【Dagger】@Provides vs @Binds


久々に Dagger を使うとかなりの不自由感。



👉 Dagger 2 @Binds vs @Provides – Elye – Medium 

👉 Why is @Binds different from @Provides? – Frequently Asked Questions 

Android が登場してから10年程度ですが、経緯というか、伝統というか、歴史というか。


Toolbar / ActionBar もそうでしょう?


この、Dagger2 2019-08-15現在 もそれと同じ雰囲気なのでしょう。


@Binds methods must have only one parameter whose type is assignable to the return type

So only a single parameter, and the type return is typically the interface of the given parameter object.

Having said that, the other tips is consider using static function for @Provides which would help also reduce some generated codes.




@Provides, the most common construct for configuring a binding, serves three functions:
1. Declare which type (possibly qualified) is being provided — this is the return type
2. Declare dependencies — these are the method parameters
3. Provide an implementation for exactly how the instance is provided — this is the method body

While the first two functions are unique and critical to every @Provides method, the third can often be tedious and repetitive. So, whenever there is a @Provides whose implementation is simple and common enough to be inferred by Dagger, it makes sense to just declare that as a method without a body (an abstract method) and have Dagger apply the behavior.

But, if we were to just say that abstract @Provides methods should be treated as we do for @Binds methods, the specification of @Provides would basically be two specifications with a bunch of conditional logic. For example, a @Provides method can have any number of parameters of any type, but a @Binds method can only have a single parameter whose type is assignable to the return type. Separating those specifications makes it easier to reason about correctness because the annotation determines the constraints.



しかし、abstract @Provides methodsを@Binds methodsのように扱うべきであると言うだけなら、@Providesの仕様は基本的に条件付きロジックの束を持つ2つの仕様になります。たとえば、@Provides methodは、任意の型の任意の数のパラメーターを持つことができますが、a @Binds methodは、型が戻り型に割り当て可能な単一のパラメーターのみを持つことができます。これらの仕様を分離すると、注釈によって制約が決定されるため、正確性についての推論が容易になります。


kotlin でいうとこの

1. 基本 @Binds を使う。
2. Application 起動の object の場合(static な singleton メソッドの場合)は @Provides を使う。


👉 ATM – Dagger Tutorial