在前面的博客中,有一个栗子,是点击按钮转跳的监听器。
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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
快更新啊
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
这就是个lambda咯
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
监听器里回调来的咯
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。
太6了老哥
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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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