Xamarin.Forms Device Class

Artikel
07/20/2020

The Device class contains a number of properties and methods to help developers customize layout and functionality on a per-platform basis.

In addition to methods and properties to target code at specific hardware types and sizes, the Device class includes methods that can be used to interact with UI controls from background threads. For more information, see Interact with the UI from background threads.

Provide platform-specific values

Prior to Xamarin.Forms 2.3.4, the platform the application was running on could be obtained by examining the Device.OS property and comparing it to the TargetPlatform.iOS, TargetPlatform.Android, TargetPlatform.WinPhone, and TargetPlatform.Windows enumeration values. Similarly, one of the Device.OnPlatform overloads could be used to provide platform-specific values to a control.

However, since Xamarin.Forms 2.3.4 these APIs have been deprecated and replaced. The Device class now contains public string constants that identify platforms – Device.iOS, Device.Android, Device.WinPhone(deprecated), Device.WinRT (deprecated), Device.UWP, and Device.macOS. Similarly, the Device.OnPlatform overloads have been replaced with the OnPlatform and On APIs.

In C#, platform-specific values can be provided by creating a switch statement on the Device.RuntimePlatform property, and then providing case statements for the required platforms:

double top;
switch (Device.RuntimePlatform)
{
  case Device.iOS:
    top = 20;
    break;
  case Device.Android:
  case Device.UWP:
  default:
    top = 0;
    break;
}
layout.Margin = new Thickness(5, top, 5, 0);

The OnPlatform and On classes provide the same functionality in XAML:

<StackLayout>
  <StackLayout.Margin>
    <OnPlatform x:TypeArguments="Thickness">
      <On Platform="iOS" Value="0,20,0,0" />
      <On Platform="Android, UWP" Value="0,0,0,0" />
    </OnPlatform>
  </StackLayout.Margin>
  ...
</StackLayout>

The OnPlatform class is a generic class that must be instantiated with an x:TypeArguments attribute that matches the target type. In the On class, the Platform attribute can accept a single string value, or multiple comma-delimited string values.

Important

Providing an incorrect Platform attribute value in the On class will not result in an error. Instead, the code will execute without the platform-specific value being applied.

Alternatively, the OnPlatform markup extension can be used in XAML to customize UI appearance on a per-platform basis. For more information, see OnPlatform Markup Extension.

Device.Idiom

The Device.Idiom property can be used to alter layouts or functionality depending on the device the application is running on. The TargetIdiom enumeration contains the following values:

Phone – iPhone, iPod touch, and Android devices narrower than 600 dips^
Tablet – iPad, Windows devices, and Android devices wider than 600 dips^
Desktop – only returned in UWP apps on Windows 10 desktop computers (returns Phone on mobile Windows devices, including in Continuum scenarios)
TV – Tizen TV devices
Watch – Tizen watch devices
Unsupported – unused

^ dips is not necessarily the physical pixel count

The Idiom property is especially useful for building layouts that take advantage of larger screens, like this:

if (Device.Idiom == TargetIdiom.Phone) {
    // layout views vertically
} else {
    // layout views horizontally for a larger display (tablet or desktop)
}

The OnIdiom class provides the same functionality in XAML:

<StackLayout>
    <StackLayout.Margin>
        <OnIdiom x:TypeArguments="Thickness">
            <OnIdiom.Phone>0,20,0,0</OnIdiom.Phone>
            <OnIdiom.Tablet>0,40,0,0</OnIdiom.Tablet>
            <OnIdiom.Desktop>0,60,0,0</OnIdiom.Desktop>
        </OnIdiom>
    </StackLayout.Margin>
    ...
</StackLayout>

The OnIdiom class is a generic class that must be instantiated with an x:TypeArguments attribute that matches the target type.

Alternatively, the OnIdiom markup extension can be used in XAML to customize UI appearance based on the idiom of the device the application is running on. For more information, see OnIdiom Markup Extension.

Device.FlowDirection

The Device.FlowDirection value retrieves a FlowDirection enumeration value that represents the current flow direction being used by the device. Flow direction is the direction in which the UI elements on the page are scanned by the eye. The enumeration values are:

In XAML, the Device.FlowDirection value can be retrieved by using the x:Static markup extension:

<ContentPage ... FlowDirection="{x:Static Device.FlowDirection}"> />

The equivalent code in C# is:

this.FlowDirection = Device.FlowDirection;

For more information about flow direction, see Right-to-left Localization.

Device.Styles

The Styles property contains built-in style definitions that can be applied to some controls' (such as Label) Style property. The available styles are:

BodyStyle
CaptionStyle
ListItemDetailTextStyle
ListItemTextStyle
SubtitleStyle
TitleStyle

Device.GetNamedSize

GetNamedSize can be used when setting FontSize in C# code:

myLabel.FontSize = Device.GetNamedSize (NamedSize.Small, myLabel);
someLabel.FontSize = Device.OnPlatform (
      24,         // hardcoded size
      Device.GetNamedSize (NamedSize.Medium, someLabel),
      Device.GetNamedSize (NamedSize.Large, someLabel)
);

Device.GetNamedColor

Xamarin.Forms 4.6 introduces support for named colors. A named color is a color that has a different value depending on which system mode (for example, light or dark) is active on the device. On Android, named colors are accessed via the R.Color class. On iOS, named colors are called system colors. On the Universal Windows Platform, named colors are called XAML theme resources.

The GetNamedColor method can be used to retrieve named colors on Android, iOS, and UWP. The method takes a string argument and returns a Color:

// Retrieve an Android named color
Color color = Device.GetNamedColor(NamedPlatformColor.HoloBlueBright);

Color.Default will be returned when a color name cannot be found, or when GetNamedColor is invoked on an unsupported platform.

Note

Because the GetNamedColor method returns a Color that's specific to a platform, it should typically be used in conjunction with the Device.RuntimePlatform property.

The NamedPlatformColor class contains the constants that define the named colors for Android, iOS, and UWP:

Android	iOS	macOS	UWP
`BackgroundDark`	`Label`	`AlternateSelectedControlTextColor`	`SystemAltHighColor`
`BackgroundLight`	`Link`	`ControlAccent`	`SystemAltLowColor`
`Black`	`OpaqueSeparator`	`ControlBackgroundColor`	`SystemAltMediumColor`
`DarkerGray`	`PlaceholderText`	`ControlColor`	`SystemAltMediumHighColor`
`HoloBlueBright`	`QuaternaryLabel`	`DisabledControlTextColor`	`SystemAltMediumLowColor`
`HoloBlueDark`	`SecondaryLabel`	`FindHighlightColor`	`SystemBaseHighColor`
`HoloBlueLight`	`Separator`	`GridColor`	`SystemBaseLowColor`
`HoloGreenDark`	`SystemBlue`	`HeaderTextColor`	`SystemBaseMediumColor`
`HoloGreenLight`	`SystemGray`	`HighlightColor`	`SystemBaseMediumHighColor`
`HoloOrangeDark`	`SystemGray2`	`KeyboardFocusIndicatorColor`	`SystemBaseMediumLowColor`
`HoloOrangeLight`	`SystemGray3`	`Label`	`SystemChromeAltLowColor`
`HoloPurple`	`SystemGray4`	`LabelColor`	`SystemChromeBlackHighColor`
`HoloRedDark`	`SystemGray5`	`Link`	`SystemChromeBlackLowColor`
`HoloRedLight`	`SystemGray6`	`LinkColor`	`SystemChromeBlackMediumColor`
`TabIndicatorText`	`SystemGreen`	`PlaceholderText`	`SystemChromeBlackMediumLowColor`
`Transparent`	`SystemIndigo`	`PlaceholderTextColor`	`SystemChromeDisabledHighColor`
`White`	`SystemOrange`	`QuaternaryLabel`	`SystemChromeDisabledLowColor`
`WidgetEditTextDark`	`SystemPink`	`QuaternaryLabelColor`	`SystemChromeHighColor`
	`SystemPurple`	`SecondaryLabel`	`SystemChromeLowColor`
	`SystemRed`	`SecondaryLabelColor`	`SystemChromeMediumColor`
	`SystemTeal`	`SelectedContentBackgroundColor`	`SystemChromeMediumLowColor`
	`SystemYellow`	`SelectedControlColor`	`SystemChromeWhiteColor`
	`TertiaryLabel`	`SelectedControlTextColor`	`SystemListLowColor`
		`SelectedMenuItemTextColor`	`SystemListMediumColor`
		`SelectedTextBackgroundColor`
		`SelectedTextColor`
		`Separator`
		`SeparatorColor`
		`ShadowColor`
		`SystemBlue`
		`SystemGray`
		`SystemGreen`
		`SystemIndigo`
		`SystemOrange`
		`SystemPink`
		`SystemPurple`
		`SystemRed`
		`SystemTeal`
		`SystemYellow`
		`TertiaryLabel`
		`TertiaryLabelColor`
		`TextBackgroundColor`
		`TextColor`
		`UnderPageBackgroundColor`
		`UnemphasizedSelectedContentBackgroundColor`
		`UnemphasizedSelectedTextBackgroundColor`
		`UnemphasizedSelectedTextColor`
		`WindowBackgroundColor`
		`WindowFrameTextColor`

Device.StartTimer

The Device class also has a StartTimer method which provides a simple way to trigger time-dependent tasks that works in Xamarin.Forms common code, including a .NET Standard library. Pass a TimeSpan to set the interval and return true to keep the timer running or false to stop it after the current invocation.

Device.StartTimer (new TimeSpan (0, 0, 60), () =>
{
    // do something every 60 seconds
    return true; // runs again, or false to stop
});

If the code inside the timer interacts with the user-interface (such as setting the text of a Label or displaying an alert) it should be done inside a BeginInvokeOnMainThread expression (see below).

Note

The System.Timers.Timer and System.Threading.Timer classes are .NET Standard alternatives to using the Device.StartTimer method.

Interact with the UI from background threads

Most operating systems, including iOS, Android, and the Universal Windows Platform, use a single-threading model for code involving the user interface. This thread is often called the main thread or the UI thread. A consequence of this model is that all code that accesses user interface elements must run on the application's main thread.

Applications sometimes use background threads to perform potentially long running operations, such as retrieving data from a web service. If code running on a background thread needs to access user interface elements, it must run that code on the main thread.

The Device class includes the following static methods that can be used to interact with user interface elements from backgrounds threads:

Method	Arguments	Returns	Purpose
`BeginInvokeOnMainThread`	`Action`	`void`	Invokes an `Action` on the main thread, and doesn't wait for it to complete.
`InvokeOnMainThreadAsync<T>`	`Func<T>`	`Task<T>`	Invokes a `Func<T>` on the main thread, and waits for it to complete.
`InvokeOnMainThreadAsync`	`Action`	`Task`	Invokes an `Action` on the main thread, and waits for it to complete.
`InvokeOnMainThreadAsync<T>`	`Func<Task<T>>`	`Task<T>`	Invokes a `Func<Task<T>>` on the main thread, and waits for it to complete.
`InvokeOnMainThreadAsync`	`Func<Task>`	`Task`	Invokes a `Func<Task>` on the main thread, and waits for it to complete.
`GetMainThreadSynchronizationContextAsync`		`Task<SynchronizationContext>`	Returns the `SynchronizationContext` for the main thread.

The following code shows an example of using the BeginInvokeOnMainThread method:

Device.BeginInvokeOnMainThread (() =>
{
    // interact with UI elements
});

Device API

Delen via

Xamarin.Forms Device Class

Provide platform-specific values

Device.Idiom

Device.FlowDirection

Device.Styles

Device.GetNamedSize

Device.GetNamedColor

Device.StartTimer

Interact with the UI from background threads

Aanvullende resources

Delen via

Xamarin.Forms Device Class

Provide platform-specific values

Device.Idiom

Device.FlowDirection

Device.Styles

Device.GetNamedSize

Device.GetNamedColor

Device.StartTimer

Interact with the UI from background threads

Related links

Aanvullende resources