Dagger2自定义@Qualifier用法 [英] Dagger2 custom @Qualifier usage

问题描述

假设我正在制造汽车并且我有几个具有不同实施的制动豆

Suppose I'm building a car and I have several Brake beans with different implementations

class Car {
    @Inject
    Car(@BrakeType(value="abs")Brake frontBrake, @BrakeType(value="nonabs")Brake rearBrake) { }
}

@Qualifier
@Retention(RetentionPolicy.RUNTIME)
public @interface BrakeType {
    String value();
}

interface Brake {}

@BrakeType(value="abs")
class AbsBrakeImpl implements Brake {
    @Inject AbsBrakeImpl() {}
}

@BrakeType(value="nonabs")
class BrakeImpl implements Brake {
    @Inject BrakeImpl() {}
}

为什么我的CarModule必须为特定的Brake类型定义@Provides？自定义注释类型@BrakeType不应该足以确定要注入哪个impl？或者是否需要使用反射，dagger2不使用？

why does my CarModule have to define @Provides for the specific Brake types? Shouldn't the custom annotation type @BrakeType be enough to determine which impl to inject? Or would that require using reflection, which dagger2 does not use?

@Module
public class CarModule {
    @Provides @BrakeType("abs")
    public Brake absBrake() {
        return new AbsBrakeImpl();
    }
    @Provides @BrakeType("nonabs")
    public Brake nonabsBrake() {
        return new BrakeImpl();
    }
}

推荐答案

Dagger不会在类上查看限定符注释，只能在 @Provides 或 @Binds 方法上查看。所以你班上的 @BrakeType（value =abs）注释没有任何效果。

Dagger doesn't look at qualifier annotations on classes, only on @Provides or @Binds methods. So the @BrakeType(value="abs") annotations on your classes don't have any effect.

编写代码的更规范的方法是：

A more canonical way of writing your code is:

class AbsBrakeImpl implements Brake {
    @Inject AbsBrakeImpl() {}
}

class BrakeImpl implements Brake {
    @Inject BrakeImpl() {}
}

@Module
abstract class CarModule {
    @Binds @BrakeType("abs")
    abstract Brake absBrake(AbsBrakeImpl impl);

    @Binds @BrakeType("nonabs")
    abstract Brake nonabsBrake(BrakeImpl impl);
}

请注意，因为你有 @Inject 在你的实现的构造函数上，你可以简单地使用Dagger的 @Bind 将实现直接绑定到适当的限定接口。

Note that since you have @Inject on the constructors of your implementations, you can simply use Dagger's @Bind to bind the implementations directly to the appropriately qualified interface.

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