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https://github.com/afollestad/ulfberht
🗡️ A small but powerful & opinionated DI library. Written in Kotlin, and powered by annotation processing.
https://github.com/afollestad/ulfberht
annotation-processor dependency-injection di kotlin lifecycle viewmodel
Last synced: 9 days ago
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🗡️ A small but powerful & opinionated DI library. Written in Kotlin, and powered by annotation processing.
- Host: GitHub
- URL: https://github.com/afollestad/ulfberht
- Owner: afollestad
- License: apache-2.0
- Archived: true
- Created: 2019-07-15T18:13:00.000Z (over 5 years ago)
- Default Branch: main
- Last Pushed: 2023-01-05T22:10:34.000Z (almost 2 years ago)
- Last Synced: 2024-08-01T19:46:41.502Z (4 months ago)
- Topics: annotation-processor, dependency-injection, di, kotlin, lifecycle, viewmodel
- Language: Kotlin
- Homepage: https://af.codes
- Size: 672 KB
- Stars: 251
- Watchers: 10
- Forks: 8
- Open Issues: 6
-
Metadata Files:
- Readme: README.md
- Funding: .github/FUNDING.yml
- License: LICENSE.md
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- awesome-list - afollestad/ulfberht - 🗡️ A small but powerful & opinionated DI library. Written in Kotlin, and powered by annotation processing. (Kotlin)
README
Note: Dagger2 and [Anvil](https://github.com/square/anvil) should be used in production apps.
This is now mostly just an experiment with kapt and Kotlinpoet,
IR should be used rather than Kapt these days.
# Ulfberht
[  ](https://repo1.maven.org/maven2/com/afollestad/ulfberht)
[](https://github.com/afollestad/ulfberht/actions?query=workflow%3A%22Android+CI%22)
[](https://opensource.org/licenses/Apache-2.0)
> The Vikings were among the fiercest warriors of all time. Yet only a select few carried the
> ultimate weapon of their era: the feared Ulfberht sword. Fashioned using a process that would
> remain unknown to the Vikings' rivals for centuries, the Ulfberht was a revolutionary high-tech
> tool as well as a work of art.
*A little more bad-ass than a Dagger.* Dependency injection is a technique in which an application
supplies dependencies of an object. "Dependencies" in this context are not dependencies like in a
Gradle file. Dependencies are services (i.e. APIs, classes) that are needed in certain parts of
your code.
---
# Table of Contents
1. [Why Choose Ulfberht?](#why-choose-ulfberht)
2. [Gradle Dependency](#gradle-dependency)
3. [Modules](#modules)
1. [Binding](#binding)
2. [Providing](#providing)
3. [Singletons](#singletons)
4. [Qualifiers](#qualifiers)
4. [Components](#components)
1. [Basics](#basics)
2. [Child Components](#child-components)
4. [Scoping](#scoping)
5. [Putting Modules and Components to Use - Injection](#putting-modules-and-components-to-use---injection)
6. [Runtime Dependencies](#runtime-dependencies)
7. [Android](#android)
1. [ScopeOwners](#scope-owners)
2. [ViewModels](#viewmodels)
---
# Why Choose Ulfberht?
I wrote Ulfberht as an experiment to see if I could make [Dagger](https://github.com/google/dagger)
style dependency injection a bit easier, and quicker to pickup for newbies. I wanted something
lightweight, with less Boilerplate code and more automation. I wanted something annotation-processor
based rather than reflection-based, while still being written in Kotlin. And I wanted better built-in
scoping support, especially on Android. _This is the result._
You may be wondering what makes this library different than KOIN or other Kotlin "DI" libraries?
Libraries like KOIN are just service locators - you need to build the dependency graph manually,
filling in constructors, etc. Annotation processor based DI libraries handle this for you with
code generation, so you *don't* need to write boilerplate and you *don't* need to use reflection.
---
# Gradle Dependency
```gradle
dependencies {
implementation "com.afollestad:ulfberht:0.6.0"
kapt "com.afollestad:ulfbert-processor:0.6.0"
}
```
---
# Modules
A module is a class that gives instructions for dependency instantiation.
### Binding
One way a module can instruct object instantiation is through binding. If you've used Dagger, this
should be straightforward.
In this example below, we bind an interface with a concrete implementation. Whenever you inject
`Demo`, you inject the `DemoImpl` implementation of it.
Taking this interface and implementation...
```kotlin
interface Demo {
fun myMethod()
}
class DemoImpl : Demo {
override fun myMethod() {
...
}
}
```
...they can be bound in a `@Module` interface:
```kotlin
@Module
interface DemoModule {
@Binds
fun demoClass(impl: DemoImpl): Demo
}
```
---
If `DemoImpl` itself had dependencies in its constructor, those must be bound or provided as well
so that they can be injected too...
```kotlin
interface SomethingElse
class SomethingElseImpl : SomethingElse
interface Demo {
fun myMethod()
}
class DemoImpl(val somethingElse: SomethingElse) : Demo {
override fun myMethod() {
...
}
}
```
...with a module setup like this:
```kotlin
@Module
interface DemoModule {
@Binds
fun demoClass(impl: DemoImpl): Demo
@Binds
fun somethingElse(impl: SomethingElseImpl): SomethingElse
}
```
### Providing
Another way a module can instruct object instantiation is through providing. This should also be a
familiar concept for Dagger users.
Providing is more flexible than binding. You tell the library how instantiation should happen, and
what you provide _does not_ need all of its constructor parameters to be injectable. Notice that
a module which can use `@Provides` must be a `abstract class` rather than an `interface`.
```kotlin
class SomethingElse
class Demo(val somethingElse: SomethingElse)
@Module
abstract class DemoModule {
@Provides
fun demoClass(): Demo {
val somethingElse = SomethingElse()
return Demo(somethingElse)
}
}
```
`@Provides` methods can have parameters, which the library will fill from the dependency graph
as well. This could be a dependency provided in another module or even another component's module.
```kotlin
class SomethingElse
class Demo(val somethingElse: SomethingElse)
@Module
interface DemoModule {
@Provides
fun demoClass(somethingElse: SomethingElse): Demo {
return Demo(somethingElse)
}
@Provides
fun somethingElse(): SomethingElse {
return SomethingElse()
}
}
```
### Singletons
There's a `@Singleton` annotation that can be used to mark `@Binds` and `@Provides` methods. When
it's used, a module in a specific component will hold the same instance of the provided object until
the module is destroyed. *If you were to use the same module in two different components,
`ComponentA` and `ComponentB`, each component would have a separate singleton instance of what
you're providing. They wouldn't share between each other.*
```kotlin
@Module
interface DemoModule1 {
@Binds @Singleton
fun demoClass1(impl: Demo1Impl): Demo1
}
@Module
abstract class DemoModule2 {
@Provides @Singleton
fun demoClass2(): Demo2 {
return Demo2Impl()
}
}
```
Every time injection pulls `Demo1` and `Demo2`, it will be the same cached instances.
---
# Components
A component is a class that takes a set of [modules](#modules), and knows how to inject what they
collectively bind/provide into a target object.
### Basics
A basic component looks like this:
```kotlin
@Component(modules = [DemoModule::class])
interface DemoComponent {
fun inject(target: SomeClass)
}
```
You could include multiple items in the array of the `@Component`'s `modules` parameter.
You can also define a void (no return type) `inject` method for every class that the component
can inject into.
### Child Components
In a real application, you'd probably have a hierarchy of components. Components operate at a
certain level - for an example you could have a component that's alive for the entire lifetime of
the application, while you would have more short-lived components that are alive when specific
screens of the application are. *You build this hierarchy by assigning child components.*
When you inject something at the bottom of the chain, you're able to inject things that are
bound/provided throughout the chain all the way up to the top.
---
A component hierarchy is built by assigning *children* to components. The code below mimics the
diagram above.
```kotlin
@Component(
children = [Component2::class, Component3::class],
modules = [Module1::class, Module2::class]
)
interface Component1
@Component(
children = [Component4::class, Component5::class],
modules = [Module3::class, Module4::class]
)
interface Component2
@Component(
children = [Component6::class, Component7::class],
modules = [Module5::class, Module6::class]
)
interface Component3
@Component(modules = [Module7::class, Module8::class])
interface Component4
@Component(modules = [Module9::class, Module10::class])
interface Component5
@Component(modules = [Module11::class, Module12::class])
interface Component6
@Component(modules = [Module13::class, Module14::class])
interface Component7
```
### Scoping
In many applications, especially mobile applications, you generally do not want to keep things around
for the entire lifecycle of the app.
Say you're on a login screen, and need to inject an authentication dependency -- you probably only
need access to that object on the login screen. Once you leave, that authenticator should go away.
Scoping allows you to achieve this easily.
---
`Scope`'s are associated with components. When a scope is exited, components in that scope destroy
themselves along with their modules, and anything that the modules may be storing. To scope a component,
there's a simple parameter to add on the `@Component` annotation.
```kotlin
// Using constants is encouraged so you can share values throughout your app.
const val FIRST_SCOPE = "first scope"
@Component(
scope = FIRST_SCOPE,
modules = [MyModule::class]
)
interface MyComponent
```
You can then retrieve an instance of this scope. There are a few things you can do with it:
```kotlin
val scope: Scope = getScope(FIRST_SCOPE)
// Hook into its lifecycle, which currently is just an exit event.
scope.addObserver(object : ScopeObserver {
override fun onExit() {
...
}
})
// You can tell the scope to exit, destroying its children.
scope.exit()
```
When you use the `component<>()` method to retrieve a component with a scope, that component is
attached to its scope. There can be multiple components in a scope.
When you call `exit()` on that scope, every component attached to it is destroyed. All modules in
those components go with them, along with any stored singletons amd child components. The next time
you call `component<>()` for a destroyed component, a new instance is created.
---
If you use parenting _and_ scoping together:
```kotlin
const val PARENT_SCOPE = "i'm a parent"
@Component(
scope = PARENT_SCOPE,
children = [ChildComponent::class],
modules = [Module1::class]
)
interface ParentComponent
@Component(modules = [Module2::class])
interface ChildComponent
// This would destroy both components
getScope(PARENT_SCOPE).exit()
```
*Parent components will destroy all of their children (components and their modules) as well.*
---
# Putting Modules and Components to Use - Injection
To perform injection, you need to retrieve the component that's able to inject into your target.
```kotlin
@Component(...)
interface Component5 {
fun inject(activity: MyActivity)
}
class MyActivity : AppCompatActivity() {
@Inject lateinit var someDependency: NeededClass
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
// Perform injection, `SomeComponent` would have an inject() method defined for `MyActivity`
component().inject(this)
// Use the injected dependency!
someDependency.helloWorld()
}
}
```
This code assumes that one of the modules going up the graph from `Component5` can supply
`NeededClass` with a `@Binds` or `@Provides` method.
---
# Qualifiers
Qualifiers are simple identifiers that associate a type with a very specific bound or provided
instance. A qualifier is a special type of annotation, which is defined like this:
```kotlin
@Qualifier
annotation class DemoQualifier1
@Qualifier
annotation class DemoQualifier2
```
You use it to mark `@Binds` and `@Provides` functions:
```kotlin
@Module
interface DemoModule1 {
@Binds @Singleton @DemoQualifier1
fun demoClass(impl: Demo1Impl): Demo1
}
@Module
abstract class DemoModule2 {
@Provides @Singleton @DemoQualifier2
fun demoClass(): Demo1 {
return Demo1Impl()
}
}
```
Then, you can mark constructor parameters with it...
```kotlin
class SomeInjectedClass(
@DemoQualifier1 val someDependency: Demo1,
@DemoQualifier2 val anotherDependency: Demo1
) {
...
}
```
...along with `@Inject` targets (the `field:` prefix on the annotation name is important in Kotlin):
```kotlin
class SomeClass {
@Inject @field:DemoQualifier1
lateinit var someDependency1: Demo1
@Inject @field:DemoQualifier2
lateinit var someDependency2: Demo1
init {
component().inject(this)
}
}
```
You will get two completely separate instances of `Demo1`, since two different qualifiers are being
used. `@Singleton` was applied for demo purposes to show that it'll store two different instances.
But even without that, you're providing two different things. This could be useful if you were
providing primitives, like strings, or an interface for something like preferences. There's a lot of
possibilities.
---
# Runtime Dependencies
Sometimes your app may need to be able to inject something that is defined at runtime, something
that cannot be constructed in a module. A good example of when this would be necessary is in
an Android application, like if you needed to inject the Application context.
First, you tag constructor parameters or fields that need to be provided at runtime with a
qualifier annotation, which is discussed in [Qualifiers](#qualifiers) above.
```kotlin
@Qualifier
annotation class AppContext
@Qualifier
annotation class ApiKey
class StringRetriever(
@AppContext val appContext: Context,
@ApiKey val apiKey: String
) {
fun getString(@IdRes res: Int): String {
return appContext.resources.getString(res)
}
}
```
At injection time, you pass mapped runtime dependencies into the `component<>()` method. They are
available for injection until the component is destroyed, or its parents destroy it.
```kotlin
class LoginActivity : AppCompatActivity() {
@Inject
lateinit var stringRetriever: StringRetriever
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
component(
AppContext::class to applicationContext,
ApiKey::class to "hello, world!"
).inject(this)
}
}
```
Runtime dependencies in a component are made available to all of the component's children too.
In an Android application, providing the application context at the `Application` level will make
it available to all Activities and Fragments that use child components.
---
# Android
### ScopeOwners
On Android, you can automatically attach scopes to `LifecycleOwner`'s, such as:
* `Fragment` (from `androidx.app`)
* `AppCompatActivity`/`FragmentActivity`
* `androidx.lifecycle.ViewModel`
_(these all implement the `LifecycleOwner` interface)_
---
First, setup your components and modules as you would normally:
```kotlin
object ScopeNames {
const val LOGIN_SCOPE = "scope_login"
const val MAIN_SCOPE = "scope_main"
}
@Component(
children = [LoginComponent::class, MainComponent::class],
modules = [AppModule::class]
)
interface AppComponent {
fun inject(app: Application)
}
@Component(
scope = ScopeNames.LOGIN_SCOPE,
modules = [LoginModule::class]
)
interface LoginComponent {
fun inject(activity: LoginActivity)
}
@Component(
scope = ScopeNames.MAIN_SCOPE,
modules = [MainModule::class]
)
interface MainComponent {
fun inject(activity: MainActivity)
}
```
Then, you annotate your `LifecycleOwner`'s with the `ScopeOwner` annotation:
```kotlin
@ScopeOwner(LOGIN_SCOPE)
class LoginActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
component().inject(this)
}
}
```
Since `LoginComponent` is being injected, _and_ because it's marked as being in `LOGIN_SCOPE`, it
will automatically destroy itself when `LoginActivity` is destroyed.
---
### ViewModels
On Android, injecting AndroidX `ViewModel`'s is supported. You don't have to do anything special,
just inject the `ViewModel` as you would inject anything else.
However, you can only inject a `ViewModel` into an `androidx.fragment.app.Fragment` or
`androidx.fragment.app.FragmentActivity` (includes `AppCompatActivity` and descendants). Why?
Internally, Ulfberht's generated code delegates through `ViewModelProviders` which must attach to
an Activity or Fragment.