让你的代码减少三倍！使用kotlin开发Android(五) 监听器

wing％1 $ S

在前面的博客中，有一个栗子，是点击按钮转跳的监听器。

让你的代码量减少3倍！使用kotlin开发Android(四) kotlin bean背后的秘密

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button.setOnClickListener {
val user = User("name")
user.id = "100"
SecondActivity.startActivity(this,user)
}
 

可以看到，不用再new一个OnClickListener了。实际上这种写法是lambda的一种简略模式，通常情况下匿名内部类可以以如下表现形式展示：

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button.setOnClickListener {
view-> toast(view.id.toString())
}
 

简单来说就是->左边为参数，右面为结果。其过程相当于

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<br />button.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View view){
toast(view.id.toString())
}
})
 

从以上栗子可以看出，lambda可以极大简化我们的代码。当左面参数没有使用到的时候，可以省略掉整个左半部分”view->”，就如同第一个代码片一样。

给RecyclerView加监听

在MainActivity里添加一个RecyclerView。并且新建一个适配器起名MainAdapter

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<br />import kotlinx.android.synthetic.main.item_main.view.*
 
class MainAdapter: RecyclerView.Adapter() {
override fun onBindViewHolder(holder: ViewHolder?, position: Int) {
 
//改变作用域，with{}都是属于itemView
with(holder?.itemView!!){
tv_content.text = "第 $position 条数据"
}
}
 
override fun getItemCount(): Int {
return 5
}
 
override fun onCreateViewHolder(parent: ViewGroup?, viewType: Int): ViewHolder {
return ViewHolder(View.inflate(parent?.context, R.layout.item_main,null))
}
class ViewHolder(itemView: View?) : RecyclerView.ViewHolder(itemView) {
 
}
}
 

像往常一样写一个RecyclerView的布局

### 注意这个适配器里使用的一些小技巧

1.with改变作用域，如果你有一大堆需要重复写的前缀，则可以使用with改变作用域。如:

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mBinding?.tvName?.text = "张三"
mBinding?.tvContent?.text = "张三跟李四有py交易"
mBinding?.ivIcon.resouce = R.drawable.icon
 
 

可以改为

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with(mBinding){
tvName?.text = "张三"
tvContent?.text = "张三跟李四有py交易"
ivIcon.resouce = R.drawable.icon
}
 

2.在头部导入

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import kotlinx.android.synthetic.main.item_main.view.*
 

这样就可以免去万恶的findviewbyid

3.View.infalte()代替LayoutInfalter.form().infalte()
这个没啥好说的，省点代码而已~~

4.String模板输出，类似于C语言的print

添加监听器

大家都知道，有个简便的方法就是直接给itemView添加个OnClickListener()~

所以在onBindViewHolder()开刀,注意写法，这里与java有不同

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class MainAdapter : RecyclerView.Adapter() {
var mListener: ((pos: Int) -> Unit)? = null
 
fun setOnItemClickListener(listener: ((pos: Int) -> Unit)){
mListener = listener
}
override fun onBindViewHolder(holder: ViewHolder?, position: Int) {
with(holder?.itemView!!) {
tv_content.text = "第 $position 条数据"
setOnClickListener { mListener?.invoke(position) }
}
}
 

其中给了一个函数变量mListener， 他的输入为int，返回值为Unit(kotlin的null，不过是个具体的对象)

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var mListener: ((pos: Int) -> Unit)? = null
 

接下来给个set函数

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fun setOnItemClickListener(listener: ((pos: Int) -> Unit)){
mListener = listener
}
 

这样就完成了item监听器的编写，所以我们的Activity代码如下：

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val adapter = MainAdapter()
recyclerView.adapter = adapter
recyclerView.layoutManager = LinearLayoutManager(this)
adapter.setOnItemClickListener { pos->toast("$pos") }
 

本文这就结束了(嫌文字太短？不要急啦，先去动手实践下咯。。系列文章，既然挖了坑，就不会拖更的(吧..))

欢迎加入Kotlin小乐园群（这是一个高质量、优秀的圈子）：479109923

如果你是Android开发者，那么你还可以来 wing的酒馆:425983695来分享你的开发经验哦

 

本项目使用的代码地址

51 thoughts on “让你的代码减少三倍！使用kotlin开发Android(五) 监听器”

  2. var mListener: ((pos: Int) -> Unit)? = null

    fun setOnItemClickListener(listener: ((pos: Int) -> Unit)){
    mListener = listener
    } 请分析一下 ((pos: Int) -> Unit) 这是个什么类型

    回复

  3. adapter.setOnItemClickListener { pos->toast(“$pos”) }
    activity里面的这个代码 pos->toast(“$pos”) } 这个参数 pos怎么来的啊

    回复

      1. 这不叫回调把，就是把adatpter的监听器初始化为pos->toast(“$pos”) ，然后当你点击adapter里面item,会调用该监听器的执行方法。
        回调的意思A调用B的方法，然后在B的方法中又调用了A的方法，此时才叫回调。

        回复

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  26. listener: ((pos: Int) -> Unit)
    大神这个看不懂啊，能否详细解释一下，我觉得整篇文章就在这里

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