Android Dagger2
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The document explains Dependency Injection (DI) using Dagger2 for Android, emphasizing how DI allows classes to obtain their dependencies externally rather than instantiating them internally, improving reusability and testability. Dagger2, a static, compile-time DI framework, generates dependency graphs and checks for errors at build time using annotations. It outlines the types of injections (constructor, field, method), along with examples of dependency consumers and providers, and highlights limitations and important considerations when using Dagger2.
Android Dagger2
- 1. ©W3Engineers2018 Android-DI Dagger2 Md. Moniruzzaman (Monir) Sr. Software Engineer Sk Arman Junior Software Engineer 13-09-2018
- 2. Android-dagger2 Monir & Arman What is DI Dependency Injection in build upon the concept of Inversion of Control. Which says that a class should get its dependencies from outside. In simple words, no class should instantiate another class but should get the instances from a configuration class.
- 3. Android-dagger2 Monir & Arman Why do we need Dependency Injection? If a java class creates an instance of another class via the new operator, then it cannot be used and tested independently from that class and is called a hard dependency.
- 4. Android-dagger2 Monir & Arman Benefits So, what are the benefits of providing the dependencies from outside the class? ● The most important advantage is that it increases the possibility of reusing the class and to be able to test them independent of other classes. ● This sounds awesome to create a class that is not a specific implementation of a business logic.
- 5. Android-dagger2 Monir & Arman How do we do DI To answer this question, we have to look back into the past: A framework class called dependency container was used to analyzes the dependencies of a class. With this analysis, it was able to create an instance of the class and inject the objects into the defined dependencies via Java Reflections. This eliminated the hard dependencies. That way the class could be tested in isolation, ex. by using mock objects. This was Dagger1.
- 6. Android-dagger2 Monir & Arman Disadvantages of past process Main disadvantages of this process were two folds. First, the Reflection is slow in itself and second, it used to perform dependency resolution at runtime, leading to unexpected crashes. This lead to the birth of Dagger2 forked from Square Dagger1 by Google.
- 7. Android-dagger2 Monir & Arman What is Dagger2 Dagger is a fully static, compile-time dependency injection framework for both Java and Android. It is an adaptation of an earlier version created by Square and now maintained by Google.— Dagger Team
- 8. Android-dagger2 Monir & Arman What new in dagger2? ● The big changes that were brought in Dagger2 were generating the dependency graph using annotation processor. The classes providing the dependencies were now generated at build time using javax inject package. This facilitated the possible error checking before the application runs. The generated code is highly readable as if written by hand. Note: Annotation Processor is a way to read the compiled files during build time to generate source code files to be used in the project.
- 9. Android-dagger2 Monir & Arman Mode of Injection The standard java annotations for describing the dependencies of a class are defined in the Java Specification Request 330 (JSR 330) ● Constructor Injection: Injecting the method parameters. ● Field Injection: Injecting the member variable (must not be private). ● Method Injection: Injecting the method parameter.
- 10. Android-dagger2 Monir & Arman Order of dependency injection Order of dependency injection according to JSR330: 1. Constructor 2. Field 3. Method
- 11. Android-dagger2 Monir & Arman Important things to remember ● The order in which the methods or fields annotated with @Inject are called is not defined by JSR330. You cannot assume that the methods or fields are called in the order of their declaration in the class. ● As fields and method parameters are injected after the constructor is called, you cannot use injected member variables in the constructor.
- 12. Android-dagger2 Monir & Arman Now proceed to understand Dagger2 A dependency consumer asks for the dependency(Object) from a dependency provider through a connector. Dependency consumer: The @Inject annotation is used to define a dependency. .
- 13. Android-dagger2 Monir & Arman Example of consumer @Singleton public class DataManager { private Context mContext; private DbHelper mDbHelper; private SharedPrefsHelper mSharedPrefsHelper; @Inject public DataManager(@ApplicationContext Context context, DbHelper dbHelper, SharedPrefsHelper sharedPrefsHelper) { mContext = context; mDbHelper = dbHelper; mSharedPrefsHelper = sharedPrefsHelper; } public void saveAccessToken(String accessToken) { mSharedPrefsHelper.put(SharedPrefsHelper.PREF_KEY_ACCESS_TOKEN, accessToken); } public Long createUser(User user) throws Exception { return mDbHelper.insertUser(user); }}
- 14. Android-dagger2 Monir & Arman Now proceed to understand Dagger2 Dependency provider: Classes annotated with @Module are responsible for providing objects which can be injected. Such classes define methods annotated with @Provides. The returned objects from these methods are available for dependency injection.
- 15. Android-dagger2 Monir & Arman Example of Provider @Module public class ActivityModule { private Activity mActivity; public ActivityModule(Activity activity) { mActivity = activity; } @Provides @ActivityContext Context provideContext() { return mActivity; } @Provides Activity provideActivity() { return mActivity; } }
- 16. Android-dagger2 Monir & Arman Now proceed to understand Dagger2 Connecting consumer and producer (Connector): A @Component annotated interface defines the connection between the provider of objects (modules) and the objects which express a dependency. The class for this connection is generated by the Dagger.
- 17. Android-dagger2 Monir & Arman Example of Connector @Singleton @Component(modules = ActivityModule.class) public interface ActivityComponent { void inject(DemoApplication demoApplication); @ApplicationContext Context getContext(); Application getApplication(); DataManager getDataManager(); SharedPrefsHelper getPreferenceHelper(); DbHelper getDbHelper(); }
- 18. Android-dagger2 Monir & Arman Limitations of Dagger2 ● Dagger2 does not inject fields automatically. ● It cannot inject private fields. ● If you want to use field injection you have to define a method in your @Component annotated interface which takes the instance of the class into which you want to inject the member variable.
- 19. Android-dagger2 Monir & Arman References ● https://blog.mindorks.com/introduction-to-dagger-2-using-dependency- injection-in-android-part-1-223289c2a01b ● https://medium.com/@harivigneshjayapalan/dagger-2-for-android-beginners- di-part-i-f5cc4e5ad878 ● https://android.jlelse.eu/the-simplest-dagger2-dependency-injection-sample- 80a0eb60e33b ● https://www.simplifiedcoding.net/dagger-2-android-example/