Cross-Platform OS Theme APIs

[jfversluis]

Description of Change

Adds the cross-platform APIs to support different OS Themes

Background

See #9804 for the relevant APIs and how to use it. Because of timing I have removed the dependency on Essentials from this (see #9926) so we can get this merged in time for 4.6. After that, we can easily swap out this implementation for the Essentials one. By keeping naming and enum values, etc. the same this should be an easy task.

Issues Resolved

• related to [Spec] OS Themes (Dark Mode support) #9804

API Changes

Added:

• public enum AppTheme { Unspecified, Light, Dark }
• public class AppThemeColor
• public class OnAppTheme
• AppTheme Application.RequestedTheme { get; }
• public event EventHandler Application.RequestedThemeChanged;
• public void Application.OnRequestedThemeChanged(AppThemeChangedEventArgs args)
• public class AppThemeChangedEventArgs
• protected virtual void OnRequestedThemeChanged(AppTheme newValue)

Platforms Affected

• Core/XAML (all platforms)
• iOS
• Android
• UWP
• MacOS
• WPF
• Tizen

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

I have added a gallery page which shows (nearly) all ways to use this new functionality. Check if that works as expected. Whenever the values are used as a DynamicResource they should update whenever the theme is changed while the app is running. Check out the gallery page or, to take it a step further, download the NuGets and take it for a spin in your own (sample) app.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[jsuarezruiz]

I have done some tests. If you change the system theme, when initializing a page we initialize OnAppTheme and load the correct theme color. However, if we move the App to the background, change the theme and return to the App, the change will not be detected until navigating, etc.

Still missing to implement an event to detect change in appearance mode, right?. Is the PR in progress or are we going to divide the Spec into different PRs?.

[jsuarezruiz]

The failed tests seem unrelated.

[StephaneDelcroix]

only reviewed the Core part, some minor remarks here and there

[StephaneDelcroix]

are we sure we won't need to extend this list ? if so, we might want to use strings...

[MagicAndre1981]

@StephaneDelcroix Essentials defined it first in this way, so I think this was done to keep this in sync

[jfversluis]

Indeed follow the Essentials way for now. I'd like to think this is like the OS level theming and on top of that people can have their own custom theming, probably as an extension of this (naming will be a hell...). But if you implement your own theme as a user, you still might want to also follow the light and dark mode by the OS and show different colors based on that.

[StephaneDelcroix]

this definitely doesn't look like a BindableObject. It's never parented, so it probably never gets a BindingContext set...

[jfversluis]

Because I'm using bindable properties? Should I be using something different?

[StephaneDelcroix]

not a BO. look at OnPlatform, OnDevice

[StephaneDelcroix]

also, you might want a xaml markup language extension

[jfversluis]

Because I'm using bindable properties? Should I be using something different? And there is an extension as well; https://github.com/xamarin/Xamarin.Forms/pull/9958/files#diff-c65a7e8e0eed508c58cd0802b61b2cb1

[StephaneDelcroix]

I'd like to avoid doing that

[jfversluis]

In the latest changes it has at least changed to internal. I know it's not pretty and open to alternatives. But the problem is that the actual value doesn't really change. Depending on the theme the implicit operator will return a different color and that is why it's not picked up normally.

The only time this happens is when the theme changes on OS level and then a number of expensive operations already happen to redraw the whole screen already.

[PureWeen]

#10202