Chapter 13. Inter-Process Communication

Android is designed to host a variety of applications and to maximize user choice. The platform is intended to eliminate the duplication of functionality in different applications, to allow functionality to be discovered and invoked on the fly, and to let users replace applications with others that offer similar functionality. Applications must have as few dependencies as possible, and must be able to contract out operations to other applications that may change at the user's discretion.

Inter-process communication (IPC) is thus the basis of key features of the Android programming model. The techniques we'll look at in this chapter are:

Intents

These enable an application to select an Activity based on the action you want to invoke and the data on which they operate. In other words, you don't need a hardcoded path to an application to use its functions and exchange data with it. Data can be passed in both directions using Intent objects, and this enables a convenient, high-level system of inter-process communication.

Remote methods

This feature resembles the remote procedure calls (RPCs) offered by other systems: it makes APIs accessible remotely. Remote objects allow you to make method calls that look "local" but are executed in another process. They involve the use of Android's interface definition language (AIDL).

In this chapter, we will see how these features work and how they can be used in applications.

Android applications could avoid inter-process communication and provide functions in packages loaded by the applications that need them. If applications had to exchange data, they could use the filesystem or other traditional Unix/Linux IPC mechanisms (sockets, shared memory, etc.). But these practices are error prone and hard to maintain. In particular, some of the problems include:

• Libraries are difficult to share among multiple Java processes. Java was designed to have threads, not processes, share common code resources.

• Sharing address space easily leads to errors and inappropriate access to private data.

Consequently, modern programming environments have moved on to more robust component-like systems. Intents and remote methods fit the bill excellently for Android.