Inversion of Control

An abstract principle describing an aspect of some software architecture designs in which the flow of control of a system is inverted in comparison to procedural programming

Huh?

Inversion of Control

A chunk of code that uses, or consumes, another chunk of code is in control of the process, not implementation

Inversion of Control

Purpose

Decouple Your App

• Dependency Contracts
• Allows for interchangeable code
• Encapsulation of modules

Facilitate Testing

• Dependencies with assertions
• Mock services
• Skip over implementation
• True unit testing

Inversion of Control

Dependencies

Constructor Injector

Dependencies are provided via constructor arguments

Example

interface ISettingsProvider {
    getSetting<SettingType>(settingKey: String): SettingType
}

class TestSettingsProvider : ISettingsProvider {
    getSetting<SettingType>(settingKey:String) =
        when(settingKey) {
            SETTING_IS_PRIVATE -> true
            SETTING_ACCOUNT_NAME -> "MyUser"
            SettING_SHARE_LOCATION -> false
        }
}

class AccountSettingsActivity(val settingsProvider: ISettingsProvider) {
    ...
    fun initialize() =
        settingsProvider.run {
            username = getSetting<String>(SETTING_ACCOUNT_NAME)
            privacy_toggle = getSetting<Boolean>(SETTING_IS_PRIVATE)
            share_location = getSettings<Boolean>(SETTING_SHARE_LOCATION)
        }
    ....
}

Property Injection

Dependencies are set via accessors

Example

interface ISettingsProvider {
    getSetting<SettingType>(settingKey: String): SettingType
}

class TestSettingsProvider : ISettingsProvider {
    getSetting<SettingType>(settingKey:String) =
        when(settingKey) {
            SETTING_IS_PRIVATE -> true
            SETTING_ACCOUNT_NAME -> "MyUser"
            SettING_SHARE_LOCATION -> false
        }
}

class AccountSettingsActivity() {
    var settingsProvider: ISettingsProvider? = null
    ...
    fun initialize() =
        settingsProvider?.run {
            username = getSetting<String>(SETTING_ACCOUNT_NAME)
            privacy_toggle = getSetting<Boolean>(SETTING_IS_PRIVATE)
            share_location = getSettings<Boolean>(SETTING_SHARE_LOCATION)
        }
    ....
}

Service Locator

Locates a service, usually by type or name

Example

interface IRESTServiceLocator {
    resolve(serviceClass: KClass<IRESTService>): IRestService
}

interface IRetrieveUserService : IRESTService {
    ...
}

class RetrieveUserService_v1 : IRetrieveUserService {
    getCurrentUser(): User?
    getUser(userId: Long): User?
}

class UserManager(val serviceLocator: IRESTServiceLocator) {
    fun setUsername(username: String) = 
        serviceLocator.resolve(IRetrieveUserService::class)
                      .getCurrentUser()
                      .setUsername(username)
    ...
}

Inversion of Control

Containers

A Container is...

A framework, of sorts, that can create a given type with all necessary dependencies

Roll Your Own

class PreferenceWrapper : IPreferenceProvider {
    constructor(sharedPreferences: SharedPreferences) {...}
    constructor(mockPreferences: MockPreferences) {...}

    fun getBoolean(prefName: String): Boolean { ... }
    fun getString(prefName: String): String { ... }
}

interface ISingletonScope {
    fun getApplicationContext(): Context
    fun getPreferences(): IPreferenceProvider
}

class ApplicationContainer(private val context: Context) 
        : ISingletonScope {
    private lateinit var preferences: SharedPreferences

    init {
        preferences = 
            PreferenceWrapper(
                PreferenceManager.getDefaultSharedPreferences(context))
    }

    fun getApplicationContext() = context

    fun getPreferences() = preferences
}

Roll Your Own

interface IConfigurationScope {
    fun getUserSettings(): UserSettings
    fun getPushNotificationSettings(): NotificationSettings
}

class PreferenceConfigurationContainer(private var singletons: ISingletonScope) 
        : IConfigurationScope {
    fun setSingletons(singletonScope: ISingletonScope) {
        singletons = singletonScope
    }

    fun getUserSettings() = with(singletons.getPreferences()) {
        UserSettings(getString(PREF_USERNAME), getString(PREF_PASSWORD))
    }

    fun getPushNotificationSettings() = with(singletons.getPreferences()) {
        NotificationSettings(getBoolean(PREF_PUSH_ENABLED), 
                             getBoolean(PREF_PUSH_VIBRATE), 
                             getString(PREF_PUSH_SOUND))
    }
}

Dagger

IoC Container Builder