第五章探究Fragment

添加Fragment

XML中添加

<?xml version="1.0" encoding="utf-8"?>
<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:layout_height="match_parent"
    android:orientation="horizontal"
    tools:context=".chapter5.FragmentActivity">

    <fragment
        android:id="@+id/leftFragment"
        android:name="com.youngly.firstlineofcode.chapter5.LeftFragment"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1" />

    <FrameLayout
        android:id="@+id/right_layout"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="2" />

</LinearLayout>

动态添加

class FragmentActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_fragment)

        replaceFragment(RightFragment.newInstance())
    }

    fun changeFragment(view: View) {
        replaceFragment(AnotherRightFragment.newInstance())
    }

    private fun replaceFragment(newInstance: Fragment) {
        val beginTransaction = supportFragmentManager.beginTransaction()
        beginTransaction.replace(R.id.right_layout, newInstance)
        beginTransaction.commit()
    }
}

实现返回栈
```
beginTransaction.addToBackStack(null)
```

生命周期
1. 回调方法
  - onAttach(context: Context)：当Fragment和Activity建立关联时调用
  - onCreateView()：为Fragment创建视图（加载布局）时调用
  - onActivityCreated()：确保与Fragment相关联的Activity已经创建完毕时调用
  - onDestoryView()：当与Fragment关联的视图被移除时调用
  - onDetach()：当Fragment和Activity解除关联时调用
动态加载布局技巧
1. 使用限定符
  
  layout-large文件夹放置平板资源
2. 使用最小宽度限定符
  
  最小宽度限定符（smallest-width qualifier）这个对应的是开发者选项中最小宽度
  
  最小宽度限定符允许我们对屏幕的宽度指定一个最小值（以dp为单位），以这个最小值为临界点，屏幕宽度大于这个值就加载这个目录下的布局。

最佳实践

实现一个app，手机端水果列表页面展示水果，点击某个水果跳转到水果详情页面。pad端左侧展示水果列表，右侧显示选中的水果详情。

实现两个Fragment：水果列表Fragment、水果详情Fragment

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".chapter5.FruitListFragment">

    <ListView
        android:id="@+id/fruit_listview"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</FrameLayout>

class FruitListFragment : Fragment() {
    private var fruitList = ArrayList<FruitListActivity.Fruit>()

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

    private fun initFruits() {
        repeat(20) {
            fruitList.add(FruitListActivity.Fruit(R.drawable.apple_pic, "Apple"))
            fruitList.add(FruitListActivity.Fruit(R.drawable.banana_pic, "Banana"))
            fruitList.add(FruitListActivity.Fruit(R.drawable.orange_pic, "Orange"))
            fruitList.add(FruitListActivity.Fruit(R.drawable.watermelon_pic, "Watermelon"))
            fruitList.add(FruitListActivity.Fruit(R.drawable.pear_pic, "Pear"))
            fruitList.add(FruitListActivity.Fruit(R.drawable.grape_pic, "Grape"))
            fruitList.add(FruitListActivity.Fruit(R.drawable.pineapple_pic, "PineApple"))
            fruitList.add(FruitListActivity.Fruit(R.drawable.strawberry_pic, "Strawberry"))
            fruitList.add(FruitListActivity.Fruit(R.drawable.cherry_pic, "Cherry"))
            fruitList.add(FruitListActivity.Fruit(R.drawable.mango_pic, "Mango"))
        }
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_fruit_list, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        view?.let {
            val fruitListView = it.findViewById<ListView>(R.id.fruit_listview)
            fruitListView.adapter =
                context?.let { it1 -> FruitAdapter(it1, R.layout.fruit_item, fruitList) }
            fruitListView.setOnItemClickListener { _, _, position, _ ->
                (context as FruitActivity).showFruit(
                    fruitList[position].imgId,
                    fruitList[position].name
                )
            }
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:id="@+id/constraint_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".chapter5.FruitFragment">


    <ImageView
        android:id="@+id/fruit_img"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="268dp"
        android:src="@drawable/apple_pic"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/fruit_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/fruit_img"
        app:layout_constraintVertical_bias="0.118" />
</androidx.constraintlayout.widget.ConstraintLayout>

class FruitFragment : Fragment() {

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_fruit, container, false)
    }

    fun refreshView(resourceId: Int, name: String) {
        view?.apply {
            findViewById<ImageView>(R.id.fruit_img).setImageResource(resourceId)
            findViewById<TextView>(R.id.fruit_name).text = name
        }
    }
}

实现Activity

手机布局

<androidx.constraintlayout.widget.ConstraintLayout 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=".chapter5.FruitActivity">

    <fragment
        android:id="@+id/chat_history"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:name="com.youngly.firstlineofcode.chapter5.FruitListFragment"/>

</androidx.constraintlayout.widget.ConstraintLayout>

平板布局

<?xml version="1.0" encoding="utf-8"?>
<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:layout_height="match_parent"
    android:orientation="horizontal"
    tools:context=".chapter5.FruitActivity">

    <fragment
        android:id="@+id/fruit_list_fragment"
        android:name="com.youngly.firstlineofcode.chapter5.FruitListFragment"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1" />

    <fragment
        android:id="@+id/fruit_fragment"
        android:name="com.youngly.firstlineofcode.chapter5.FruitFragment"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="2" />

</LinearLayout>

Activity代码

class FruitActivity : AppCompatActivity() {
    private var mIsLarge: Boolean = false
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_fruit)

      	// 通过判断右侧Fragment是否存在来判断是否是平板
        mIsLarge = supportFragmentManager.findFragmentById(R.id.fruit_fragment) != null
    }

    fun showFruit(resourceId: Int, name: String) {
        if (mIsLarge) {
            (supportFragmentManager.findFragmentById(R.id.fruit_fragment) as FruitFragment).refreshView(resourceId, name)
        } else {
            val intent = Intent(this, FruitDetailActivity::class.java)
            intent.putExtra("resourceId", resourceId)
            intent.putExtra("name", name)
            startActivity(intent)
        }
    }
}

Kotlin课堂

扩展函数

需求：统计字符串中字母个数
1. Java中编写Util类，定义函数获取字母个数
2. Kotlin中扩展函数
  
  由于希望向String类中添加一个扩展函数，因此创建一个String.kt文件
```
fun String.letterCount(): Int {
    var count = 0
    for (char in this) {
        if (char.isLetter())
            count++
    }
    return count
}
```

运算符重载

运算符重载用的是operator关键字

+运算符对应的是plus()函数

下例是不同货币之间相加，得到同一货币

定义父类

/**
 * Creater:     yanglei
 * Date:        2021/7/2  下午11:59
 * Desc:
 * rate:汇率
 */
open class Money(var value: Float, var rate: Float) {

    var symbol: String = ""

    operator fun plus(money: Money): Money {
        val newMoney = this::class.java.newInstance()
        this::class.java.superclass.getDeclaredField("value").set(newMoney, this.value + money.value * (money.rate / this.rate))
        this::class.java.superclass.getDeclaredField("rate").set(newMoney, this.rate)
        newMoney.symbol = this.symbol
        return newMoney
    }

    override fun toString(): String {
        return "${this::class.java.simpleName}($symbol $value)"
    }
}

定义人名币（汇率以人民币为基准）

class RMB(value: Float, rate: Float) : Money(value, rate) {

    constructor() : this(0f, 0f)

    init {
        symbol = "¥"
    }
}

定义美元

class Dollar(value: Float, rate: Float) : Money(value, rate) {
    constructor() : this(0f, 0f)

    init {
        symbol = "$"
    }
}

测试

fun main() {
    val fl = RMB(7f, 1f) + Dollar(1f, 7f)
    println(fl)

    val money = Dollar(1f, 7f) + RMB(7f, 1f)
    println(money)
}

输出

RMB(¥ 14.0) Dollar($ 2.0)

第五章 探究Fragment

添加Fragment

XML中添加

动态添加

实现返回栈

生命周期

回调方法

动态加载布局技巧

使用限定符

使用最小宽度限定符

最佳实践

实现两个Fragment：水果列表Fragment、水果详情Fragment

实现Activity

手机布局

Kotlin课堂

扩展函数

Java中编写Util类，定义函数获取字母个数

Kotlin中扩展函数

运算符重载

定义父类

定义人名币（汇率以人民币为基准）

定义美元

测试

输出

第五章探究Fragment