
}

The equivalent C# code is:

PointerGestureRecognizer pointerGestureRecognizer = new PointerGestureRecognizer();
pointerGestureRecognizer.PointerEntered += (s, e) =>
{
    // Handle the pointer entered event
};
pointerGestureRecognizer.PointerExited += (s, e) =>
{
    // Handle the pointer exited event
};
pointerGestureRecognizer.PointerMoved += (s, e) =>
{
    // Handle the pointer moved event
};

Image image = new Image();
image.GestureRecognizers.Add(pointerGestureRecognizer);

Get the gesture position

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

void OnPointerExited(object sender, PointerEventArgs 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 pointer gesture inside the window.

Megosztás a következőn keresztül:

Recognize a pointer gesture

Create a PointerGestureRecognizer

Get the gesture position

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