搭建自己的框架之3:项目中引入Dagger2&Dagger.android

后台开发Spring很早很早就有依赖注入，Dragger(2) 出现后大型的Android 项目开发依赖管理也美好了。

以前在学习使用Dagger2的时候感觉理解绕，相似的模版代码还很多，哪里需要注入还要写DaggerXXX..inject(this);而且也违背依赖注入的核心原则：一个类不应该知道如何实现依赖注入；它要求注射类型知道其注射器, 即使这是通过接口而不是具体类型完成的。

谷歌大神们又研究出一套专门用于Android的注入方式,拓展Dagger2.android https://google.github.io/dagger//android.html

开始使用

首先在Gradle 中引入依赖，目前dagger-android 还是Beta版本，建议及时更新最新

compile 'com.google.dagger:dagger-android:2.13'
compile 'com.google.dagger:dagger-android-support:2.13'
annotationProcessor 'com.google.dagger:dagger-compiler:2.13'
annotationProcessor 'com.google.dagger:dagger-android-processor:2.13'

#为什么不用Dagger2，要等Dagger2.android 出来才用

项目规模小，即使使用Dagger2 也不能显著提高生产力

dagger 难以上手，项目组可能难以推广，或者都是Copy

违背依赖注入核心原则，模版代码多

dagger2.Android 的出现很大程度的解决/缓解了以上问题

#在项目中快速使用
建议小范围或demo 大胆使用，修改添加一些东西慢慢尝试，注释都在代码里面；基本上的根据已有的规则慢慢熟悉就回了，demo 链接 https://github.com/AnyLifeZLB/MVP-Dagger2-Rxjava2，都在代码里详细的注释了。

，，，，

这里有空再写
。。，，

#下面是官方翻译 https://google.github.io/dagger//android.html

Dagger 2相比比其他大多数依赖注入框架的主要优势之一就是其严格生成的实现（无反射）意味着它可以在Android应用程序中使用。然而，在Android开发中使用Dagger还是有一些注意事项。

One of the primary advantages of Dagger 2 over most other dependency injection frameworks is that its strictly generated implementation (no reflection) means that it can be used in Android applications. However, there are still some considerations to be made when using Dagger within Android applications.

哲学问题 Philosophy

虽然为编写Android 程序也是使用的Java，但在风格方面通常是完全不同的。通常情况下，这种差异是为了适应移动平台独特的 性能考虑。

While code written for Android is Java source, it is often quite different in terms of style. Typically, such differences exist to accomodate the unique performance considerations of a mobile platform.

但是，通常应用于Android代码的许多模式与应用于其他Java代码的模式相反。甚至很多Effective Java这本书上的建议 对于Android来说都是不合适的。

But many of the patterns commonly applied to code intended for Android are contrary to those applied to other Java code. Even much of the advice in Effective Java is considered inappropriate for Android.

为了达到符合习惯和可移植代码的目标，Dagger依靠ProGuard来后处理编译的字节码。这使得Dagger能够在服务器和Android开发生成的源代码看起来和感觉都是非常的相近而又自然，同时使用不同的工具链来产生在两个环境中都能有效执行的字节码。此外，Dagger有一个明确的目标，确保它生成的Java源代码与ProGuard优化保持一致。

In order to achieve the goals of both idiomatic and portable code, Dagger relies on ProGuard to post-process the compiled bytecode. This allows Dagger to emit source that looks and feels natural on both the server and Android, while using the different toolchains to produce bytecode that executes efficiently in both environements. Moreover, Dagger has an explicit goal to ensure that the Java source that it generates is consistently compatible with ProGuard optimizations.

当然，并不是所有的问题都可以用这种方式来解决，但它是提供Android特定兼容性的主要机制。

Of course, not all issues can be addressed in that manner, but it is the primary mechanism by which Android-specific compatbility will be provided.

tl;dr

所以在Android开发中使用Dagger最好你也使用ProGuard
Dagger assumes that users on Android will use ProGuard.

推荐的ProGuard设置 Recommended ProGuard Settings

注意在ProGuard设置有对应的考虑你使用了Dagger。
Watch this space for ProGuard settings that are relevant to applications using Dagger.

让Dagger2飞起来的东西-

dagger.android

在Android开发中使用Dagger的主要困难是之一是许多Android框架类是由操作系统本身进行实例化的，像 Activity和Fragment，但是如果Dagger可以创建所有的注入对象那就再好不过了。悲剧的是您必须在生命周期方法中执行成员注入。这意味着许多类最终看起来像这个鬼样子：
One of the central difficulties of writing an Android application using Dagger is that many Android framework classes are instantiated by the OS itself, like 

Activity

 and 

Fragment

, but Dagger works best if it can create all the injected objects. Instead, you have to perform members injection in a lifecycle method. This means many classes end up looking like:

public class FrombulationActivity extends Activity {
@Inject Frombulator frombulator;

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// DO THIS FIRST. Otherwise frombulator might be null!
//悲剧的是您必须在生命周期方法中执行成员注入
((SomeApplicationBaseType) getContext().getApplicationContext())
.getApplicationComponent()
.newActivityComponentBuilder()
.activity(this)
.build()
.inject(this);
// ... now you can write the exciting code
}
}

这样就会有些问题
This has a few problems:

1.即使Dagger使我们的代码耦合性更低，但是如果要面临重构，我们仍然不得不去面对每个Activity中这样数行需要我们「复制」+「粘贴」的代码，这会给我们的重构带来一定的难度（试想一下，如果我们的应用有数十个乃至上百个这样的Activity或者Fragment容器，我们的重构计划，首先就要面对这样数百行的代码）。
并且随着新的开发人员加入（他们也许并不知道这些代码的意义，但是他们会复制粘贴），越来越少的人知道这些代码都干了些什么。
2.更重要的是，它要求注射类型（FrombulationActivity）知道其注射器。 即使这是通过接口而不是具体类型完成的，它打破了依赖注入的核心原则：一个类不应该知道如何实现依赖注入。

Copy-pasting code makes it hard to refactor later on. As more and more developers copy-paste that block, fewer will know what it actually does.（项目组中太少人去研究Dagger）

More fundamentally, it requires the type requesting injection (

FrombulationActivity

) to know about its injector. Even if this is done through interfaces instead of concrete types, it breaks a core principle of dependency injection: a class shouldn’t know anything about how it is injected.
这打破了依赖注入的核心原则：一个类不应该知道如何依赖注入，最好在需要的地方@Inject就注入完成了✅

The classes in 

dagger.android

 offer one approach to simplify this pattern.

Injecting 

Activity

 objects

Install 

AndroidInjectionModule

 in your application component to ensure that all bindings necessary for these base types are available.

Start off by writing a 

@Subcomponent

 that implements 

AndroidInjector<YourActivity>

, with a 

@Subcomponent.Builder

that extends 

AndroidInjector.Builder<YourActivity>

:

@Subcomponent(modules = ...)
public interface YourActivitySubcomponent extends AndroidInjector<YourActivity> {
@Subcomponent.Builder
public abstract class Builder extends AndroidInjector.Builder<YourActivity> {}
}

After defining the subcomponent, add it to your component hierarchy by defining a module that binds the subcomponent builder and adding it to the component that injects your 

Application

:

@Module(subcomponents = YourActivitySubcomponent.class)
abstract class YourActivityModule {
@Binds
@IntoMap
@ActivityKey(YourActivity.class)
abstract AndroidInjector.Factory<? extends Activity>
bindYourActivityInjectorFactory(YourActivitySubcomponent.Builder builder);
}

@Component(modules = {..., YourActivityModule.class})
interface YourApplicationComponent {}

Pro-tip: If your subcomponent and its builder have no other methods or supertypes than the ones mentioned in step #2, you can use

@ContributesAndroidInjector

 to generate them for you. Instead of steps 2 and 3, add an 

abstract

 module method that returns your activity, annotate it with 

@ContributesAndroidInjector

, and specify the modules you want to install into the subcomponent. If the subcomponent needs scopes, apply the scope annotations to the method as well.

@ActivityScope
@ContributesAndroidInjector(modules = { /* modules to install into the subcomponent */ })
abstract YourActivity contributeYourActivityInjector();

Next, make your 

Application

 implement 

HasActivityInjector

 and 

@Inject

 a

DispatchingAndroidInjector<Activity>

 to return from the 

activityInjector()

 method:

public class YourApplication extends Application implements HasActivityInjector {
@Inject DispatchingAndroidInjector<Activity> dispatchingActivityInjector;

@Override
public void onCreate() {
super.onCreate();
DaggerYourApplicationComponent.create()
.inject(this);
}

@Override
public AndroidInjector<Activity> activityInjector() {
return dispatchingActivityInjector;
}
}

Finally, in your 

Activity.onCreate()

 method, call 

AndroidInjection.inject(this)

 before calling 

super.onCreate();

:

public class YourActivity extends Activity {
public void onCreate(Bundle savedInstanceState) {
AndroidInjection.inject(this);
super.onCreate(savedInstanceState);
}
}

Congratulations!

How did that work?

AndroidInjection.inject()

 gets a 

DispatchingAndroidInjector<Activity>

 from the 

Application

 and passes your activity to 

inject(Activity)

. The 

DispatchingAndroidInjector

 looks up the 

AndroidInjector.Factory

 for your activity’s class (which is 

YourActivitySubcomponent.Builder

), creates the 

AndroidInjector

 (which is 

YourActivitySubcomponent

), and passes your activity to 

inject(YourActivity)

.

Injecting 

Fragment

 objects

Injecting a 

Fragment

 is just as simple as injecting an 

Activity

. Define your subcomponent in the same way, replacing 

Activity

 type parameters with 

Fragment

, 

@ActivityKey

 with 

@FragmentKey

, and 

HasActivityInjector

 with 

HasFragmentInjector

.

Instead of injecting in 

onCreate()

 as is done for 

Activity

 types, inject 

Fragment

s to in 

onAttach()

.

Unlike the modules defined for 

Activity

s, you have a choice of where to install modules for 

Fragment

s. You can make your 

Fragment

component a subcomponent of another 

Fragment

 component, an 

Activity

 component, or the 

Application

 component — it all depends on which other bindings your 

Fragment

 requires. After deciding on the component location, make the corresponding type implement 

HasFragmentInjector

. For example, if your 

Fragment

 needs bindings from 

YourActivitySubcomponent

, your code will look something like this:

public class YourActivity extends Activity
implements HasFragmentInjector {
@Inject DispatchingAndroidInjector<Fragment> fragmentInjector;

@Override
public void onCreate(Bundle savedInstanceState) {
AndroidInjection.inject(this);
super.onCreate(savedInstanceState);
// ...
}

@Override
public AndroidInjector<Fragment> fragmentInjector() {
return fragmentInjector;
}
}

public class YourFragment extends Fragment {
@Inject SomeDependency someDep;

@Override
public void onAttach(Activity activity) {
AndroidInjection.inject(this);
super.onAttach(activity);
// ...
}
}

@Subcomponent(modules = ...)
public interface YourFragmentSubcomponent extends AndroidInjector<YourFragment> {
@Subcomponent.Builder
public abstract class Builder extends AndroidInjector.Builder<YourFragment> {}
}

@Module(subcomponents = YourFragmentSubcomponent.class)
abstract class YourFragmentModule {
@Binds
@IntoMap
@FragmentKey(YourFragment.class)
abstract AndroidInjector.Factory<? extends Fragment>
bindYourFragmentInjectorFactory(YourFragmentSubcomponent.Builder builder);
}

@Subcomponent(modules = { YourFragmentModule.class, ... }
public interface YourActivityOrYourApplicationComponent { ... }

Base Framework Types

Because 

DispatchingAndroidInjector

 looks up the appropriate 

AndroidInjector.Factory

 by the class at runtime, a base class can implement 

HasActivityInjector

/

HasFragmentInjector

/etc as well as call 

AndroidInjection.inject()

. All each subclass needs to do is bind a corresponding 

@Subcomponent

. Dagger provides a few base types that do this, such as 

DaggerActivity

 and 

DaggerFragment

, if you don’t have a complicated class hierarchy. Dagger also provides a 

DaggerApplication

 for the same purpose — all you need to do is to extend it and override the 

applicationInjector()

 method to return the component that should inject the

Application

.

The following types are also included:

DaggerService

 and 

DaggerIntentService

DaggerBroadcastReceiver

DaggerContentProvider

Note: 

DaggerBroadcastReceiver

 should only be used when the 

BroadcastReceiver

 is registered in the 

AndroidManifest.xml

. When the 

BroadcastReceiver

 is created in your own code, prefer constructor injection instead.

Support libraries

For users of the Android support library, parallel types exist in the 

dagger.android.support

 package. Note that while support 

Fragment

 users have to bind 

AndroidInjector.Factory<? extends android.support.v4.app.Fragment>

, AppCompat users should continue to implement 

AndroidInjector.Factory<? extends Activity>

 and not 

<? extends AppCompatActivity>

 (or 

FragmentActivity

).

How do I get it?

Add the following to your build.gradle:

dependencies {
compile 'com.google.dagger:dagger-android:2.x'
compile 'com.google.dagger:dagger-android-support:2.x' // if you use the support libraries
annotationProcessor 'com.google.dagger:dagger-android-processor:2.x'
}

When to inject

Constructor injection is preferred whenever possible because 

javac

 will ensure that no field is referenced before it has been set, which helps avoid 

NullPointerException

s. When members injection is required (as discussed above), prefer to inject as early as possible. For this reason, 

DaggerActivity

 calls 

AndroidInjection.inject()

 immediately in 

onCreate()

, before calling

super.onCreate()

, and 

DaggerFragment

 does the same in 

onAttach()

, which also prevents inconsistencies if the 

Fragment

 is reattached.

It is crucial to call 

AndroidInjection.inject()

 before 

super.onCreate()

 in an 

Activity

, since the call to 

super

 attaches 

Fragment

s from the previous activity instance during configuration change, which in turn injects the 

Fragment

s. In order for the 

Fragment

 injection to succeed, the 

Activity

 must already be injected. For users of ErrorProne, it is a compiler error to call 

AndroidInjection.inject()

 after 

super.onCreate()

.

FAQ

Scoping 

AndroidInjector.Factory

AndroidInjector.Factory

 is intended to be a stateless interface so that implementors don’t have to worry about managing state related to the object which will be injected. When 

DispatchingAndroidInjector

 requests a 

AndroidInjector.Factory

, it does so through a 

Provider

 so that it doesn’t explicitly retain any instances of the factory. Because the 

AndroidInjector.Builder

implementation that is generated by Dagger does retain an instance of the 

Activity

/

Fragment

/etc that is being injected, it is a compile-time error to apply a scope to the methods which provide them. If you are positive that your 

AndroidInjector.Factory

 does not retain an instance to the injected object, you may suppress this error by applying

@SuppressWarnings("dagger.android.ScopedInjectoryFactory")

 to your module method.