匕首2 - 无法注射物体

Question

我试图在Android应用程序中进行非常简单的依赖注入.我使用匕首2作为DI工具.

问题是没有发生注射:

这是我的代码:

//不料Motor.java敬畏.

public class Motor {

    private int rpm;

    public Motor(){
        this.rpm = 10; //default will be 10
    }

    public int getRpm(){
        return rpm;
    }

    public void accelerate(int value){
        rpm = rpm + value;
    }

    public void brake(){
        rpm = 0;
    }
}

这是实现电机类的Vehicle.java:

import javax.inject.Inject;


public class Vehicle {

    private Motor motor;

    @Inject
    public Vehicle(Motor motor){
        this.motor = motor;
    }

    public void increaseSpeed(int value){
        motor.accelerate(value);
    }

    public void stop(){
        motor.brake();
    }

    public int getSpeed(){
        return motor.getRpm();
    }
}

然后我创建了一个VehicleModule.java类来定义我的提供者:

import javax.inject.Singleton;

import dagger.Module;
import dagger.Provides;



@Module
public class VehicleModule {

    @Provides
    @Singleton
    Motor provideMotor(){
        return new Motor();
    }

    @Provides @Singleton
    Vehicle provideVehicle(){
        return new Vehicle(new Motor());
    }
}

然后,我有一个注释的接口组件,定义如下:

import javax.inject.Singleton;

import Modules.VehicleModule;
import dagger.Component;

@Singleton
@Component(modules = {VehicleModule.class})
public interface VehicleComponent {

    Vehicle provideVehicle();

}

这是我的Android mainactivity类应该注入,但不是,任何人都可以帮助:

import javax.inject.Inject;

public class MainActivity extends ActionBarActivity {

    @Inject
    Vehicle vehicle;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        Toast.makeText(this, String.valueOf(vehicle.getSpeed()), Toast.LENGTH_SHORT).show();
    }
}

我在车辆上得到一个空指针异常:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int com.example.uen229.myapplication.Vehicle.getSpeed()' on a null object reference

顺便说一句,我的gradle依赖项看起来像这样:

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:22.2.0'
    compile 'com.google.dagger:dagger:2.0'
}

Answer 1

这是我应该注入的Android mainactivity类,但不是

当你注释某些东西时,你期待发生神奇的事情@Inject.虽然神奇的事情会发生,但并不是那么神奇.您需要通过实例化Dagger生成的组件实现来自己完成.

您可以通过几种方式完成此操作,我将介绍两种方法.

首先,在你MainActivity的onCreate:

private Vehicle vehicle; // Note, no @Inject annotation

public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);

  VehicleComponent vehicleComponent = DaggerVehicleComponent.create();
  this.vehicle = vehicleComponent.provideVehicle();

  Toast.makeText(this, String.valueOf(vehicle.getSpeed()), Toast.LENGTH_SHORT).show();
}

在这种情况下,您创建一个VehicleComponent由Dagger实现的Vehicle实例,并从中获取实例.该vehicle字段未注释@Inject.这有一个优势,即该领域可以private,这是一件好事.

其次,如果你想让Dagger注入你的字段,你需要为你的inject方法添加一个方法VehicleComponent:

@Singleton
@Component(modules = {VehicleModule.class})
public interface VehicleComponent {

  Vehicle provideVehicle();

  void inject(MainActivity mainActivity);
}

在你的MainActivity课堂上,你打电话inject(this),这将填补该vehicle领域:

@Inject
Vehicle vehicle; // Note, package-local

public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);

  VehicleComponent vehicleComponent = DaggerVehicleComponent.create();
  vehicleComponent.inject(this);

  Toast.makeText(this, String.valueOf(vehicle.getSpeed()), Toast.LENGTH_SHORT).show();
}

这带来了一些额外的配置,但有时是必要的.但我喜欢第一种方法.

作为最后的评论,让我们来看看你的VehicleModule,并真正使用Dagger的力量.

您可以自己制作Dagger,而不是使用模块自己创建实例.你已经用Vehicle构造函数注释了@Inject,所以Dagger会知道使用这个构造函数.但是,它需要一个Motor它不知道的实例.如果你也将@Inject注释添加到构造函数中Motor,并使用它注释Motor类@Singleton,那么你可以VehicleModule完全摆脱它!

例如:

@Singleton
public class Motor {

    private int rpm;

    @Inject // Just an annotation to let Dagger know how to retrieve an instance of Motor.
    public Motor(){
        this.rpm = 10; //default will be 10
    }

    public int getRpm(){
        return rpm;
    }

    public void accelerate(int value){
        rpm = rpm + value;
    }

    public void brake(){
        rpm = 0;
    }
}

你的Vehicle班级:

@Singleton
public class Vehicle {

    private Motor motor;

    @Inject
    public Vehicle(Motor motor){
        this.motor = motor;
    }

    public void increaseSpeed(int value){
        motor.accelerate(value);
    }

    public void stop(){
        motor.brake();
    }

    public int getSpeed(){
        return motor.getRpm();
    }
}

您现在可以安全地删除VehicleModule该类,并在您的类中删除对它的引用VehicleComponent.