So the Windows Phone reviews are coming out and I keep reading complaints that the apps have to restart when the phone is unlocked. That complaint is confused for a lack of multitasking. Not the case. By default Microsoft has set a line of code to deactivate idle applications and the OS will detect an idle application if the lock screen is disabled unless the dev changes one line of code:
When application idle detection is enabled, the operating system will deactivate idle applications. Disabling application idle detection requests that the operating system not perform this service. This feature should be used with caution as applications that run while the phone is locked continue to drain the device’s battery. In this case, the user may not realize that the application is running because the screen is off and the phone looks idle.
In the current release, an application is considered to be idle if the user has allowed the phone to lock. In future releases, the conditions that qualify an application as idle may change. Even if application idle detection is disabled, the operating system may deactivate an application for other reasons, such as a depleted battery. For more information on using this feature, see Idle Detection for Windows Phone.
Here are the details from MSDN for devs to use to disable app idle detection.
The MS certification requirements cover this as well:
6.3 Applications Running under a Locked Screen
An application in the foreground can continue to run when the phone screen is locked by setting the PhoneApplicationService.ApplicationIdleDetectionMode property.
An application that is running under a locked screen must meet the following requirements:
6.3.1 Configurable Functionality
In the UI or settings menu, an application must provide the user with the ability to prevent the application from running under a locked screen.
6.3.2 Launch Configuration of Running under a Locked Screen
On first use of ApplicationIdleDetectionMode, an application must ask the user for explicit permission to run under a locked screen.
Microsoft recommends that these applications minimize power usage as much as possible while running under a locked screen.
In other words, if an app needs to restart after the lock screen is enabled, the dev needs to be asked to change the code. So don’t blame the OS or the phone. This is done to protect battery life so apps don’t run in the background and some apps that should continue to run in the background can take advantage of this. It is not a multitasking limitation though.