Dagger2 Cheat Sheet #1
- What is dependency injection?
- What is Dagger2?
- Should I use it everywhere?
- Dagger Fundamentals: What is Inject, Module, Provider, Component?
- What is constructor injection?
- What is Field Injection?
- What is Method Injection?
- How do I add Dagger to an Android project?
- Can I use base classes in inject methods of Component interface?
- What is @Named annotation?
- I did not like @Named annotation, is there an alternative?
- How many Module classes do I need?
- Does Dagger support Assisted Injection?
- Is there a basic example project?
Check out Part 2 for more.
1. What is dependency injection?
“Dependency Injection” is a 25-dollar term for a 5-cent concept. It means giving an object its instance variables. Really. That’s it.
2. What is Dagger2?
Dagger-2 is a dependency injection framework for Java and Android.
3. Should I use it everywhere?
[This one is subjective]
DI frameworks should be used in order to resolve critical dependencies in “top level” components (Service, Activity, Fragment, etc.) right after creation (because you can’t use “constructor” injection in Android’s “top-level” components), and that’s it.
4. Dagger Fundamentals: What is Inject, Module, Provider, Component?
@Inject: Marks those dependencies which should be provided by Dependency Injection framework.
@Module: Marks classes which provide dependencies.
@Provides: Used inside module classes to mark methods that return dependencies.
@Component: Used to build an interface that Dagger will use to generate the code that will do the dependency injection for us.
5. What is constructor injection?
- Mark your constructor with @Inject annotation and all of its dependencies are provided by Dagger. Best way to use dependency injection.
- In a class you can only mark one constructor with @Inject.
- An injected constructor magically makes the class injectable too!
7. What is Method Injection?
If a method is markes with @Inject annotation all of its parameters are provided by Dagger. Only valid use case is passing this instance to one of the injected parameters.
8. How do I add Dagger to an Android project?
Basic Dagger dependencies for gradle are:
implementation 'com.google.dagger:dagger-android:2.11'
implementation 'com.google.dagger:dagger-android-support:2.11'
annotationProcessor 'com.google.dagger:dagger-compiler:2.14.1'If you want to use dagger-android components:
implementation 'com.google.dagger:dagger:2.14.1'
implementation 'com.google.dagger:dagger-android-support:2.13'
annotationProcessor 'com.google.dagger:dagger-compiler:2.14.1'
annotationProcessor 'com.google.dagger:dagger-android-processor:2.13'9. Can I use base classes in inject methods of Component interface?
Inject method has a strong type association with the injection target so you cannot. If you want a work around you can check out Annoyance #1 on Dagger 2: Even sharper, less square post.
10. What is @Named annotation?
Sometimes the type alone is insufficient to identify a dependency. For example if you need a Refrofit instance with GsonConverterFactory and another one ScalarConverterFactory you will end up with 2 provide methods that have the same return type: Retrofit. In this case, you can use @Named annotation to differentiate two Retrofit instances.
Then when you need the Retrofit you can request it with its name.
11. I did not like @Named annotation, is there an alternative?
You can create your own qualifiers annotations and use them.
12. How many Module classes do I need?
Google creates a module for each Activity.
An alternative is to create a module for app level dependencies, a module for Activities, a module for services… you get the idea. Then you can create component interfaces for module groups.
13. Does Dagger support Assisted Injection?
No. Assisted injection means you can send a method parameter to a provides annotated method that is not supplied by Dagger. In the below example first method provides a Validator instance. Second method provides a Util instance using Validator instance. When a Util is needed dagger automatically finds the Validator because it knows a method that provides a Validator instance. But the 3rd methods is not valid because Dagger does not know where to get the String object.
// Module class
@Provides
@Singleton
Validator provideValidator() {
return Validator.getInstance();
}@Provides
Util provideUtil(Validator validator) {
return new Utils(validator);
}@Provides
Util provideUtil(Validator validator, String address) {
return new Utils(validator, address);
}
Subject discussed here:
14. Is there a basic example project?
Try these
[Turkish] Blog post and repo:
[English] Blog post and repo: