How does the operating system prioritize applications?

The Android operating system uses various heuristics and mechanisms to determine process priority, primarily based on the application's activity state and its components.

Main application states affecting priority:

• Foreground: The application actively interacts with the user or performs visible tasks. This has the highest priority. Examples: Active Activity, launched foreground service, attached Input Method.
• Visible: The application is visible to the user but not actively interacting. Examples: Application with a visible Activity, partially covered by a dialog, or a launched Bound Service.
• Service: The application performs background operations not visible to the user. Priority is lower than Visible. Examples: A running Service.
• Cached: The application is in memory but not performing active tasks and not visible to the user. Considered the lowest priority and may be terminated by the system to free resources.

Factors influencing priority:

1. Process state: The main factor. Determined based on the application's active components in this process.
2. Importance: The system assigns an importance level to each process.
   • Foreground process: Highest priority. The system tries to keep it at all costs.
   • Visible process: Slightly below Foreground but still high. Terminated only when resources are needed for Foreground processes.
   • Service process: A process running startService(). Terminated if the system needs resources for more important processes.
   • Cached process: A process with no active components. The lowest priority. Terminated when memory is low.
3. Recent usage: Recently used applications have a slightly higher priority within the same importance level.
4. Dependencies: If process A uses a component from process B (e.g., Bound Service), then process B's priority is increased.

The system monitors the current importance of each process and tries to keep the most important processes in memory. When memory is needed, it terminates processes in reverse order of importance: first Cached, then Service, Visible, and finally Foreground.

Example:

// Example foreground service
ContextCompat.startForegroundService(this, Intent(this, MyForegroundService::class.java))