Sharpen your Code with Dagger

Sharpen your code
with Dagger
Aung Kyaw Paing

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Dependency?
public class A {
B b;
public A() {
this.b = new B();
}
}

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creates
public class A {
B b;
public A() {
this.b = new B();
}
}
A B

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Dependency Injection
public class A {
B b;
public A(B b) {
this.b = b;
}
}

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public class A {
B b;
public A(B b) {
this.b = b;
}
}
Injected into
A B

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S.O.L.I.D Principle
• Single Responsibility Principle
• Open/Closed Principle
• Liskov Substitution Principle
• Interface Segregation Principle
• Dependency Inversion Principle

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Dependency Inversion Principle
• Depend on abstractions. Do not depend
on concretions.

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Network
Abstraction
Retroﬁt
Volley Mock
App

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Dependency Injection
-A pattern

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public class FacebookService {
public void postStatus(String status) {
InterfaceFacebookApi apiInterface= RetrofitProvider.createFacebookApi();
apiInterface.postStatus(status);
}
}

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public class PostStatus {
public void execute(String status) {
FacebookService service = new FacebookService();
service.postStatus(status);
}
}
}
}

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PostStatus postStatus = new PostStatus();
postStatus.execute("Hello DevFest Yangon 2017");
postStatus.execute("Nice to Meet you All");

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}
}

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InterfaceFacebookApi apiInterface = RetrofitProvider.createFacebookApi();
}
}

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InterfaceFacebookApi apiInterface;
public FacebookService(InterfaceFacebookApi apiInterface) {
this.apiInterface = apiInterface;
}
}
}

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FacebookService service = new FacebookService();
}
}

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FacebookService service = new FacebookService(RetrofitProvider.createFacebookApi());
}
}

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}
}

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PostStatus
Facebook
Service
Retroﬁt

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PostImage postImage = new PostImage();
postImage.execute("www.example.com/boston.jpg");
postImage.execute("www.example.com/paris.jpg");

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}
}
}

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}
}
public void postImage(String imageLink) {
apiInterface.postImage(imageLink);
}
}

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public class PostImage {
public void execute(String imageUrl) {
service.postImage(imageUrl);
}
}

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}
}
}
}

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FacebookService service;
public PostImage(FacebookService service) {
this.service = service;
}
}
}
FacebookService service;
public PostStatus(FacebookService service) {
}
}
}

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PostImage postImage = new PostImage();

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InterfaceFacebookApi apiInterface = RetrofitProvider.createFacebookApi();
FacebookService service = new FacebookService(apiInterface);
PostStatus postStatus = new PostStatus(service);
PostImage postImage = new PostImage(service);

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PostStatus
Facebook
Service
Retroﬁt

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PostStatus
Facebook
Service
Retroﬁt
PostImage

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DI comes at the cost of boilerplate

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Spring
DI Framework

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- Developed by Pivotal Software
- Written in XML
- Runtime crashes
- Untraceable
Spring

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Spring Guice
DI Framework

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- Developed at Google by Bob Lee, Jessie Wilson and others
- Maintained by Java Core Lib team
- Written in Java
- Heavy Initialization
- Untraceable
- Runtime crashes
Guice

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DI Framework
Spring Guice Dagger 1

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- Developed by Jessie Wilson(Square)
- Advised By Bob Lee
- Written in Java
- Targeted for resource limited environment
- Uses reﬂection
- Runtime crashes
- No scoping
Dagger 1

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DI Framework
Spring Guice Dagger 1 Dagger 2

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- Developed by Java Core Lib Team
- Written with Java
- Compile time crashes
- Eliminated overheads
- Support Scoping
Dagger 2

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Dagger 2 API
@Module / @Provides
- Provide dependencies

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Dagger 2 API
@Module / @Provides
@Inject
- Request dependencies

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Dagger 2 API
@Module / @Provides
@Inject
- Request dependencies
@Component
- Bridge modules and injections

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[email protected] is a class that holds related dependencies
[email protected] is the method of module classes
Dagger 2 API

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@Module public class NetworkModule {
@Provides @Singleton Retrofit providesRetrofit() {
return new Retrofit.Builder().build();
}
@Provides @Singleton FacebookService providesFacebookService(Retrofit retrofit) {
return new FacebookService(retrofit.create(InterfaceFacebookApi.class));
}
}

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FacebookService
NetworkModule#providesRetrofit
NetworkModule#providesFacebookService
Retrofit

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Retrofit
}
FacebookService NetworkModule#providesFacebookService

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Retrofit
}

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}
}
}

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@Provides @Singleton OkHttpClient providesOkHttpClient() {
return new OkHttpClient.Builder().build();
}
@Provides @Singleton Retrofit providesRetrofit(OkHttpClient okHttpClient) {
return new Retrofit.Builder().client(okHttpClient)
.build();
}
}
}

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Retrofit
NetworkModule#providesOkHttpClient
OkHttpClient

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[email protected](s) can be used together
Dagger 2 API

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@Module public class FacebookModule {
}

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@Provides @Singleton PostStatus providesPostStatus(FacebookService facebookService) {
return new PostStatus(facebookService);
}
}

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Retrofit
PostStatus FacebookModule#providesPostStatus

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}
Retrofit

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Retrofit
}

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}
}

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}
@Provides @Singleton PostImage providesPostImage(FacebookService facebookService) {
return new PostImage(facebookService);
}
}

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PostImage FacebookModule#providesPostImage
Retrofit

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Retrofit
@Provides @Singleton PostImage providesPostImage(FacebookService facebookService) {
}

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Retrofit

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[email protected] requests dependencies
-Constructor/Method/Field Injection
Dagger 2 API

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[email protected] on one constructor
-Constructor parameters = dependencies
-Can store dependencies as immutable
-Can use for downstream injection
Constructor Injection

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}
}
}

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private final InterfaceFacebookApi apiInterface;
}
}
}
}

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@Inject public FacebookService(Retrofit retrofit) {
this.apiInterface = retrofit.create(InterfaceFacebookApi.class);
}
}
}
}

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@Singleton
}
}
}
}

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Method Injection
[email protected] on methods
-Method parameters = dependencies
-Injection after object is fully instantiated
-Use when you need to pass an instance of itself into
dependency

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Method Injection
public class MapActivity extends AppCompatActivity {
@Inject void subscribeGPSChange(LocationCallback locationCallback) {
locationCallback.subscribe(this);
}
}

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Field Injection
[email protected] on ﬁelds
-Fields = dependencies
-Injection after object is fully instantiated
-Risk of Mutability
-Use in System objects

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Field Injection
public class MainActivity extends AppCompatActivity {
@Inject PostStatus postStatus;
@Inject PostImage postImage;
@Override protected void onCreate(@Nullable Bundle savedInstanceState) {
//Inject here
super.onCreate(savedInstanceState);
}
}

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[email protected] bridges @Inject and @Module/
@Provide
Dagger 2 API

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Components
@Component
public interface FacebookComponent {
}

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Components
@Component(modules = {
NetworkModule.class,
FacebookModule.class
})
}

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Components
})
PostStatus postStatus();
PostImage postImage();
}

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Components
@Singleton
})
}

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Retrofit retrofit = RetrofitProvider.get();
FacebookService service = new FacebookService(retrofit);
PostStatus postStatus = new PostStatus(service);
PostImage postImage = new PostImage(service);

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FacebookComponent facebookComponent = DaggerFacebookComponent.builder()
.facebookModule(new FacebookModule())
.networkModule(new NetworkModule())
.build();
PostStatus postStatus = facebookComponent.postStatus();
PostImage postImage = facebookComponent.postImage();

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class FacebookActivity extends AppCompatActivity {
PostStatus postStatus;
PostImage postImage;
.build();
this.postStatus = facebookComponent.postStatus();
this.postImage = facebookComponent.postImage();
}
}

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Components
})
}

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Components
@Singleton
})
void inject(Activity activity);
}

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PostStatus postStatus;
PostImage postImage;
.build();
this.postStatus = facebookComponent.postStatus();
this.postImage = facebookComponent.postImage();
}
}

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DaggerFacebookComponent.builder()
.build()
.inject(this);
}
}

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Scoping
@Singleton
})
}

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Scoping
@Singleton
})
}
NetworkModule.class
})
@Singleton
public class NetworkComponent {
}

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@Singleton
@Component(
dependencies = { NetworkComponent.class },
modules = { FacebookModule.class }
)
}
NetworkModule.class
})
@Singleton
public class NetworkComponent {
}
Scoping

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@Singleton
@Component(
)
}
NetworkModule.class
})
@Singleton
public interface NetworkComponent {
FacebookService facebookService();
}
Scoping

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@Component(
)
}
NetworkModule.class
})
@Singleton
public interface NetworkComponent {
FacebookService facebookService();
}
Scoping

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@Component(
)
}
@Provides @Singleton PostStatus providesPostStatus(FacebookService
facebookService) {
}
@Provides @Singleton PostImage providesPostImage(FacebookService
facebookService) {
}
}
Scoping

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@Component(
)
}
@Provides PostStatus providesPostStatus(FacebookService facebookService) {
}
@Provides PostImage providesPostImage(FacebookService facebookService) {
}
}
Scoping

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Scoping
@Scope
public @interface StatusUpdateScreen {
}

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Scoping
@StatusUpdateScreen
@Component(
)
}

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Scoping
@Provides PostStatus providesPostStatus(FacebookService facebookService) {
}
@Provides PostImage providesPostImage(FacebookService facebookService) {
}
}

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Scoping
@Provides @StatusUpdateScreen PostStatus providesPostStatus(FacebookService
facebookService) {
}
@Provides @StatusUpdateScreen PostImage providesPostImage(FacebookService
facebookService) {
}
}

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Scoping
class StatusUpdateActivity extends AppCompatActivity {
NetworkComponent daggerNetworkComponent = DaggerNetworkComponent.create();
DaggerStatusUpdateComponent.builder()
.networkComponent(daggerNetworkComponent)
.build()
.inject(this);
}
}

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Network Component

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Network Component
Network
Module
Retroﬁt
Facebook
Service

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Network Component
Network
Module
Retroﬁt
Facebook
Service
Facebook
Service

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Network Component
Network
Module
Retroﬁt
Facebook
Service
Facebook
Service
Post
Status Component

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Network Component
Network
Module
Retroﬁt
Facebook
Service
Facebook
Service
Post
Status Component
FacebookModule
PostStatus
PostImage

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Network Component
Network
Module
Retroﬁt
Facebook
Service
Facebook
Service

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Generated Code Under The Hood
}
return new Retrofit.Builder().client(okHttpClient)
.build();
}
@Provides @Singleton FacebookService providesFaceookService(Retrofit retrofit) {
return new FacebookService(retrofit);
}
}

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public final class NetworkModule_ProvidesOkHttpClientFactory implements
Factory {
private final NetworkModule module;
public NetworkModule_ProvidesOkHttpClientFactory(NetworkModule module) {
assert module != null;
this.module = module;
}
@Override public OkHttpClient get() {
return Preconditions.checkNotNull(module.providesOkHttpClient(),
"Cannot return null from a [email protected] @Provides method");
}
public static Factory create(NetworkModule module) {
return new NetworkModule_ProvidesOkHttpClientFactory(module);
}
}

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public final class NetworkModule_ProvidesRetrofitFactory implements
Factory {
private final NetworkModule module;
private final Provider okHttpClientProvider;
public NetworkModule_ProvidesRetrofitFactory(
NetworkModule module, Provider okHttpClientProvider) {
assert module != null;
this.module = module;
assert okHttpClientProvider != null;
this.okHttpClientProvider = okHttpClientProvider;
}
@Override
public Retrofit get() {
return Preconditions.checkNotNull(
module.providesRetrofit(okHttpClientProvider.get()),
"Cannot return null from a [email protected] @Provides method");
}
public static Factory create(
NetworkModule module, Provider okHttpClientProvider) {
return new NetworkModule_ProvidesRetrofitFactory(module, okHttpClientProvider);
}
}

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public final class DaggerNetworkComponent implements NetworkComponent {
private Provider providesOkHttpClientProvider;
private Provider providesRetrofitProvider;
private Provider providesFaceookServiceProvider;
private DaggerNetworkComponent(Builder builder) {
assert builder != null;
initialize(builder);
}
public static Builder builder() {
return new Builder();
}
public static NetworkComponent create() {
return new Builder().build();
}
@SuppressWarnings("unchecked")
private void initialize(final Builder builder) {
this.providesOkHttpClientProvider =
DoubleCheck.provider(
NetworkModule_ProvidesOkHttpClientFactory.create(builder.networkModule));
this.providesRetrofitProvider =
NetworkModule_ProvidesRetrofitFactory.create(
builder.networkModule, providesOkHttpClientProvider));
this.providesFaceookServiceProvider =
NetworkModule_ProvidesFaceookServiceFactory.create(
builder.networkModule, providesRetrofitProvider));
}
@Override
public FacebookService facebookService() {
return providesFaceookServiceProvider.get();
}
}

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NetworkModule_ProvidesOkHttpClientFactory.create(builder.networkModule);
builder.networkModule, providesOkHttpClientProvider);
this.providesFacebookServiceProvider =
NetworkModule_ProvidesFacebookServiceFactory.create(
builder.networkModule, providesRetrofitProvider);
}

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NetworkModule_ProvidesOkHttpClientFactory.create(builder.networkModule);
builder.networkModule, providesOkHttpClientProvider);
this.providesFacebookServiceProvider =
NetworkModule_ProvidesFacebookServiceFactory.create(
builder.networkModule, providesRetrofitProvider);
}
@Override
public FacebookService facebookService() {
return providesFacebookServiceProvider();
}

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.build()
.inject(this);
}
}

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public final class StatusUpdateActivity_MembersInjector
implements MembersInjector {
private final Provider postStatusProvider;
private final Provider postImageProvider;
public StatusUpdateActivity_MembersInjector(
Provider postStatusProvider, Provider postImageProvider) {
assert postStatusProvider != null;
this.postStatusProvider = postStatusProvider;
assert postImageProvider != null;
this.postImageProvider = postImageProvider;
}
public static MembersInjector create(
Provider postStatusProvider, Provider postImageProvider) {
return new StatusUpdateActivity_MembersInjector(postStatusProvider, postImageProvider);
}
@Override
public void injectMembers(StatusUpdateActivity instance) {
if (instance == null) {
throw new NullPointerException("Cannot inject members into a null reference");
}
instance.postStatus = postStatusProvider.get();
instance.postImage = postImageProvider.get();
}
public static void injectPostStatus(
StatusUpdateActivity instance, Provider postStatusProvider) {
}
public static void injectPostImage(
StatusUpdateActivity instance, Provider postImageProvider) {
}
}

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public void injectMembers(StatusUpdateActivity instance) {
if (instance == null) {
throw new NullPointerException("Cannot inject members into a null reference");
}
}

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General Rules of Thumbs

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Favour Constructor Injection

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@Inject private InterfaceFacebookApi apiInterface;
public FacebookService() {
//Inject here
}
}
}

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private DbFacebook dbFacbook;
//Inject here
}
//Save on fail
}
public void saveStatus(String status) {
dbFacbook.saveStatus(status);
}
}

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private DbFacebook dbFacbook;
//Inject here
}
//Save on fail
}
//NULL POINTER EXECPTION
}
}

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@Inject private DbFacebook dbFacbook;
//Inject here
}
//Save on fail
}
}
}
}

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private final DbFacebook dbFacbook;
//Inject here
}
//Save on fail
}
}
}
}

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private final DbFacebook dbFacbook;
@Inject public FacebookService(InterfaceFacebookApi apiInterface,
DbFacebook dbFacbook) {
this.dbFacbook = dbFacbook;
}
//Save on fail
}
}
}
}

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.build()
.inject(this);
}
}

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Don’t use Singleton everywhere
-Use Singleton only when the state is mutable
-Don’t use if there’s no mutable object
-Use if expensive

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-Use Singleton only when the state is mutable
@Singleton
public class DbConnectionCounter {
public Integer connectionCount = 0;
@Inject public DbConnectionCounter(Context context) {
//Implementation
}
void increment() {
connectionCount = connectionCount++;
}
void decerement() {
if (connectionCount > 0) {
connectionCount = connectionCount--;
}
}
}

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-Immutable, don’t use
private final FacebookService service;
@Inject public PostStatus(FacebookService service) {
}
}
}

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-Use if the object is expensive to create
}
return new Retrofit.Builder().client(okHttpClient).build();
}
}

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Use @Inject when you can
}
return new Retrofit.Builder().client(okHttpClient).build();
}
return new FacebookService(retrofit);
}
}

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Use @Inject when you can
}
}
}
}

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Use static @Provides Methods
@Module public class AppModule {
@Provides ErrorMessageFactory errorMessageFactory() {
return new ErrorMessageFactory();
}
}
@Component(modules = AppModule.class)
public interface AppComponent {
ErrorMessageFactory errorMessageFactory();
}

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public final class DaggerAppComponent implements AppComponent {
private Provider errorMessageFactoryProvider;
//...
@Override public ErrorMessageFactory errorMessageFactory() {
return errorMessageFactoryProvider.get();
}
//...
}

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@Provides ErrorMessageFactory errorMessageFactory() {
}
}
}

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@Provides static ErrorMessageFactory errorMessageFactory() {
}
}
}

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public final class DaggerAppComponent implements AppComponent {
//...
@Override
public ErrorMessageFactory errorMessageFactory() {
return AppModule.errorMessageFactory();
}
//...
}

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}
}

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@Module public abstract class AppModule {
}
}

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Avoid @Singleton on providing abstractions
@Provides @Singleton
NetworkManager provideNetworkManager(NetworkManagerImpl networkManagerImpl) {
return networkManagerImpl;
}

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Avoid @Singleton on providing abstractions
@Provides
NetworkManager provideNetworkManager(NetworkManagerImpl networkManagerImpl) {
return networkManagerImpl;
}
@Singleton
public class NetworkManagerImpl implements NetworkManager
{
}

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Don’t Stress on scoping

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Q & A

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Sharpen your Code with Dagger

Sharpen your Code with Dagger

Aung Kyaw Paing

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