App Themes for Xamarin.Forms

This post has been republished via RSS; it originally appeared at: Microsoft Developer Blogs.

All major OSes now support dark and light app themes, and Xamarin.Forms 4.7 has arrived to make this easy to add to your applications. In face, if you do nothing at all, your Xamarin.Forms apps will respect the user's OS preference. Why stop there? You can also customize the light and dark colors used throughout your app UI, and even give the user a choice to control their own app theme. Let's start at the beginning.

Default Platform Colors

When you set no styles or colors, your UI will default to the theme native to the platform the app runs on. For example, look at how this new "Blank App" template looks on iOS:

<StackLayout>
            <Frame BackgroundColor="#2196F3" Padding="36,48,36,36" CornerRadius="0">
                <Label Text="Welcome to Xamarin.Forms!" HorizontalTextAlignment="Center" TextColor="White" FontSize="36" />
            </Frame>
            <Label Text="Start developing now" FontSize="Title" Padding="30,10,30,10" />
            <Label Text="Make changes to your XAML file and save to see your UI update in the running app with XAML Hot Reload. Give it a try!" FontSize="16" Padding="30,0,30,0" />
            <Label FontSize="16" Padding="30,24,30,0">
                <Label.FormattedText>
                    <FormattedString>
                        <FormattedString.Spans>
                            <Span Text="Learn more at " />
                            <Span Text="https://aka.ms/xamarin-quickstart" FontAttributes="Bold" />
                        </FormattedString.Spans>
                    </FormattedString>
                </Label.FormattedText>
            </Label>
        </StackLayout>

Image apptheme default

When you toggle the iOS simulator between dark and light modes (CMD+SHFT+A) you can see the ContentPage background shift from white to black, and the text from black to white. Those are default platform colors. Contrast that with the header which remains blue and the header text that remains white. Those are explicit colors set in code.

Take Control of the Dark

To now control the colors for the dark and light of the head and text, you can replace the static colors with an AppThemeBinding that will react at runtime to the OS theme settings. First enable this preview feature by adding the flag to your App.xaml.cs:

public App()
{
    Device.SetFlags(new string[]{ "AppTheme_Experimental" });

    InitializeComponent();
}

Updating just the header, this looks like:

<Frame BackgroundColor="{AppThemeBinding Dark=#2196F3, Light=#2196F3}" Padding="36,48,36,36" CornerRadius="0">
                <Label Text="Welcome to Xamarin.Forms!" HorizontalTextAlignment="Center" TextColor="{AppThemeBinding Dark=DarkBlue, Light=White}" FontSize="36" />
            </Frame>

Image apptheme explicit

You can of course refactor these to styles like this:

<ContentPage.Resources>
    <Style x:Key="HeaderBg" TargetType="Frame">
        <Setter Property="BackgroundColor" Value="{AppThemeBinding Dark=#1d1d1d, Light=#2196F3}"/>
        <Setter Property="Padding" Value="36,48,36,36"/>
        <Setter Property="CornerRadius" Value="0"/>
    </Style>

    <Style x:Key="HeaderTitle" TargetType="Label">
        <Setter Property="TextColor" Value="{AppThemeBinding Dark=#F1F1F1, Light=White}"/>
        <Setter Property="HorizontalTextAlignment" Value="Center"/>
        <Setter Property="FontSize" Value="36"/>
    </Style>
</ContentPage.Resources>

<Frame Style="{StaticResource HeaderBg}">
    <Label
        Style="{StaticResource HeaderTitle}"
        Text="Welcome to Xamarin.Forms!" />
</Frame>

And if you wish to use predefined color styles, it might look like this:

<Color x:Key="Background_Dark">#1d1d1d</Color>
<Color x:Key="Background_Light">#1d1d1d</Color>
<Style x:Key="HeaderBg" TargetType="Frame">
    <Setter Property="BackgroundColor" Value="{AppThemeBinding Dark={StaticResource Background_Dark}, Light={StaticResource Background_Light}}"/>

Letting the User Choose

Sometime you may want to give the app user control over the theme instead of relying on the OS theme. To do this, you need only to provide a way to set the App.Current.UserAppTheme like this:

App.Current.UserAppTheme = OSAppTheme.Dark;

app themes

Then, to reset the app to respond automatically to OS theme changes, you can set it back to "Unspecified":

App.Current.UserAppTheme = OSAppTheme.Unspecified;

Now create the above experience, add three checkboxes to your UI for default, dark, and light.

<StackLayout Orientation="Horizontal" Spacing="10">
    <CheckBox IsChecked="{Binding UseDeviceThemeSettings}" VerticalOptions="Center" />
    <Label Text="Use device settings"
            VerticalOptions="Center"/>
</StackLayout>

<StackLayout
    IsVisible="{Binding UseDeviceThemeSettings, Converter={StaticResource InvertedBoolConverter}}"
    Orientation="Horizontal"
    Spacing="10">
    <CheckBox IsChecked="{Binding UseDarkMode}" VerticalOptions="Center"/>
    <Label Text="Dark Theme"
            VerticalOptions="Center"/>
</StackLayout>

<StackLayout
    IsVisible="{Binding UseDeviceThemeSettings, Converter={StaticResource InvertedBoolConverter}}"
    Orientation="Horizontal"
    Spacing="10">
    <CheckBox IsChecked="{Binding UseLightMode}" VerticalOptions="Center"/>
    <Label Text="Light Theme"
            VerticalOptions="Center"/>
</StackLayout>

Then add the public properties to the BindingContext for the page, in this case the page handles its own state.

public MainPage()
{
    BindingContext = this;
    InitializeComponent();
}

private bool _useDarkMode;
public bool UseDarkMode {
    get {
        return _useDarkMode;
    }
    set {
        _useDarkMode = value;
        if(_useDarkMode)
        {
            UseLightMode = UseDeviceThemeSettings = false;
            App.Current.UserAppTheme = OSAppTheme.Dark;
        }

    }
}

private bool _useLightMode;
public bool UseLightMode
{
    get
    {
        return _useLightMode;
    }
    set
    {
        _useLightMode = value;
        if (_useLightMode)
        {
            UseDarkMode = UseDeviceThemeSettings = false;
            App.Current.UserAppTheme = OSAppTheme.Light;
        }
    }
}

private bool _useDeviceThemeSettings = true;
public bool UseDeviceThemeSettings
{
    get
    {
        return _useDeviceThemeSettings;
    }
    set
    {
        _useDeviceThemeSettings = value;
        if(_useDeviceThemeSettings)
        {
            App.Current.UserAppTheme = OSAppTheme.Unspecified;
        }
    }

}

The code above handles toggling between dark and light according to the user's preference in the app, and then switching between user preference and OS theme preference.

That's a Wrap

This new dark and light mode theme helper, AppThemeBinding along with UserAppTheme make it really easy to handle theme modes in your Xamarin.Forms apps. This works not only for colors, but images and other resources as well. As shown previously, it even works with the new Shapes and Paths introduced in Xamarin.Forms 4.7 too!

Image apptheme shapes

For more information on app themes, check out the system theme changes documentation. If you want to go even further than dark and light themes, then check out using dynamic resources and even loading themes at runtime to theme your applications.

REMEMBER: these articles are REPUBLISHED. Your best bet to get a reply is to follow the link at the top of the post to the ORIGINAL post! BUT you're more than welcome to start discussions here:

This site uses Akismet to reduce spam. Learn how your comment data is processed.