Wallpaper, Screenshots, and Application Switching

To get around some of the Today Screen API limitations (see this post for a Today Screen rant), I've played around quite a bit with programatically switching the wallpaper on a WM device. However, there a number of problems with this approach, most significantly: changing the device wallpaper is slow and once the device has been signaled that the wallpaper has been updated, it does not actually redraw the wallpaper until the desktop is visible (e.g., the desktop is the top level window). For more about this issue, see my .NET CF newsgroup post. Nonetheless, changing the device wallpaper is one of the easiest ways to display ambient information.

A project at Intel Research required that a screenshot be taken of a specified application, cropped, and displayed as the wallpaper on the device. However, as mentioned above, setting the wallpaper on a user's device can be an intrusive operation--not only can it take between 10-30 seconds but the wallpaper update does not actually happen until all applications are minimized making the desktop visible.

I created a little test application which fulfills our requirements and attempts to avoid disrupting the user by waiting to execute until the device is idle. Here's how it works:

1. Waits for device idle
2. Finds handle to specified application (based on its window title)
3. Switches from current app to specified application (saving current foreground window handle)
4. Takes screenshot
5. Crops screenshot (if necessary)
6. Sets background wallpaper to screenshot
7. Minimizes all open applications so phone background refreshes
8. After background refresh, restores state of open applications
9. Returns to saved foreground window handle
10. Puts device back to sleep

Yes, it's hacky but I could think of no better way to do this. Note that in this case it was not possible to create a Today Screen plug-in.

Download the source code and the binary. You should be able to compile and deploy as long as you have the Smart Device Framework 2.0 from the OpenNETCF team installed. This sample will probably work with the SDF 2.1 as well but I haven't tested it. Alternatively, you can download the binary zip file and add those .dll references to the source code project. I've also added this sample code to the samples directory in Roam (see SourceForge). Roam is a Windows Mobile open source library which provides functionality like a SQL connection pool, power management functions, and a whole slew of UI code.

Note: this code has been tested on the Cingular 2125 and the T-Mobile SDA device. In addition, I also ran it on the Cingular 8125 Pocket PC Phone and everything worked except for the wallpaper changing code--this is because Roam's wallpaper changer methods only work on the SmartPhone.