Activity-＞Activity中动态添加Fragment-＞Fragment回退栈BackStack

`Fragment`回退栈

Fragment回退栈用于管理Fragment的导航历史(添加、删除、替换)。每个Activity都有一个包含其所有Fragment的FragmentManager，调用其addToBackStack方法时，这个事务就会被添加到FragmentManager的回退栈中
当用户按下返回键时，系统就会从回退栈中弹出并反向执行最近的事务。如果你替换了一个Fragment，并将这个操作添加到了回退栈，那么按下返回键时，原来的Fragment会再次出现
commitNow()方法不能和addToBackStack()方法一起使用

`XML`文件

Activity 布局文件R.layout.activity_main

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/replace_child_ll"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@android:color/holo_green_light"
    android:gravity="center"
    android:orientation="vertical">
</LinearLayout>

第一次添加的MyFragmentA布局R.layout.inflate_layout_a

<?xml version="1.0" encoding="utf-8"?>
<TextView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/inflate_tv_a"
    android:layout_width="match_parent"
    android:layout_height="100dp"
    android:background="@android:color/holo_blue_light" />

第二次添加的MyFragmentB布局R.layout.inflate_layout_b

<?xml version="1.0" encoding="utf-8"?>
<TextView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/inflate_tv_b"
    android:layout_width="match_parent"
    android:layout_height="100dp"
    android:background="@android:color/holo_orange_light"/>

`Activity`代码和`Fragment`代码

MyFragmentA代码

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

MyFragmentB代码

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

Activity代码

const val TAG = "Yang"
class MainActivity : AppCompatActivity() {
    var replaceLl : LinearLayout? = null
    var mMainHandler = Handler(Looper.getMainLooper())
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        replaceLl = findViewById(R.id.replace_child_ll) as? LinearLayout
        // 添加第一个Fragment
        mMainHandler.postDelayed({
            val firstFragment = MyFragmentA()
            replaceLl?.let {
                replaceFragmentAddToStack(firstFragment, it)
            }
        }, 1000)
        // 添加第二个Fragment
        mMainHandler.postDelayed({
            val secondFragment = MyFragmentB()
            replaceLl?.let {
                replaceFragmentAddToStack(secondFragment, it)
            }
        }, 2000)
    }


    private fun replaceFragmentAddToStack(fragment: Fragment, targetView: View) {
        val transaction = supportFragmentManager.beginTransaction()
        transaction?.replace(targetView.id, fragment)
            ?.addToBackStack(null)
            ?.commitAllowingStateLoss()
    }
}

效果图

3s后添加蓝色背景的MyFragmentA，6s后添加橘色背景的MyFragmentB
按下第一次返回键后，最上层的橘色背景的MyFragmentB销毁，下层蓝色背景的MyFragmentA显示
按下第二次返回键后，最上层的蓝色背景的MyFragmentA销毁，下层绿色背景的Activity显示
按下第三次返回键后，最上层的绿色背景的Activity执行onPause()和onStop()，应用进入后台

`FragmentManger.popBackStack()`

如果在Activity添加Fragment时，通过addToBackStack添加到回退栈，popBackStack的作用和按下返回键一样

`replace`方式添加+`addToBackStack`+`popBackStack`

class MainActivity : AppCompatActivity() {
    var replaceLl : LinearLayout? = null
    var mMainHandler = Handler(Looper.getMainLooper())
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        replaceLl = findViewById(R.id.replace_child_ll) as? LinearLayout
        // 添加第一个Fragment
        mMainHandler.postDelayed({
            val firstFragment = MyFragmentA()
            replaceLl?.let {
                replaceFragmentAddToStack(firstFragment, it)
            }
        }, 1000)

        // 添加第二个Fragment
        mMainHandler.postDelayed({
            val secondFragment = MyFragmentB()
            replaceLl?.let {
                replaceFragmentAddToStack(secondFragment, it)
            }
        }, 2000)

        // 移除第二个Fragment
        mMainHandler.postDelayed({
            supportFragmentManager.popBackStack()
        }, 3000)

        // 移除第一个Fragment
        mMainHandler.postDelayed({
            supportFragmentManager.popBackStack()
        }, 4000)
    }

    private fun replaceFragmentAddToStack(fragment: Fragment, targetView: View) {
        val transaction = supportFragmentManager.beginTransaction()
        transaction?.replace(targetView.id, fragment)
            ?.addToBackStack(null)
            ?.commit()
    }
}

效果图

1s添加蓝色背景的MyFragmentA
2s添加橘色背景的MyFragmentB，移除蓝色背景的MyFragmentA，此时屏幕上只有MyFragmentB
3s移除橘色背景的MyFragmentB，显示蓝色背景的MyFragmentA，此时屏幕上只有MyFragmentA
4s移除橘色背景的MyFragmentA，移除蓝色背景的MyFragmentA，此时屏幕上没有任何Fragment

`add`方式添加+`addToBackStack`+`popBackStack`

class MainActivity : AppCompatActivity() {
    var replaceLl : LinearLayout? = null
    var mMainHandler = Handler(Looper.getMainLooper())
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        replaceLl = findViewById(R.id.replace_child_ll) as? LinearLayout
        // 添加第一个Fragment
        mMainHandler.postDelayed({
            val firstFragment = MyFragmentA()
            replaceLl?.let {
                addFragmentAddToStack(firstFragment, it)
            }
        }, 1000)

        // 添加第二个Fragment
        mMainHandler.postDelayed({
            val secondFragment = MyFragmentB()
            replaceLl?.let {
                addFragmentAddToStack(secondFragment, it)
            }
        }, 2000)

        // 移除第二个Fragment
        mMainHandler.postDelayed({
            supportFragmentManager.popBackStack()
        }, 3000)

        // 移除第一个Fragment
        mMainHandler.postDelayed({
            supportFragmentManager.popBackStack()
        }, 4000)
    }

    private fun addFragmentAddToStack(fragment: Fragment, targetView: View) {
        val transaction = supportFragmentManager.beginTransaction()
        transaction?.add(targetView.id, fragment)
            ?.addToBackStack(null)
            ?.commit()
    }
}