Dagger2是一个Java和Android的依赖注入框架.
本文介绍Android中dagger2的基本使用.
其中包括@Inject, @Component, @Module和@Provides注解的使用.

使用依赖注入的好处

1.使用类和被依赖的对象构造分开,这样如果我们需要改变被依赖类的构造方法,不必改动每一个使用类.
2.对各种被依赖类的实例,可以只构造一次.
3.当我们需要更换一种实现时,只需要保证接口一致.
4.利于单元测试,我们可以方便地mock依赖类的对象.

优点总结: 创建对象和使用对象分离, 模块化增强.

Dagger2的使用

Set Up

在项目的build.gradle里加这个:
classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'

然后app的build.gradle:

apply plugin: 'com.android.application'
apply plugin: 'com.neenbedankt.android-apt'

android {
    compileSdkVersion 24
    buildToolsVersion "24.0.0"

    defaultConfig {
        applicationId "com.ddmeng.dagger2sample"
        minSdkVersion 16
        targetSdkVersion 24
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:24.0.0'
    compile 'javax.annotation:jsr250-api:1.0'
    compile 'com.google.dagger:dagger:2.2'
    apt 'com.google.dagger:dagger-compiler:2.2'
}

常用注解

最常使用的主要是以下这几个注解:

@Component
Annotates an interface or abstract class for which a fully-formed, dependency-injected implementation is to be generated from a set of modules(). The generated class will have the name of the type annotated with @Component prepended with Dagger. For example, @Component interface MyComponent {...} will produce an implementation named DaggerMyComponent.

@Module
Annotates a class that contributes to the object graph.

@Inject
Dagger constructs instances of your application classes and satisfies their dependencies. It uses the javax.inject.Inject annotation to identify which constructors and fields it is interested in.

Use @Inject to annotate the constructor that Dagger should use to create instances of a class. When a new instance is requested, Dagger will obtain the required parameters values and invoke this constructor.

@Provides
Annotates methods of a module to create a provider method binding. The method's return type is bound to its returned value. The component implementation will pass dependencies to the method as parameters.

Dagger2基本使用

最简单的一个实例

首先写一个Component

@Component(modules = MyApplicationModule.class)
public interface MyApplicationComponent {
    // this should be an interface or abstract class

    // write like this, and Make Project, then a DaggerMyApplicationComponent class will be generated
}

此时里面的Module内容可以暂时为空:

@Module
public class MyApplicationModule {
}

写好后make一下,就生成了

package com.ddmeng.dagger2sample.component;

import com.ddmeng.dagger2sample.module.MyApplicationModule;
import dagger.internal.Preconditions;
import javax.annotation.Generated;

@Generated(
  value = "dagger.internal.codegen.ComponentProcessor",
  comments = "https://google.github.io/dagger"
)
public final class DaggerMyApplicationComponent implements MyApplicationComponent {
  private DaggerMyApplicationComponent(Builder builder) {
    assert builder != null;
  }

  public static Builder builder() {
    return new Builder();
  }

  public static MyApplicationComponent create() {
    return builder().build();
  }

  public static final class Builder {
    private Builder() {}

    public MyApplicationComponent build() {
      return new