第一行代码第三版java版资源第一行代码第三版pdf

转载

mob6454cc613c41 2024-05-15 13:33:43

文章标签 第一行代码第三版java版资源 android ui kotlin xml 文章分类 Java 后端开发

第四章 UI

注：因为和第一行代码第二版高度重合，所以我就记了一些我不太熟的地方和不一样的地方

4.2常用控件

4.2.2Button

android:textAllCaps="false"//取消默认大写字体

设计点击事件:

匿名内部类式

button.setOnClickListener{
            
        }

实现接口式

class MainActivity : AppCompatActivity() , View.OnClickListener{
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        button.setOnClickListener(this) 
    }

    override fun onClick(p0: View?) {
        when(p0?.id){
            R.id.button ->{
                
            }
        }
    }
}

补充：kotlin 的set/get方法的语法糖，一般直接写带有它们的格式然后利用代码提示tab来获取简便用法

4.2.4 ImageView

imageView.setImageResource(R.drawable.img_2)

4.2.5 ProgressBar

4.2.6 AlertDialog

AlertDialog.Builder(this).apply {
                setTitle("This is a Title")
                setMessage("This is a Message")
                setCancelable(false)
                setPositiveButton("OK"){
                    dialog,which ->
                }
                setNegativeButton("Cancel"){
                    dialog,which ->
                }
                show()
            }

4.3 布局

4.3.1 线性布局

4.3.2 相对布局

相对父布局位置

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:text="button1"
        />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_alignParentTop="true"
        android:text="button2"
        />
    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="button3"
        />
    <Button
        android:id="@+id/button4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentBottom="true"
        android:text="button4"
        />
    <Button
        android:id="@+id/button5"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_alignParentBottom="true"
        android:text="button5"
        />

</RelativeLayout>

相对其他控件位置

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@id/button3"
        android:layout_toLeftOf="@id/button3"
        android:text="button1"
        />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@id/button3"
        android:layout_toRightOf="@id/button3"
        android:text="button2"
        />
    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="button3"
        />
    <Button
        android:id="@+id/button4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/button3"
        android:layout_toLeftOf="@id/button3"
        android:text="button4"
        />
    <Button
        android:id="@+id/button5"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/button3"
        android:layout_toRightOf="@id/button3"
        android:text="button5"
        />

</RelativeLayout>

边缘对齐

android:layout_alignLeft 表示一个控件和另一个控件左边缘对齐
右边，底部和顶部类似

4.3.3 FrameLayout

控件默认摆放在左上角，xml中放在下面的控件压在上层，用layout_gravity定位

4.4 创建自定义控件

4.4.1 引入布局

写布局文件
利用引入

补充：supportActioinBar?.hide( )隐藏标题栏

4.4.3 创建自定义控件

自定义控件布局

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:orientation="horizontal"
    android:layout_height="wrap_content">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/titleBack"
        android:layout_gravity="center"
        android:layout_margin="5dp"
        android:text="Back"
        android:textColor="#fff"
        ></Button>

    <TextView
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:id="@+id/titleText"
        android:layout_weight="1"
        android:gravity="center"
        android:text="Title Text"
        android:textColor="#fff"
        android:textSize="24sp"
        ></TextView>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/titleEdit"
        android:layout_gravity="center"
        android:layout_margin="5dp"
        android:text="Edit"
        android:textColor="#fff"
        ></Button>


</LinearLayout>

编写自定义控件代码

class TitleLayout(context: Context, attrs: AttributeSet):LinearLayout(context,attrs) {
    init{
        LayoutInflater.from(context).inflate(R.layout.title,this)//动态加载布局
        titleBack.setOnClickListener {
            val activity = context as Activity //as是kotlin中的强制类型转换
            activity.finish()
        }
        titleEdit.setOnClickListener {
            Toast.makeText(context,"You clicked Edit button",Toast.LENGTH_LONG).show()
        }
    }

}

引入控件

<android.bignerdranch.uilayouttest.TitleLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"></android.bignerdranch.uilayouttest.TitleLayout>

##4.5 ListView

4.5.1 简单用法

布局放入listview
代码

private val data = listOf("苹果","梨子","香蕉","西瓜","凤梨","火龙果","龙眼","枸杞","水蜜桃","樱桃")

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val adapter = ArrayAdapter<String>(this, androidx.appcompat.R.layout.support_simple_spinner_dropdown_item,data)
        listView.adapter = adapter
    }

4.5.2 定制 ListView 的界面

准备一组图片,放下android-xxhdpi下
定义实体类

class Fruit(val name:String,val imageId:Int) {
}

定义子项布局 fruit_item.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="60dp">

    <ImageView
        android:id="@+id/fruitImage"
        android:layout_width="40dp"
        android:layout_height="40dp"
        android:layout_gravity="center_vertical"
        android:layout_marginLeft="10dp"/>

    <TextView
        android:id="@+id/fruitName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_vertical"
        android:layout_marginLeft="10dp" />

</LinearLayout>

自定义适配器

class FruitAdapter(activity: Activity, val resourceId: Int,data:List<Fruit>) : ArrayAdapter<Fruit>(activity,resourceId,data){

    //每个子项被滚动到屏幕内的时候会被调用
    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {
        val view = LayoutInflater.from(context).inflate(resourceId,parent,false)//false 只让我们在父布局中声明的layout起作用，不会为这个view生命父布局。
        val fruitImage : ImageView = view.findViewById(R.id.fruitImage)
        val fruitName : TextView = view.findViewById(R.id.fruitName)
        val fruit =  getItem(position)
        if (fruit != null) {
            fruitImage.setImageResource(fruit.imageId)
            fruitName.text = fruit.name
        }
        return view
    }
}

编写代码

class MainActivity : AppCompatActivity() {

    private val fruitList = ArrayList<Fruit>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initFruits()
        val adapter = FruitAdapter(this, R.layout.fruit_item,fruitList)
        listView.adapter = adapter
    }

    private fun initFruits(){
        repeat(2){//kotlin中的标准函数，把Lambda中的代码执行两遍
            fruitList.add(Fruit("Apple",R.drawable.apple_pic))
            fruitList.add(Fruit("Banana",R.drawable.banana_pic))
            fruitList.add(Fruit("Orange",R.drawable.orange_pic))
            fruitList.add(Fruit("Watermelon",R.drawable.watermelon_pic))
            fruitList.add(Fruit("Pear",R.drawable.pear_pic))
            fruitList.add(Fruit("Grape",R.drawable.grape_pic))
            fruitList.add(Fruit("Pineapple",R.drawable.pineapple_pic))
            fruitList.add(Fruit("Strawberry",R.drawable.strawberry_pic))
            fruitList.add(Fruit("Cherry",R.drawable.cherry_pic))
            fruitList.add(Fruit("Mango",R.drawable.mango_pic))
        }
    }

}

4.5.3 提升 ListView 的运行效率

class FruitAdapter(activity: Activity, val resourceId: Int,data:List<Fruit>) : ArrayAdapter<Fruit>(activity,resourceId,data){

    //每个子项被滚动到屏幕内的时候会被调用
    //convertView 用于将之前加载好的布局进行缓存，以便重用
    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {
        val view:View
        val  viewHolder: ViewHolder
        if (convertView == null) {
            view = LayoutInflater.from(context).inflate(resourceId,parent,false)
            val fruitImage : ImageView = view.findViewById(R.id.fruitImage)
            val fruitName : TextView = view.findViewById(R.id.fruitName)
            //将实例存在ViewHolder里，然后再存在Tag里
            viewHolder = ViewHolder(fruitImage,fruitName)
            view.tag = viewHolder
        }else{
            view = convertView
          	//有缓存后从Tag里取出
            viewHolder = view.tag as ViewHolder
        }

        val fruit =  getItem(position)
        if (fruit != null) {
            viewHolder.fruitImage.setImageResource(fruit.imageId)
            viewHolder.fruitName.text = fruit.name
        }
        return view
    }

  
    inner class ViewHolder(val fruitImage: ImageView, val fruitName: TextView)
}

4.5.4 点击事件

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initFruits()
        val adapter = FruitAdapter(this, R.layout.fruit_item,fruitList)
        listView.adapter = adapter
        listView.setOnItemClickListener { _, _, position, _ ->
            val fruit = fruitList[position]
            Toast.makeText(this,fruit.name,Toast.LENGTH_LONG).show()
        }
    }

补充

如果需要知道传入什么参数可以点进去看源码
kotlin中可以将没使用的参数用下划线替代

4.6 RecycleView

准备一组图片，子项视图，水果类，放置RecycleView
编写适配器

class FruitAdapter(val fruitList:List<Fruit>): RecyclerView.Adapter<FruitAdapter.ViewHolder>() {
    //创建ViewHolder实例
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): FruitAdapter.ViewHolder {
        val view = LayoutInflater.from(parent.context).inflate(R.layout.fruit_item, parent, false)
        return ViewHolder(view)
    }
    //给子项赋值，滚动到屏幕中间的时候执行
    override fun onBindViewHolder(holder: FruitAdapter.ViewHolder, position: Int) {
        val fruit = fruitList[position]
        holder.fruitImage.setImageResource(fruit.imageId)
        holder.fruitName.text = fruit.name
    }
    //告诉有多少子项
    override fun getItemCount() = fruitList.size

    inner class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        val fruitImage: ImageView = view.findViewById(R.id.fruitImage)
        val fruitName : TextView = view.findViewById(R.id.fruitName)
    }
}

编写代码

class MainActivity : AppCompatActivity() {
    private val fruitList = ArrayList<Fruit>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initFruits()
        val layoutManager = LinearLayoutManager(this)
        recycleView.layoutManager = layoutManager
        val adapter = FruitAdapter(fruitList)
        recycleView.adapter = adapter
    }

    private fun initFruits(){
        repeat(2){
            fruitList.add(Fruit("Apple",R.drawable.apple_pic))
            fruitList.add(Fruit("Banana",R.drawable.banana_pic))
            fruitList.add(Fruit("Orange",R.drawable.orange_pic))
            fruitList.add(Fruit("Watermelon",R.drawable.watermelon_pic))
            fruitList.add(Fruit("Pear",R.drawable.pear_pic))
            fruitList.add(Fruit("Grape",R.drawable.grape_pic))
            fruitList.add(Fruit("Pineapple",R.drawable.pineapple_pic))
            fruitList.add(Fruit("Strawberry",R.drawable.strawberry_pic))
            fruitList.add(Fruit("Cherry",R.drawable.cherry_pic))
            fruitList.add(Fruit("Mango",R.drawable.mango_pic))
        }
    }
}

4.6.2 横向滚动和瀑布流布局

横向布局

横向布局

//布局
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="80dp"
    android:orientation="vertical"
    android:layout_height="wrap_content">

    <ImageView
        android:id="@+id/fruitImage"
        android:layout_width="40dp"
        android:layout_height="40dp"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="10dp"/>

    <TextView
        android:id="@+id/fruitName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="10dp" />

</LinearLayout>

代码

layoutManager.orientation = LinearLayoutManager.HORIZONTAL

瀑布流布局

布局

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_margin="5dp"
    android:orientation="vertical"
    android:layout_height="wrap_content">
		<--宽度由布局的列数自动适配，而不是固定值,所以宽度设为填充父布局-->
    <ImageView
        android:id="@+id/fruitImage"
        android:layout_width="40dp"
        android:layout_height="40dp"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="10dp"/>

    <TextView
        android:id="@+id/fruitName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="left"
        android:layout_marginTop="10dp" />

</LinearLayout>

代码

class MainActivity : AppCompatActivity() {
    private val fruitList = ArrayList<Fruit>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initFruits()
        val layoutManager = StaggeredGridLayoutManager(3,StaggeredGridLayoutManager.VERTICAL)
        recycleView.layoutManager = layoutManager
        val adapter = FruitAdapter(fruitList)
        recycleView.adapter = adapter
    }

    private fun initFruits(){
        repeat(2){
            fruitList.add(Fruit(getRandomLengthString("Apple"),R.drawable.apple_pic))
            fruitList.add(Fruit(getRandomLengthString("Banana"),R.drawable.banana_pic))
            fruitList.add(Fruit(getRandomLengthString("Orange"),R.drawable.orange_pic))
            fruitList.add(Fruit(getRandomLengthString("Watermelon"),R.drawable.watermelon_pic))
            fruitList.add(Fruit(getRandomLengthString("Pear"),R.drawable.pear_pic))
            fruitList.add(Fruit(getRandomLengthString("Grape"),R.drawable.grape_pic))
            fruitList.add(Fruit(getRandomLengthString("Pineapple"),R.drawable.pineapple_pic))
            fruitList.add(Fruit(getRandomLengthString("Strawberry"),R.drawable.strawberry_pic))
            fruitList.add(Fruit(getRandomLengthString("Cherry"),R.drawable.cherry_pic))
            fruitList.add(Fruit(getRandomLengthString("Mango"),R.drawable.mango_pic))
        }
    }

    private fun getRandomLengthString(str:String):String{
        val n = (1..20).random()
        val builder = StringBuilder()
        repeat(n){
            builder.append(str)
        }
        return builder.toString()
    }
}

4.6.3 点击事件

viewHolder.itemView.setOnClickListener{
            val position = viewHolder.adapterPosition
            val fruit = fruitList[position]
            Toast.makeText(parent.context,"you clicked view ${fruit.name}",Toast.LENGTH_SHORT).show()
        }
        viewHolder.fruitImage.setOnClickListener{
            val position = viewHolder.adapterPosition
            val fruit = fruitList[position]
            Toast.makeText(parent.context,"you clicked view ${fruit.imageId}",Toast.LENGTH_SHORT).show()
        }

补充：dp是一种和屏幕密度无关的尺寸单位，可以保证在不同手机上显示效果尽可能的一致。

4.7 编写界面的实践

4.7.1 制作 9-Patch 图片

获得原始图片

[外链图片转存中…(img-5KFLVQC2-1660655504008)]

右击图片 Create 9 - Patch file
给四个边框绘制小黑点，左边框和上边框为设置需要拉伸就拉伸的区域。另外两个是放置内容的区域。shift擦除
删除原来的图片

4.7.2 编写精美的聊天界面

主界面

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:orientation="vertical"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:background="#d8e0e8">

    <androidx.recyclerview.widget.RecyclerView
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:id="@+id/recycleView"
        ></androidx.recyclerview.widget.RecyclerView>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <EditText
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:id="@+id/inputText"
            android:layout_weight="1"
            android:hint="Type something here"
            android:maxLines="2"
            ></EditText>

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Send"
            android:id="@+id/send"
            ></Button>

    </LinearLayout>

</LinearLayout>

定义实体类Msg

class Msg(val content:String,val type:Int) {
    companion object{
        //常量关键字只能在单例类、companion object或顶层方法中才能用
        const val TYPE_RECEIVED = 0
        const val TYPE_SEND = 1
    }
}

编写子项布局 msg_left.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="10dp"
    android:orientation="vertical">

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="left"
        android:background="@drawable/message_left_original"
        >


        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/leftMsg"
            android:layout_gravity="center"
            android:layout_margin="10dp"
            android:textColor="#fff"
            ></TextView>
    </LinearLayout>

</FrameLayout>

4.编写子项布局 msg_right.xml 。图片资源为第一行代码配套资源

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="10dp"
    android:orientation="vertical">

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="right"
        android:background="@drawable/message_right"
        >


        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/rightMsg"
            android:layout_gravity="center"
            android:layout_margin="10dp"
            android:textColor="#000"
            ></TextView>
    </LinearLayout>

</FrameLayout>

编写适配器

class MsgAdapter(val msgList:List<Msg>) : RecyclerView.Adapter<RecyclerView.ViewHolder>() {
		//编写两个ViewHolder分别用于缓存两个布局中的控件
    inner class LeftViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        val leftMsg: TextView = view.findViewById(R.id.leftMsg)
    }

    inner class RightViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        val rightMsg: TextView = view.findViewById(R.id.rightMsg)
    }

    
    override fun getItemViewType(position: Int): Int {
        val msg = msgList[position]
        //返回当前 position 对应的消息类型
        return msg.type
    }

    //根据不同的 viewType 加载不同的布局，并创建不同的 ViewHolder
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) = if(viewType == Msg.TYPE_RECEIVED){
        val view =
            LayoutInflater.from(parent.context).inflate(R.layout.msg_left_item, parent, false)
        LeftViewHolder(view)
    }else{
        val view =
            LayoutInflater.from(parent.context).inflate(R.layout.msg_right_item, parent, false)
        RightViewHolder(view)
    }

    override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
        val msg = msgList[position]
        when (holder) {
            is LeftViewHolder -> holder.leftMsg.text = msg.content
            is RightViewHolder -> holder.rightMsg.text = msg.content
            else -> throw  IllegalAccessException()
        }
    }

    override fun getItemCount() = msgList.size

}

编写代码

class MainActivity : AppCompatActivity() , View.OnClickListener{

    private val msgList = ArrayList<Msg>()

    private var adapter:MsgAdapter? = null
 		//问号表示该类型可为空

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initMsg()
        val layoutManager = LinearLayoutManager(this)
        recycleView.layoutManager = layoutManager
        adapter = MsgAdapter(msgList)
        recycleView.adapter = adapter
        send.setOnClickListener(this)
    }

    private fun initMsg() {
        val msg1 = Msg("Hello guy.", Msg.TYPE_RECEIVED)
        msgList.add(msg1)
        val msg2 = Msg("Hello. Who is that?",Msg.TYPE_SEND)
        msgList.add(msg2)
        val msg3 = Msg("This is Tom.Nice talking to you.",Msg.TYPE_RECEIVED)
        msgList.add(msg3)
    }

    override fun onClick(p0: View?) {
        when(p0){
            send ->{
                val content  = inputText.text.toString()
                if (content.isNotEmpty()) {
                    val msg = Msg(content, Msg.TYPE_SEND)
                    msgList.add(msg)
                  	//因为adapter是在创建变量之后别的地方初始化，所以使用里面的方法需要判空处理，否则编译不能通过
                    adapter?.notifyItemInserted(msgList.size-1)//当有消息时刷新RV中的显示
                    recycleView.scrollToPosition(msgList.size-1)//定位到最后一行
                    inputText.setText("")//清空输入框
                }
            }
        }
    }
}

4.8 Kotlin：延迟初始化和密封类

4.8.1 对变量延迟初始化

对上面的代码进行优化，利用 lateinit 延迟初始化关键字

class MainActivity : AppCompatActivity() , View.OnClickListener{

    private val msgList = ArrayList<Msg>()

    private lateinit var adapter:MsgAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initMsg()
        val layoutManager = LinearLayoutManager(this)
        recycleView.layoutManager = layoutManager
        //利用代码判断是否已经完成初始化，可以避免对一个变量的重复初始化
        if (!::adapter.isInitialized) {
            adapter = MsgAdapter(msgList)
        }
        recycleView.adapter = adapter
        send.setOnClickListener(this)
    }

    private fun initMsg() {
        val msg1 = Msg("Hello guy.", Msg.TYPE_RECEIVED)
        msgList.add(msg1)
        val msg2 = Msg("Hello. Who is that?",Msg.TYPE_SEND)
        msgList.add(msg2)
        val msg3 = Msg("This is Tom.Nice talking to you.",Msg.TYPE_RECEIVED)
        msgList.add(msg3)
    }

    override fun onClick(p0: View?) {
        when(p0){
            send ->{
                val content  = inputText.text.toString()
                if (content.isNotEmpty()) {
                    val msg = Msg(content, Msg.TYPE_SEND)
                    msgList.add(msg)
                    adapter.notifyItemInserted(msgList.size-1)//当有消息时刷新RV中的显示
                    recycleView.scrollToPosition(msgList.size-1)//定位到最后一行
                    inputText.setText("")//清空输入框
                }
            }
        }
    }
}

4.8.2 密封类

问题

interface Result
class Success(val msg:String) : Result
class Failure(val error: Exception) :Result

fun getResultMsg(result: Result) = when (result) {
    is Success -> result.msg
    is Failure -> result.error.message
    //不得不编写，否则会认为缺少条件分支，无法编译通过。这里的代码永远不会走到，只是为了满足编译检查。并且如果Result增加一个实现接口，并且没在这里添加分支，程序就会崩溃
    else -> throw IllegalArgumentException()
}

解决办法：密封类 sealed class

sealed class Result
//类可以继承，需要加括号
class Success(val msg:String) : Result()
class Failure(val error: Exception) :Result()

//当 when 语句传入密封类变量作为条件时，Kotlin 编译器会自动检查该密封类有哪些子类，并且强制要求你将每一个子类所对应的条件全部处理。当Result有新的实现类，此时这个方法就能报错，必须
//新增条件语句才能通过编译
fun getResultMsg(result: Result) = when (result) {
    is Success -> result.msg
    is Failure -> "Error is ${result.error.toString()}"
}

优化上面的适配器代码

新建一个 MsgViewHolder.kt 代码

sealed class MsgViewHolder(view: View):RecyclerView.ViewHolder(view)
    //编写两个ViewHolder分别用于缓存两个布局中的控件
    class LeftViewHolder(view: View) : MsgViewHolder(view) {
        val leftMsg: TextView = view.findViewById(R.id.leftMsg)
    }

    class RightViewHolder(view: View) :  MsgViewHolder(view) {
        val rightMsg: TextView = view.findViewById(R.id.rightMsg)
    }

修改适配器中的语句

class MsgAdapter(val msgList:List<Msg>) : RecyclerView.Adapter<MsgViewHolder>() {


 		...

    override fun onBindViewHolder(holder:MsgViewHolder, position: Int) {
        val msg = msgList[position]
        when (holder) {
            is LeftViewHolder -> holder.leftMsg.text = msg.content
            is RightViewHolder -> holder.rightMsg.text = msg.content
        }
    }
		...

}

本文章为转载内容，我们尊重原作者对文章享有的著作权。如有内容错误或侵权问题，欢迎原作者联系我们进行内容更正或删除文章。

上一篇：Hadoop集群开了防火墙 hadoop防火墙设置

下一篇：jquery写if语句 if语句javascript

提问和评论都可以，用心的回复会被更多人看到评论

发布评论

相关文章

官方博客	全部文章	热门标签	班级博客
了解我们	网站地图	意见反馈

鸿蒙开发者社区	51CTO学堂
51CTO	软考资讯

第一行代码第三版java版资源 第一行代码第三版pdf

第一行代码第三版java版资源 第一行代码第三版pdf

第四章 UI

4.2常用控件

4.2.2Button

4.2.4 ImageView

4.2.5 ProgressBar

4.2.6 AlertDialog

4.3 布局

4.3.1 线性布局

4.3.2 相对布局

4.3.3 FrameLayout

4.4 创建自定义控件

4.4.1 引入布局

4.4.3 创建自定义控件

4.5.1 简单用法

4.5.2 定制 ListView 的界面

4.5.3 提升 ListView 的运行效率

4.5.4 点击事件

4.6 RecycleView

4.6.2 横向滚动和瀑布流布局

4.6.3 点击事件

4.7 编写界面的实践

4.7.1 制作 9-Patch 图片

4.7.2 编写精美的聊天界面

4.8 Kotlin：延迟初始化和密封类

4.8.1 对变量延迟初始化

4.8.2 密封类

51CTO博客

第一行代码第三版java版资源第一行代码第三版pdf

第一行代码第三版java版资源第一行代码第三版pdf