触发Kotlin for Android的界面 [英] Triggering an Interface in Kotlin for Android

问题描述

在使用Kotlin for Android时，有一个特定的接口实现：从片段触发接口。考虑Fragment必须将UI操作传递给父Activity的常见场景。在Java中，我们将定义接口，为其创建侦听器实例，并在接口父节点中实现/覆盖操作。创建一个听众实例对我来说并不那么简单。经过一些谷歌搜索，我找到了一个实现示例，但我不明白为什么它的工作原理。有一个更好的方法吗？为什么必须以下面的方式实现？

There is a specific implementation of interface when it comes to using Kotlin for Android: triggering an interface from a fragment. Consider the common scenario where a Fragment must communicate a UI action to the parent Activity. In Java, we would define the interface, create a "listener" instance of it, and implement/override actions in interface parents. It is the creating a listener instance that is not so straightforward to me. After some googling, I found an implementation example but I do not understand why it works. Is there a better way to do this? Why must it be implemented the way it is below?

    class ImplementingFragment : Fragment(){

  private lateinit var listener: FragmentEvent
  private lateinit var vFab: FloatingActionButton

  //Here is the key piece of code which triggers the interface which is befuddling to me
  override fun onAttach(context: Context?) {
          super.onAttach(context)
          if(context is FragmentEvent) {
              listener = context
          }
      }

    //Notice the onClickListener using our interface listener member variable
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {
        val v: View? = inflater.inflate(R.layout.fragment_layout, container, false)
            vFab = v?.findViewById(R.id.fh_fab)
            vFab.setOnClickListener{listener.SomethingHappened()}
        return v
    }  


}



interface FragmentEvent{
    fun SomethingHappened()
}

推荐答案

在此代码中 lateinit var listener：FragmentEvent 应该初始化，不能 null 。所以 onAttach 应该看起来像

In this code lateinit var listener: FragmentEvent should be initialized and can't be null. So onAttach should looks like

override fun onAttach(context: Context?) {
    super.onAttach(context)
    if (context is FragmentEvent) {
        listener = context
    } else {
        throw RuntimeException(context!!.toString() + " must implement FragmentEvent")
    }
}

在这种情况下如果您忘记在活动中实施 FragmentEvent ，您将收到异常，否则回调已准备好使用。

In this case if you forget to implement FragmentEvent in Activity you'll got an exception, otherwise callback is ready to use.

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