Fragment回退棧

• Fragment回退棧用于管理Fragment的導航歷史(添加、刪除、替換)。每個Activity都有一個包含其所有Fragment的FragmentManager，調用其addToBackStack方法時，這個事務就會被添加到FragmentManager的回退棧中
• 當用戶按下返回鍵時，系統就會從回退棧中彈出并反向執行最近的事務。如果你替換了一個Fragment，并將這個操作添加到了回退棧，那么按下返回鍵時，原來的Fragment會再次出現
• 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"?>
<LinearLayoutxmlns: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"?>
<TextViewxmlns: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"?>
<TextViewxmlns: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代碼和Fragment代碼

• MyFragmentA代碼

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

• MyFragmentB代碼

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

• Activity代碼

const val TAG = "Yang"
class MainActivity : AppCompatActivity() {var replaceLl : LinearLayout? = nullvar 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// 添加第一個FragmentmMainHandler.postDelayed({val firstFragment = MyFragmentA()replaceLl?.let {replaceFragmentAddToStack(firstFragment, it)}}, 1000)// 添加第二個FragmentmMainHandler.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()}
}

效果圖

• 3s后添加藍色背景的MyFragmentA，6s后添加橘色背景的MyFragmentB
• 按下第一次返回鍵后，最上層的橘色背景的MyFragmentB銷毀，下層藍色背景的MyFragmentA顯示
• 按下第二次返回鍵后，最上層的藍色背景的MyFragmentA銷毀，下層綠色背景的Activity顯示
• 按下第三次返回鍵后，最上層的綠色背景的Activity執行onPause()和onStop()，應用進入后臺
  

FragmentManger.popBackStack()

• 如果在Activity添加Fragment時，通過addToBackStack添加到回退棧，popBackStack的作用和按下返回鍵一樣

replace方式添加+addToBackStack+popBackStack

class MainActivity : AppCompatActivity() {var replaceLl : LinearLayout? = nullvar 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// 添加第一個FragmentmMainHandler.postDelayed({val firstFragment = MyFragmentA()replaceLl?.let {replaceFragmentAddToStack(firstFragment, it)}}, 1000)// 添加第二個FragmentmMainHandler.postDelayed({val secondFragment = MyFragmentB()replaceLl?.let {replaceFragmentAddToStack(secondFragment, it)}}, 2000)// 移除第二個FragmentmMainHandler.postDelayed({supportFragmentManager.popBackStack()}, 3000)// 移除第一個FragmentmMainHandler.postDelayed({supportFragmentManager.popBackStack()}, 4000)}private fun replaceFragmentAddToStack(fragment: Fragment, targetView: View) {val transaction = supportFragmentManager.beginTransaction()transaction?.replace(targetView.id, fragment)?.addToBackStack(null)?.commit()}
}

效果圖

• 1s添加藍色背景的MyFragmentA
• 2s添加橘色背景的MyFragmentB，移除藍色背景的MyFragmentA，此時屏幕上只有MyFragmentB
• 3s移除橘色背景的MyFragmentB，顯示藍色背景的MyFragmentA，此時屏幕上只有MyFragmentA
• 4s移除橘色背景的MyFragmentA，移除藍色背景的MyFragmentA，此時屏幕上沒有任何Fragment
  

add方式添加+addToBackStack+popBackStack

class MainActivity : AppCompatActivity() {var replaceLl : LinearLayout? = nullvar 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// 添加第一個FragmentmMainHandler.postDelayed({val firstFragment = MyFragmentA()replaceLl?.let {addFragmentAddToStack(firstFragment, it)}}, 1000)// 添加第二個FragmentmMainHandler.postDelayed({val secondFragment = MyFragmentB()replaceLl?.let {addFragmentAddToStack(secondFragment, it)}}, 2000)// 移除第二個FragmentmMainHandler.postDelayed({supportFragmentManager.popBackStack()}, 3000)// 移除第一個FragmentmMainHandler.postDelayed({supportFragmentManager.popBackStack()}, 4000)}private fun addFragmentAddToStack(fragment: Fragment, targetView: View) {val transaction = supportFragmentManager.beginTransaction()transaction?.add(targetView.id, fragment)?.addToBackStack(null)?.commit()}
}

效果圖

• 1s添加藍色背景的MyFragmentA
• 2s添加橘色背景的MyFragmentB，不移除藍色背景的MyFragmentA，此時屏幕上有MyFragmentA和MyFragmentB
• 3s移除橘色背景的MyFragmentB，此時屏幕上只有MyFragmentA
• 4s移除橘色背景的MyFragmentA，此時屏幕上沒有任何Fragment