LiveData观察者的Kotlin语法? [英] Kotlin syntax for LiveData observer?

问题描述

在我的HomeActivity中有以下代码可以使用LiveData.

I have the following bit of code in my HomeActivity to use LiveData.

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    // Show the launch splash screen.
    //
    this.setContentView(R.layout.activity_home)

    this.viewModel = ViewModelProviders.of(this).get(HomeViewModel::class.java)

    this.viewModel.getUser().observe(this, Observer { user: User? ->

    });

}

这似乎可行，但以下部分是什么意思?

While this seems to work, what does the following part mean?

Observer { user: User? ->

}

这必须导致对象符合具有

This must result in an object that conforms to the Observer interface which has

void onChanged (T t)

https://developer.android.com/reference/android/arch/lifecycle/Observer.html

如何

Observer { user: User? ->

}

使用onChanged方法产生对象吗?

我不知道将接口名称放在lambda表达式前面意味着什么.

I don't know what putting the name of an interface in front of a lambda expression means.

谢谢！

推荐答案

这称为 SAM转换，该概念有助于与Java 单一抽象方法接口交互，例如你的例子.

This is called SAM Conversion, a concept that helps interacting with Java Single Abstract Method Interfaces like in your example.

以下创建了Runnable的实现，其中单个抽象方法是run():

The following creates an implementation of Runnable, where the single abstract method is run():

val runnable = Runnable { println("This runs in a runnable") }

文档中对此进行了描述: https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions

It’s described in the docs: https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions

或者，更冗长的是使用object:

Alternatively, but more verbose, would be to use an object:

val runnable2 = object : Runnable {
        override fun run() {
            println("This runs in a runnable")
        }
}

这两个都是该interface匿名实现的示例.当然也可以创建一个具体的子类，然后实例化它.

Both are examples of anonymous implementations of that interface. It's of course also possible to create a concrete subclass and instantiate it then.

class MyRunnable : Runnable {
    override fun run() {
        println("This runs in a runnable")
    }
}

val runnable3 = MyRunnable()

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