The document discusses dependency injection (DI) and how Dagger can be used to implement DI. It begins with an example Android application that manages a lock dashboard. The code to create dependencies is complex and error-prone. Dagger addresses this by generating code to manage object creation and injection. It works by analyzing how objects relate via a directed acyclic graph (DAG) of their dependencies. Modules provide object instances to the graph, and components inject them where needed. This allows clean, testable separation of concerns and simplifies object creation.

1. Keeping up
with DI and
Dagger
Matt Dupree

2. Gimli
LegolasAragorn

3. Dev
Marketing CEO

4. Keeping up
with DI and
Dagger

5. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help me keep up?

6. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help me keep up?

8. class Human {
private final Water water;
Human() {
water = new Water();
}
}

10. class Human {
private final Water water;
Human() {
water = new Water();
}
}

11. class Human {
private final Water water;
Human(Water water) {
this.water = water;
}
}

12. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help me keep up?

13. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help me keep up?

15. 1. Leverage OO for faster, cleaner implementation
2. Leverage tests to ensure changes didn’t break anything

16. 1. Leverage OO for faster, cleaner implementation
2. Leverage tests to ensure changes didn’t break anything

17. –Alan Kay, The Computer Revolution
hasn’t Happened Yet
“any particular implementation
is making pragmatic choices
and these…choices are likely
not to be able to cover all of
the cases at the level of
efficiency and…richness
required…this is standard OOP
lore…we need to have
different ways of dealing with
the same concepts in a way
that does not distract the
programmer”

19. class LockDashboardFragment : Fragment() {
override fun onViewCreated(
view: View,
savedInstanceState: Bundle?
) {
super.onViewCreated(view, savedInstanceState)
lockConnection = BleLockConnection()
lockConnection.initialize()
view.findViewById<View>(R.id.lockUnlockButton)
.setOnClickListener {
val command = Command(if (isLocked) UNLOCK else LOCK)
lockConnection.sendCommand(command)
}
}
}

20. class LockDashboardFragment : Fragment() {
override fun onViewCreated(
view: View,
savedInstanceState: Bundle?
) {
super.onViewCreated(view, savedInstanceState)
lockConnection = BleLockConnection()
lockConnection.initialize()
view.findViewById<View>(R.id.lockUnlockButton)
.setOnClickListener {
val command = Command(if (isLocked) UNLOCK else LOCK)
lockConnection.sendCommand(command)
}
}
}

21. class LockDashboardFragment : Fragment() {
override fun onViewCreated(
view: View,
savedInstanceState: Bundle?
) {
super.onViewCreated(view, savedInstanceState)
lockConnection = BleLockConnection()
lockConnection.initialize()
view.findViewById<View>(R.id.lockUnlockButton)
.setOnClickListener {
val command = Command(if (isLocked) UNLOCK else LOCK)
lockConnection.sendCommand(command)
}
}
}

22. class LockDashboardFragment : Fragment() {
override fun onViewCreated(
view: View,
savedInstanceState: Bundle?
) {
super.onViewCreated(view, savedInstanceState)
lockConnection = BleLockConnection()
lockConnection.initialize()
view.findViewById<View>(R.id.lockUnlockButton)
.setOnClickListener {
val command = Command(if (isLocked) UNLOCK else LOCK)
lockConnection.sendCommand(command)
}
}
}

24. class LockDashboardFragment : Fragment() {
override fun onViewCreated(
view: View,
savedInstanceState: Bundle?
) {
super.onViewCreated(view, savedInstanceState)
lockConnection = BleLockConnection()
lockConnection.initialize()
view.findViewById<View>(R.id.lockUnlockButton)
.setOnClickListener {
val command = Command(if (isLocked) UNLOCK else LOCK)
lockConnection.sendCommand(command)
}
}
}

25. class LockDashboardFragment : Fragment() {
override fun onViewCreated(
view: View,
savedInstanceState: Bundle?
) {
super.onViewCreated(view, savedInstanceState)
lockConnection.initialize()
view.findViewById<View>(R.id.lockUnlockButton)
.setOnClickListener {
val command = Command(if (isLocked) UNLOCK else LOCK)
lockConnection.sendCommand(command)
}
}
}

26. class LockDashboardFragment : Fragment() {
override fun onViewCreated(
view: View,
savedInstanceState: Bundle?
) {
super.onViewCreated(view, savedInstanceState)
lockConnection.initialize();
view.findViewById<View>(R.id.lockUnlockButton)
.setOnClickListener {
val command = Command(if (isLocked) UNLOCK else LOCK)
lockConnection.sendCommand(command)
}
}
}

27. 1. Leverage OO for faster, cleaner implementation
2. Leverage tests to ensure changes didn’t break anything

28. 1. Leverage OO for faster, cleaner implementation
2. Leverage tests to ensure changes didn’t break anything

29. …The trick now was to make
space for the new functionality
without breaking anything that
already worked…After the
addition of a few
unimplemented operations…
the bulk of the tests passed. By
the end of the day, all of the
tests were passing…In two days,
the potential market was
multiplied several fold…”
–Kent Beck, TDD By Example

31. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help with testing?

32. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help me keep up?

34. LockDashboard

35. LockDashboardPresenter

36. LockDashboardPresenterLocksRepository

37. LockDashboardPresenterLocksRepository
Database
LoaderManager

38. class LockDashboard extends Fragment {
Presenter locklistPresenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final Context context = getContext();
Database database = new Database(context);
LocksRepository locksRepository =
new LocksRepository(database, getLoaderManager());
lockListPresenter =
new Presenter(locksRepository);
}
}

39. class LockDashboard extends Fragment {
Presenter locklistPresenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final Context context = getContext();
Database database = new Database(context);
LocksRepository locksRepository =
new LocksRepository(database, getLoaderManager());
lockListPresenter =
new Presenter(locksRepository);
}
}

40. class LockDashboard extends Fragment {
Presenter locklistPresenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final Context context = getContext();
Database database = new Database(context);
LocksRepository locksRepository =
new LocksRepository(database, getLoaderManager());
lockListPresenter =
new Presenter(locksRepository);
}
}

41. class LockDashboard extends Fragment {
Presenter locklistPresenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final Context context = getContext();
Database database = new Database(context);
LocksRepository locksRepository =
new LocksRepository(database, getLoaderManager());
lockListPresenter =
new Presenter(locksRepository);
}
}

42. class LockDashboard extends Fragment {
Presenter locklistPresenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final Context context = getContext();
Database database = new Database(context);
LocksRepository locksRepository =
new LocksRepository(database, getLoaderManager());
lockListPresenter =
new Presenter(locksRepository);
}
}

43. static class Factory {
Presenter makePresenter() {
final Context context = getContext();
Database database = new Database(context);
LocksRepository locksRepository =
new LocksRepository(database, getLoaderManager());
return new Presenter(locksRepository);
}
}

44. –Dagger Docs
“Dagger is a replacement for
these FactoryFactory classes that implements
the dependency injection design pattern
without the burden of writing the boilerplate.”

45. class LockDashboard extends Fragment {
Presenter locklistPresenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final Context context = getContext();
Database database = new Database(context);
LocksRepository locksRepository =
new LocksRepository(database, getLoaderManager());
lockListPresenter =
new Presenter(locksRepository);
}
}

46. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
AndroidInjection.inject(this);
// ...
}
}

47. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help with testing?

48. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help me keep up?

49. How do I Dagger?

50. How do I Dagger?
@Provides
@Binds
@Component
@Module
@Inject
@BindsInstance
@Scope @Named @Qualifier
@Subcomponent @Component.Builder

51. How do I Dagger?

52. Dagger?

53. Dag?

54. Directed
acyclic
graph

56. LockDashboardPresenterLocksRepository
Database
LoaderManager

57. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

58. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

59. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

60. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

61. Components create the injected roots of the
object graph

62. @Component
interface LockListComponent {
void inject(LockDashboard lockDashboard);
}

63. @Component
interface LockListComponent {
void inject(LockDashboard lockDashboard);
}

64. @Component
interface LockListComponent {
void inject(LockDashboard lockDashboard);
}

65. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.create().inject(this);
// ...
}
}

66. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.create().inject(this);
// ...
}
}

67. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.create().inject(this);
// ...
}
}

68. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.create().inject(this);
// ...
}
}

69. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.create().inject(this);
// ...
}
}

70. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.create().inject(this);
// ...
}
}

71. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final Context context = getContext();
Database database = new Database(context);
LocksRepository locksRepository =
new LocksRepository(database, getLoaderManager());
lockListPresenter =
new Presenter(locksRepository);
// ...
}
}

72. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.create().inject(this);
// ...
}
}

73. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.create().inject(this);
// ...
}
}

74. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

75. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

76. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

77. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

78. class Presenter {
private final LocksRepository locksRepository;
Presenter(LocksRepository locksRepository) {
this.locksRepository = locksRepository;
}

79. class Presenter {
private final LocksRepository locksRepository;
@Inject
Presenter(LocksRepository locksRepository) {
this.locksRepository = locksRepository;
}

80. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

81. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

82. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

83. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

84. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

85. …But its never that easy. Why?

86. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

87. interface LocksRepository {
LockLoadResult loadLocks();
}

88. interface LocksRepository {
LockLoadResult loadLocks();
}
?

89. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

90. package android.database.sqlite;
public final class SQLiteDatabase extends SQLiteClosable {}

91. package android.database.sqlite;
public final class SQLiteDatabase extends SQLiteClosable {}
?

92. Modules have methods that know how to add
“tricky” objects to the object graph as nodes

93. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

94. @Module
class LockListModule {
@Provides Database providesDb() {
SQLiteOpenHelper helper = //...
return helper.getWritableDatabase();
}
}

95. @Module
class LockListModule {
@Provides Database providesDb() {
SQLiteOpenHelper helper = //...
return helper.getWritableDatabase();
}
}

96. @Module
class LockListModule {
@Provides Database providesDb() {
SQLiteOpenHelper helper = //...
return helper.getWritableDatabase();
}
}

97. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

98. @Module
class LockListModule {
@Provides Database providesDb() {
SQLiteOpenHelper helper = //...
return helper.getWritableDatabase();
}
}

99. @Module
class LockListModule {
@Provides Database providesDb() {
SQLiteOpenHelper helper = //...
return helper.getWritableDatabase();
}
}

100. @Module
class LockListModule {
@Provides Database providesDb(SQLiteOpenHelper helper) {
return helper.getWritableDatabase();
}
}

101. @Component
interface LockListComponent {
void inject(LockDashboard lockDashboard);
}

102. @Component(modules = LockListModule.class)
interface LockListComponent {
void inject(LockDashboard lockDashboard);
}

103. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

104. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

105. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

106. @Module
class LockListModule {
@Provides Database providesDb() {
SQLiteOpenHelper helper = //...
return helper.getWritableDatabase();
}
}

107. @Module
abstract class LockListModule {
@Provides Database providesDb() {
SQLiteOpenHelper helper = //...
return helper.getWritableDatabase();
}
@Binds abstract LocksRepository bindsLockListApiService(
CacheFirstLocksRepository locksRepository);
}

108. @Module
abstract class LockListModule {
@Provides Database providesDb() {
SQLiteOpenHelper helper = //...
return helper.getWritableDatabase();
}
@Binds abstract LocksRepository bindsLockListApiService(
CacheFirstLocksRepository locksRepository);
}

109. @Module
abstract class LockListModule {
@Provides Database providesDb() {
SQLiteOpenHelper helper = //...
return helper.getWritableDatabase();
}
@Binds abstract LocksRepository bindsLockListApiService(
CacheFirstLocksRepository locksRepository);
}

110. @Module
abstract class LockListModule {
@Provides Database providesDb() {
SQLiteOpenHelper helper = //...
return helper.getWritableDatabase();
}
@Binds abstract LocksRepository bindsLockListApiService(
CacheFirstLocksRepository locksRepository);
}

111. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

112. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

113. @Module
class LockListModule {
@Provides LoaderManager providesLoaderManager() {
return //???
}
}

114. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.create().inject(this);
// ...
}
}

115. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.builder()
.loaderManager(getLoaderManager())
.build()
.inject(this);
// ...
}
}

116. @Component
interface LockListComponent {
void inject(LockDashboard lockDashboard);
}

117. @Component
interface LockListComponent {
void inject(LockDashboard lockDashboard);
@Component.Builder interface Builder {
@BindsInstance Builder loaderManager(LoaderManager lMgr);
LockListComponent build();
}
}

118. @Component
interface LockListComponent {
void inject(LockDashboard lockDashboard);
@Component.Builder interface Builder {
@BindsInstance Builder loaderManager(LoaderManager lMgr);
LockListComponent build();
}
}

119. @Component
interface LockListComponent {
void inject(LockDashboard lockDashboard);
@Component.Builder interface Builder {
@BindsInstance Builder loaderManager(LoaderManager lMgr);
LockListComponent build();
}
}

120. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.builder()
.loaderManager(getLoaderManager())
.build()
.inject(this);
// ...
}
}

121. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

122. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.builder()
.loaderManager(getLoaderManager())
.build()
.inject(this);
// ...
}
}

123. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
AndroidInjection.inject(this);
// ...
}
}

124. class LockDashboard extends Fragment {
Presenter locklistPresenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final Context context = getContext();
Database database = new Database(context);
LocksRepository locksRepository =
new LocksRepository(database, getLoaderManager());
lockListPresenter =
new Presenter(locksRepository);
}
}

125. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
AndroidInjection.inject(this);
}
}

126. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help me keep up?

127. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help me keep up?

128. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

129. LockDashboard
Presenter
LocksRepository
DatabaseLoaderManager

130. LockDashboard
Presenter
LocksRepository
FakeDatabaseLoaderManager

131. @Component(modules = LockListModule.class)
interface LockListComponent {
void inject(LockDashboard lockDashboard);
}

132. @Component(modules = TestLockListModule.class)
interface TestLockListComponent extends LockListComponent {
void inject(LockDashboard lockDashboard);
}

133. @Component(modules = TestLockListModule.class)
interface TestLockListComponent extends LockListComponent {
void inject(LockDashboard lockDashboard);
}

134. @Component(modules = TestLockListModule.class)
interface TestLockListComponent extends LockListComponent {
void inject(LockDashboard lockDashboard);
}

135. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
DaggerLockListComponent.builder()
.loaderManager(getLoaderManager())
.build()
.inject(this);
// ...
}
}

136. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
((MyApplication)getActivity().getApplication())
.getLockDashboardComponent()
.loaderManager(getLoaderManager())
.build()
.inject(this);
// ...
}
}

137. class LockDashboard extends Fragment {
@Inject Presenter presenter;
@Override public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
((MyApplication)getActivity().getApplication())
.getLockDashboardComponent()
.loaderManager(getLoaderManager())
.build()
.inject(this);
// ...
}
}

138. What is DI?
Why would I want DI?
Why would I want Dagger to do DI?
How do I Dagger?
How can Dagger help me keep up?

140. Keeping up
with DI and
Dagger
@philosohacker
@unikeytech
(we’re hiring)