Slider

Artikkeli
08/30/2024

The .NET Multi-platform App UI (.NET MAUI) Slider is a horizontal bar that you can manipulate to select a double value from a continuous range.

Slider defines the following properties:

Minimum, of type double, is the minimum of the range, with a default value of 0.
Maximum, of type double, is the maximum of the range, with a default value of 1.
Value, of type double, is the slider's value, which can range between Minimum and Maximum and has a default value of 0.
MinimumTrackColor, of type Color, is the bar color on the left side of the thumb.
MaximumTrackColor, of type Color, is the bar color on the right side of the thumb.
ThumbColor of type Color, is the thumb color.
ThumbImageSource, of type ImageSource, is the image to use for the thumb, of type ImageSource.
DragStartedCommand, of type ICommand, which is executed at the beginning of a drag action.
DragCompletedCommand, of type ICommand, which is executed at the end of a drag action.

These properties are backed by BindableProperty objects. The Value property has a default binding mode of BindingMode.TwoWay, which means that it's suitable as a binding source in an application that uses the Model-View-ViewModel (MVVM) pattern.

Note

The ThumbColor and ThumbImageSource properties are mutually exclusive. If both properties are set, the ThumbImageSource property will take precedence.

The Slider coerces the Value property so that it is between Minimum and Maximum, inclusive. If the Minimum property is set to a value greater than the Value property, the Slider sets the Value property to Minimum. Similarly, if Maximum is set to a value less than Value, then Slider sets the Value property to Maximum. Internally, the Slider ensures that Minimum is less than Maximum. If Minimum or Maximum are ever set so that Minimum is not less than Maximum, an exception is raised. For more information on setting the Minimum and Maximum properties, see Precautions.

Slider defines a ValueChanged event that's raised when the Value changes, either through user manipulation of the Slider or when the program sets the Value property directly. A ValueChanged event is also raised when the Value property is coerced as described in the previous paragraph. The ValueChangedEventArgs object that accompanies the ValueChanged event has OldValue and NewValue properties, of type double. At the time the event is raised, the value of NewValue is the same as the Value property of the Slider object.

Slider also defines DragStarted and DragCompleted events, that are raised at the beginning and end of the drag action. Unlike the ValueChanged event, the DragStarted and DragCompleted events are only raised through user manipulation of the Slider. When the DragStarted event fires, the DragStartedCommand, of type ICommand, is executed. Similarly, when the DragCompleted event fires, the DragCompletedCommand, of type ICommand, is executed.

Warning

Do not use unconstrained horizontal layout options of Center, Start, or End with Slider. Keep the default HorizontalOptions setting of Fill, and don't use a width of Auto when putting Slider in a Grid layout.

Create a Slider

The following example shows how to create a Slider, with two Label objects:

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="SliderDemos.BasicSliderXamlPage"
             Title="Basic Slider XAML"
             Padding="10, 0">
    <StackLayout>
        <Label x:Name="rotatingLabel"
               Text="ROTATING TEXT"
               FontSize="18"
               HorizontalOptions="Center"
               VerticalOptions="Center" />
        <Slider Maximum="360"
                ValueChanged="OnSliderValueChanged" />
        <Label x:Name="displayLabel"
               Text="(uninitialized)"
               HorizontalOptions="Center"
               VerticalOptions="Center" />
    </StackLayout>
</ContentPage>

In this example, the Slider is initialized to have a Maximum property of 360. The second Label displays the text "(uninitialized)" until the Slider is manipulated, which causes the first ValueChanged event to be raised.

The code-behind file contains the handler for the ValueChanged event:

public partial class BasicSliderXamlPage : ContentPage
{
    public BasicSliderXamlPage()
    {
        InitializeComponent();
    }

    void OnSliderValueChanged(object sender, ValueChangedEventArgs args)
    {
        double value = args.NewValue;
        rotatingLabel.Rotation = value;
        displayLabel.Text = String.Format("The Slider value is {0}", value);
    }
}

The ValueChanged handler of the Slider uses the Value property of the slider object to set the Rotation property of the first Label and uses the String.Format method with the NewValue property of the event arguments to set the Text property of the second Label:

.NET MAUI Slider screenshot.

It's also possible for the event handler to obtain the Slider that is firing the event through the sender argument. The Value property contains the current value:

double value = ((Slider)sender).Value;

If the Slider object were given a name in the XAML file with an x:Name attribute (for example, "slider"), then the event handler could reference that object directly:

double value = slider.Value;

The equivalent C# code for creating a Slider is:

Slider slider = new Slider
{
    Maximum = 360
};
slider.ValueChanged += (sender, args) =>
{
    rotationLabel.Rotation = slider.Value;
    displayLabel.Text = String.Format("The Slider value is {0}", args.NewValue);
};

Data bind a Slider

The ValueChanged event handler can be eliminated by using data binding to respond to the Slider value changing:

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="SliderDemos.BasicSliderBindingsPage"
             Title="Basic Slider Bindings"
             Padding="10, 0">
    <StackLayout>
        <Label Text="ROTATING TEXT"
               Rotation="{Binding Source={x:Reference slider},
                                  Path=Value}"
               FontSize="18"
               HorizontalOptions="Center"
               VerticalOptions="Center" />
        <Slider x:Name="slider"
                Maximum="360" />
        <Label x:Name="displayLabel"
               Text="{Binding Source={x:Reference slider},
                              Path=Value,
                              StringFormat='The Slider value is {0:F0}'}"
               HorizontalOptions="Center"
               VerticalOptions="Center" />
    </StackLayout>
</ContentPage>

In this example, the Rotation property of the first Label is bound to the Value property of the Slider, as is the Text property of the second Label with a StringFormat specification. When the page first appears, the second Label displays the text string with the value. To display text without data binding, you'd need to specifically initialize the Text property of the Label or simulate a firing of the ValueChanged event by calling the event handler from the class constructor.

Precautions

The value of the Minimum property must always be less than the value of the Maximum property. The following example causes the Slider to raise an exception:

// Throws an exception!
Slider slider = new Slider
{
    Minimum = 10,
    Maximum = 20
};

The C# compiler generates code that sets these two properties in sequence, and when the Minimum property is set to 10, it is greater than the default Maximum value of 1. You can avoid the exception in this case by setting the Maximum property first:

Slider slider = new Slider
{
    Maximum = 20,
    Minimum = 10
};

In this example, setting Maximum to 20 is not a problem because it is greater than the default Minimum value of 0. When Minimum is set, the value is less than the Maximum value of 20.

The same problem exists in XAML. The properties must be set in an order that ensures that Maximum is always greater than Minimum:

<Slider Maximum="20"
        Minimum="10" ... />

You can set the Minimum and Maximum values to negative numbers, but only in an order where Minimum is always less than Maximum:

<Slider Minimum="-20"
        Maximum="-10" ... />

The Value property is always greater than or equal to the Minimum value and less than or equal to Maximum. If Value is set to a value outside that range, the value will be coerced to lie within the range, but no exception is raised. For example, the following example won't raise an exception:

Slider slider = new Slider
{
    Value = 10
};

Instead, the Value property is coerced to the Maximum value of 1.

A previous example set Maximum to 20, and Minimum to 10:

Slider slider = new Slider
{
    Maximum = 20,
    Minimum = 10
};

When Minimum is set to 10, then Value is also set to 10.

If a ValueChanged event handler has been attached at the time that the Value property is coerced to something other than its default value of 0, then a ValueChanged event is raised:

<Slider ValueChanged="OnSliderValueChanged"
        Maximum="20"
        Minimum="10" />

When Minimum is set to 10, Value is also set to 10, and the ValueChanged event is raised. This might occur before the rest of the page has been constructed, and the handler might attempt to reference other elements on the page that have not yet been created. You might want to add some code to the ValueChanged handler that checks for null values of other elements on the page. Or, you can set the ValueChanged event handler after the Slider values have been initialized.

Jaa

Slider

Create a Slider

Data bind a Slider

Precautions

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