Change Background of WP7 Application if Theme is altered

I need to change the Background Image of my Application if user changes theme from "Light" to "Dark" or vice-vesa in code behind. I hope these should be done in Page Loded even开发者_如何学Ct

@TimDams pointed you to one of the nice ways to detect what-theme-is-now-set, but I didn't notice there any information how to detect a change to the theme during the application runtime. The user could start your app, then bump forward to the menu, change the theme, and get back to your app. While you may think that your app will be tombstoned and then restarted and renavigated to your page with full cycle with all pageloads -- it is not 100% true.

Firstly, the PageLoaded is NOT a good place to do the initial check-and-set-styles, because, if you get that event called, then the page, probably, has already rendered once. If I remember well, the PageLoaded is invoked just after the first render. If this is true, then you will have to detect the colors earlier, for example in the LayoutUpdated (warning: this event is a great spammer. I mean, it gets called gazillions times. Attach a single-shot handler, you know, such that will instantly unattach-self on first invocation). Maybe you will be able to do it in the Page's .ctor, just after InitializeComponent. Or in the OnApplyTemplate or MeasureOverride, or at least ArrangeOverride -- the visuals should be mostly/fully available there.

Buuuut. I've intentionally 'bolded-out' the word "initial". With Mango, there's some multitasking getting more and more common, but even the pre-Mango 7.0 version does not guarantee that your app will be tombstoned. From my observations in early 7.0, for example, starting the MediaPlayer from WebBrowser component does not tombstone your app:) If you have some time to read, check WP7 recover from Tombstone and return to page for details on the "pause" vs "tombstone".

Anyways, if your app gets "paused" and the user switches themes in the meantime, I think (I've NOT checked) that your page will (in most of the cases) be just temporarily hidden and upon returning to the screen, it will probably not be re-created and will not be re-(Page)Loaded. If it is true, then you will have not so easy problem to solve, because your app may be paused, the OS rethemed, and your app unpased virtually in any moment of time and the only events you will get in the mean time are .... global events of App.Deactivated and App.Activated. It is possible that completely none of per-page events will fire [but I've not checked - before you do anything I suggest below, CHECK IT].

If this pessimistic view is really true, than in those events, you will have to detect the current theme (->Tim's post), then somehow inform your current Page that themes changed - or not. If you have your ViewModel at least a bit separated from the rest of the app (as it should be:) ) you have an easy option to do it: create in that ViewModel a set of properties (dp or inotif) like Brush Background, Brush Foreground, Brush Hightlight, and other that you need, and instead of harcoding colours in your XAML - bind to those properties. You may event want to create a separate class for all those many Brushes and other Styles, let's say "pub class MyCurrentAppTheme" and keep that props there, and expose such object from ViewModel - whatever. Just Bind your colors to whatever -- but whatever that will be "logically global" and that will be easily accessible from the App.Acticated event handler. Having that done, in the App.Activated, detect the current theme and if changed, so through all the colors kept in VM and set them appropriatelly. Voil'a, whole your App gets recoloured properly.

But mind that still - there might be some transient blinks and flashes between the rendering of cached old theme, refreshing databound objects, and redrawing fresh theme. I hope not, but I sense it may occur, especially when returning from fast-switch tool (long back press): I think that the device captures the 'last screenshot' of your app in the backbuffer and uses that throughout all the time the app is 'minimalized' to do transitions animations, to show the fast-app-switch overview and so on.. again, I've not checked, but I doubt that during such animations the pagecontents are 'live', it could be very demaning on CPU/GPU resources. Any one knows anything on that? It could be a nice test to have some looping animation on the page and then to switch out and check in the fast-switch overview, whether the animation moves or is halted!:)

If your application is tombstoned, all your controls will be recreated and the new theme will be applied. If you'd like to manage your light/dark specific styles in a similar vein to normal themes, you might want to take a look at a custom ResourceDictionary I blogged about a while back.

Unfortunately, as of Mango there, is a bug (?) related to fast application switching that causes the theme to remain unchanged in your application. The bug is outlined in this question and its linked posts:

Is there a bug when changing themes when app is deactivated and reactivated in Windows Phone Mango

My ResourceDictionary is still useful for the initial startup but it appears, unfortunately, that nothing can be done to workaround the Mango bug.

For this no event exists. You need to figure this out manually by comparing the PhoneBackgroundBrush color to see if the user has the dark or light theme and compare it with your last stored value.

Were you able to do some test on the App.Activate - Deactivate?

I decided to use a different path to solve the issue of dynamic theme change.

I've assigned to all text and buttons only system resource colors. For the icons inside the buttons in the window instead of using PNG images-icons I used the in XAML and assing it a Foreground color from system resource.

For the buttons in the SystemMenuBar there is no issue as they are always on a dark gray background so the black PNG images work just fine.

Hoping this helps.

You can check if the dark theme is in use with this simple check:

public static bool CurrentThemeIsDark
{
  get
    {
      return (Visibility)Application.Current.Resources["PhoneDarkThemeVisibility"] == Visibility.Visible;
    }
}