This document discusses common patterns for implementing Android services using AIDL for inter-process communication. It begins by introducing services and AIDL, then discusses the challenges of using AIDL across processes. It presents two common patterns - the "lock" pattern for exclusive access to shared resources, and a "broadcast" pattern for simultaneous access. The lock pattern implementation uses worker threads and locks to serialize access. The broadcast pattern maintains listener lists to share updates with multiple clients.

1. Android Service Patterns
AIDL Services Aren’t That Hard…
Shree Kumar
InnoMinds Software

2. Motivation
• Writing services is useful & fun
– Not to mention, painful !
• Documentation is present, but…
– Scattered
– Deals mostly with mechanics
• Can a 45 minute session make a difference ?
Android Service Patterns | DroidCon India 2011

3. Objectives
• Clarification on the term “Patterns”
– NOT design patterns a-la Grady Booch!
• This talk gives you
– Quick overview of AIDL based Services
– Features & Pains
– Writing useful services as patterns
Android Service Patterns | DroidCon India 2011

4. Intro to Android Services
• The “Service” concept
– Application component
– Typically used for long-running operations
– Does not automatically create
• new processes or threads
• Can be roughly classified based on
– Interaction Mechanism : Started, Bound
– Application Boundaries : In-process, IPC
Android Service Patterns | DroidCon India 2011

5. AIDL based IPC Services
• Android Interface Definition Language
• Inter-Process Communication to
– Expose interfaces to other applications
• More complicated than “Messenger”
Android Service Patterns | DroidCon India 2011

6. Why all the pain?
• “natural” interactions with the service
– Calls & callbacks
– With complex objects as parameters
• And all these across process boundaries!
Android Service Patterns | DroidCon India 2011

7. Hello, AIDL Service!
• Implements onBind()
– Exposes interface by implementing stubs
• Exports via AndroidManifest.xml
• Client apps bind via Intents
– Use interface methods
– Callbacks require app to implement stubs
corresponding to callback interface
Android Service Patterns | DroidCon India 2011

8. What’s not-so-nice there ?
• No control over binding
– onBind() return value is cached!
• No control over threading
• Code messier than in-process interface
– No support for exceptions
– Dead remote object exceptions => messy code
– Callbacks not invoked in UI context
• No concept of versioning
Android Service Patterns | DroidCon India 2011

9. Common Service Patterns
• Exclusive access to shared resource
– The “lock” pattern
• Simultaneous access to shared resource
– “serialize”, “broadcast” patterns
• Less common
– Service versioning
– Multiple objects of the same type
Android Service Patterns | DroidCon India 2011

10. The “lock” pattern
• Exclusive access using “locks”
– Take lock before use
– Release lock after use
• Looks simple ?
– Multi-app access
– Appropriate threading model
– Improper app lock handling => starvation
Android Service Patterns | DroidCon India 2011

11. Locked Service Implementation
• Associate locks with calling process
• Worker thread for serialization
– Started on service creation
– Producer-consumer modeling
• Java.util.concurrent provides ample support
• Partial solution for starvation
– Evict clients on DeadObjectException
Android Service Patterns | DroidCon India 2011

12. Shared Access Service
• “broadcast” method
– Shares a state update with multiple listeners
– Broadcast receivers don’t make the cut
• Can’t bind a Service inside them
• Implementation
– Reuse “listener” concept
• Clients add/remove listeners
– Service maintains list of listeners
• Updates listeners when needed
• Removes dead objects
– Reference : MindTheRobot Android Architecture Tutorial
Android Service Patterns | DroidCon India 2011

13. Questions ?
Shree Kumar
shree.shree@gmail.com
http:://www.shreekumar.in/