Android DI – Basic

Ref

This sample’s idea is from Google Android page (Dependency injection in Android)

From This documentation, the way of dependency injection in Android

• Constructor Injection
• Field Injection (or Setter Injection)
• Automated using library -> Dagger
• Service locator

And, if application becomes bigger, this recommends Automated injection. (over 4 screens)

In this entry, explain Non automated ones first.

Constructor Injection

Constructor injection is a constructor has dependency and when create one class, this class has arguments of other dependent class as constructor.

CPU.kt

class CPU {
    fun start() {}
}

Compuer.kt

class Computer(private val cpu: CPU) {
    fun start() {
        cpu.start()
    }
}

MainActivity.kt

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // 1. Constructor DI
        val cpu = CPU()
        val computer = Computer(cpu)
        computer.start()
}

Usage is very simple. Create CPU class first and inject using constructor. If we want to use different CPU, we can create and use. That’s it.

Field type

2nd example is field injection.

Parent has filed of injection target and create class and set using setter

FieldCPU.kt

class FieldCPU {
    fun start() {
    }
}

FieldComputer.kt

class FieldComputer {

    lateinit var cpu : FieldCPU

    fun start() {
        cpu.start()
    }
}

The key is lateinit. lateinit is inject before using.

MainActivity.kt

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val fieldcpu = FieldCPU()
        val fieldComputer = FieldComputer()
        fieldComputer.cpu = fieldcpu
        fieldComputer.start()
}

Service Locator

3rd example is Service Locator

Create injection component by Locator (kind of Factory). This code is in the target class

LocatorCPU.kt

class LocatorCPU {

    fun start() {}
}

LocatorCPU.kt

class LocatorComputer {

    private val cpu = ServiceLocator.getCPU()

    fun start() {
        cpu.start()
    }
}

ServiceLocator.kt

object ServiceLocator {
    fun getCPU(): LocatorCPU = LocatorCPU()
}