Intro To Dependency Injection

Dependency Injection continues to grow in popularity due to it's code simplification effects. If you've haven't had a chance to learn and apply DI yet, this introduction is for you.

This talk will cover the theory of "Inversion of Control", introduce Dependency injection, how it can be used within Android to simplify your App's design and help you write more testable code

Transcript

1. Intro To Dependency Injection Or Bar
2. - What is a Dependency? - Dependency Injection - Dependency Inversion Principle - Dependency Injection Frameworks Agenda
3. A depends on B
4. public class Example {
 DatabaseHelper mDatabaseHelper;
 
 public Example() {
 mDatabaseHelper = new DatabaseHelper();
 }
 
 public void doStuff() {
 ...
 mDatabaseHelper.loadUsers();
 ...
 }
 }
5. - Dependency injection is a software design pattern that implements inversion of control for resolving dependencies - An injection is the passing of a dependency to a dependent object to use
6. public class Example {
 DatabaseHelper mDatabaseHelper;
 
 public Example() {
 mDatabaseHelper = new DatabaseHelper();
 }
 }
7. public class Example {
 DatabaseHelper mDatabaseHelper;
 
 public Example() {
 mDatabaseHelper = new DatabaseHelper();
 }
 } public class Example { 
 DatabaseHelper mDatabaseHelper;
 
 public Example(DatabaseHelper databaseHelper) {
 mDatabaseHelper = databaseHelper;
 }
 }
8. public class Example {
 DatabaseHelper mDatabaseHelper;
 
 public Example() {
 mDatabaseHelper = new DatabaseHelper();
 }
 } public class Example {
 DatabaseHelper mDatabaseHelper; public Example() {
 mDatabaseHelper = new DatabaseHelper();
 }
 }
 public class Example { 
 DatabaseHelper mDatabaseHelper;
 
 public Example(DatabaseHelper databaseHelper) {
 mDatabaseHelper = databaseHelper;
 }
 } public class Example {
 DatabaseHelper mDatabaseHelper; 
 public Example(DatabaseHelper databaseHelper) {
 mDatabaseHelper = databaseHelper;
 }
 }

9. public class Example {
 DatabaseHelper mDatabaseHelper;
 
 public Example() {
 }
 
 public void setDatabaseHelper(DatabaseHelper databaseHelper) {
 mDatabaseHelper = databaseHelper;
 }
 }
10. - Shared dependencies - Configure dependencies externally - Separation of modules - Inherent testability Why?
11. public class ExampleTest {
 @Mock DatabaseHelper mockDatabaseHelper;
 
 @Test
 public void testExample_doStuff() {
 Example example = new Example(mockDatabaseHelper);
 example.doStuff();
 mockDatabaseHelper.AssertGetDataWasCalled();
 }
 } public class Example { 
 DatabaseHelper mDatabaseHelper;
 
 public Example(DatabaseHelper databaseHelper) {
 mDatabaseHelper = databaseHelper;
 } public void doStuff() {
 ...
 mDatabaseHelper.loadUsers();
 ...
 }
 }
12. - High-level modules should not depend on low-level modules. Both should depend on abstractions. - Abstractions should not depend on details. Details should depend on abstractions. Dependency Inversion Principle
13. A depends on B
14. A is coupled to B
15. - Bare bones ebook reader - Supports PDF files - Prints book contents to screen Ebook Reader
16. CoolEBookReader PDFReader DisplayPrinter
17. public class CoolEBookReader {
 public void loadPage(String bookUri, int PageNumber) {
 String pdfContent = getPdfContent(bookUri, PageNumber);
 displayPage(pdfContent);
 } }
18. public class CoolEBookReader {
 enum BookType {
 PDF,
 EPUB
 }
 
 public void loadPage(BookType bookType, String bookUri, int PageNumber) {
 String pageContent;
 if (bookType == BookType.PDF) {
 pageContent = getPdfContent(bookUri, PageNumber);
 } else if (bookType == BookType.EPUB) {
 pageContent = getEpubContent(bookUri, PageNumber);
 } else {
 throw new IllegalArgumentException("Unknown book type");
 }
 displayPage(pageContent); 
 }
 }
19. public class CoolEBookReader {
 enum BookType {
 PDF,
 EPUB
 
 }
 
 enum PrinterType {
 SCREEN,
 VOICE
 
 }
 
 public void loadPage(BookType bookType, PrinterType printerType, String bookUri, int PageNumber) {
 String pageContent;
 if (bookType == BookType.PDF) {
 pageContent = getPdfContent(bookUri, PageNumber);
 } else if (bookType == BookType.EPUB) {
 pageContent = getEpubContent(bookUri, PageNumber);
 } else {
 throw new IllegalArgumentException("Unknown book type");
 }
 
 if (printerType == PrinterType.SCREEN) {
 displayPage(pageContent);
 } else if (printerType == PrinterType.VOICE) {
 readAloudPage(pageContent);
 } else {
 throw new IllegalArgumentException("Unknown printer type");
 }
 }
 }
20. - High-level modules should not depend on low-level modules. Both should depend on abstractions. - Abstractions should not depend on details. Details should depend on abstractions. Dependency Inversion
21. CoolEBookReader PDFReader DisplayPrinter <<Interface>> Reader <<Interface>> Printer
22. interface Reader {
 String read(String bookUri, int pagNumber); }
 
 interface Printer {
 void print(String pageContent);
 } public class CoolEBookReader {
 public void loadPage(Reader reader, Printer printer, String bookUri, int pageNumber) {
 String pageContent = reader.read(bookUri, pageNumber);
 printer.print(pageContent);
 }
 }
23. interface Reader {
 String read(String bookUri, int pagNumber); }
 
 interface Printer {
 void print(String pageContent);
 } public class CoolEBookReader {
 public void loadPage(Reader reader, Printer printer, String bookUri, int pageNumber) {
 String pageContent = reader.read(bookUri, pageNumber);
 printer.print(pageContent);
 }
 } public class CoolEBookReader {
 public void loadPage(String bookUri, int PageNumber) {
 String pdfContent = getPdfContent(bookUri, PageNumber);
 displayPage(pdfContent);
 } }
24. public class CoolEBookReaderTest {
 @Mock
 Reader mockReader;
 
 @Mock
 CoolEBookReader.Printer mockPrinter;
 
 @Test 
 public void TestExample_loadPage() {
 CoolEBookReader coolEBookReader = new CoolEBookReader();
 coolEBookReader.loadPage(mockReader, mockPrinter, "", 1);
 // Assert Reader.read() is called
 // Assert Printer.print() is called
 }
 }
25. - Dependency Injection - Dependency Inversion - Where are these dependencies come from What’s next?
26. - Handles object creation - Reduces boilerplate code - A central location for organizing dependencies - Implement common patterns natively (singleton, lazy loading, etc) Why use a framework?
27. - Dagger 1 - Dagger 2 - Guice and RoboGuice - PicoContainer - Spring - Many, many, many, many more options Common Frameworks
28. History lesson of DI on Android - RoboGuice - 2009 - Spring for Android - 2012 - Dagger 1 - 2013 - Dagger 2 - 2015 - Tiger - 2016
29. - @javax.inject.Inject - @Module - @Provides - @Component Dagger uses annotations
30. public class ConnectionHelper {
 private Context mContext;
 
 public ConnectionHelper(Context context) {
 mContext = context;
 }
 
 public boolean isConnected() {
 ConnectivityManager connectivityManager =
 (ConnectivityManager) mContext.getSystemService(Context.CONNECTIVITY_SERVICE);
 return connectivityManager.getActiveNetworkInfo().isConnected();
 }
 } ConnectionHelper connectionHelper = new ConnectionHelper(context);
 
 boolean isConnected = connectionHelper.isConnected();
31. public class ConnectionHelper {
 private Context mContext;
 
 @Inject
 public ConnectionHelper(Context context) {
 mContext = context;
 }
 
 public boolean isConnected() {
 ConnectivityManager connectivityManager =
 (ConnectivityManager) mContext.getSystemService(Context.CONNECTIVITY_SERVICE);
 
 return connectivityManager.getActiveNetworkInfo().isConnected();
 }
 } @Inject
32. @Module
 public class AppModule {
 
 } @Module
33. @Provides @Module
 public class AppModule {
 private final Context mContext;
 
 public AppModule(Context context) {
 mContext = context;
 } @Provides
 public Context providesContext() {
 return mContext;
 } @Provides
 public ConnectionHelper providesConnectionHelper(Context context) {
 return new ConnectionHelper(context);
 }
 }
34. @Component(modules = {AppModule.class})
 public interface AppComponent {
 
 void inject(MainActivity activity);
 } @Component
35. public class App extends Application {
 private AppComponent component;
 @Override
 public void onCreate() {
 super.onCreate();
 
 AppComponent component = DaggerAppComponent.builder()
 .appModule(new AppModule(this))
 .build();
 }
 
 public AppComponent getComponent() {
 return component;
 }
 } public class App extends Application {
 private AppComponent component;
 @Override
 public void onCreate() {
 super.onCreate();
 
 AppComponent component = DaggerAppComponent.builder()
 .appModule(new AppModule(this))
 .build();
 }
 
 public AppComponent getComponent() {
 return component;
 }
 }
36. public class MainActivity extends AppCompatActivity {
 
 @Inject
 ConnectionHelper mConnectionHelper;
 
 @Override
 protected void onCreate(Bundle savedInstanceState) {
 ((App) getApplication()).getComponent().inject(this);
 
 boolean isConnected = mConnectionHelper.isConnected();
 
 textView.setText(isConnected ? "Connected" : "Not Connected");
 }
 } public class MainActivity extends AppCompatActivity {
 
 @Inject
 ConnectionHelper mConnectionHelper;
 
 @Override
 protected void onCreate(Bundle savedInstanceState) {
 ((App) getApplication()).getComponent().inject(this);
 
 boolean isConnected = mConnectionHelper.isConnected();
 
 textView.setText(isConnected ? "Connected" : "Not Connected");
 }
 } @Inject - again
37. - More annotations - Scopes - Lazy injections - others Fun stuff
38. @Provides
 @Singleton
 public Context providesContext() {
 return context;
 } @Singleton @Singleton
 @Component(modules = {AppModule.class})
 public interface AppComponent {
 
 void inject(MainActivity activity); }
39. @Provides
 @Reusable
 public ConnectionHelper providesConnectionHelper(Context context) {
 return new ConnectionHelper(context);
 } @Reusable @Singleton @Reusable
 @Component(modules = {AppModule.class})
 public interface AppComponent {}
40. @Inject
 Lazy<ConnectionHelper> mConnectionHelper;
 ... mConnectionHelper.get().isConnected(); Lazy and Provider Injections @Inject
 Provider<ConnectionHelper> mConnectionHelper;
 ... mConnectionHelper.get().isConnected();
41. @Target(ANNOTATION_TYPE)
 @Retention(RUNTIME)
 @Documented
 public @interface Qualifier {} @Qualifier @Qualifier
 @Documented
 @Retention(RUNTIME)
 public @interface Named {}
42. @Provides @Singleton @Named(“Adapter 1”)
 public RestAdapter providesRestAdapter1() { return …; } @Qualifier @Provides @Singleton @Named(“Adapter 2”) public RestAdapter providesRestAdapter2() { return …; }
43. @Qualifier @Inject @Named(“Adapter 1”) RestAdapter mRestAdapter;

44. Subcomponents and Scopes - Subcomponents allow you to combine different modules at runtime - Scopes let you define the lifetime of a dependency
45. Testing and dagger - Constructors used for injections can be used for testing - Test Modules for espresso
46. Questions - @or_bar - orbar@tumblr.com - orbar1.tumblr.com