Users expect some kinds of apps to run under a locked screen. These apps include music players, mapping apps, stop watches, and so on.
In the first release of the Windows Phone OS, running under the lock screen was favorable to apps that wanted to avoid being tombstoned. These were apps that were slow to load or relied on complex state models. This was alleviated, however, with the introduction of fast application switching in Windows Phone 7.5. Now apps are placed in a dormant state and remain in memory.
Running under the lock screen is not an alternative to implementing efficient transient state persistency. Recall that an app may still be tombstoned if the phone runs low on memory.
The following steps ...