Activity-＞Activity中动态添加Fragment-＞add和replace方式添加的区别

`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/root_ll"android:orientation="vertical"android:layout_width="match_parent"android:layout_height="match_parent"><LinearLayoutandroid:id="@+id/add_child_ll"android:layout_width="match_parent"android:layout_height="0dp"android:layout_weight="1"android:background="@android:color/holo_green_light"android:gravity="center"android:orientation="vertical"></LinearLayout><LinearLayoutandroid:id="@+id/replace_child_ll"android:layout_width="match_parent"android:layout_height="0dp"android:layout_weight="1"android:background="@android:color/holo_red_light"android:gravity="center"android:orientation="vertical"></LinearLayout>
</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"/>

`Activtiy`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 addLl : LinearLayout? = nullvar replaceLl : LinearLayout? = nullvar mMainHandler = Handler(Looper.getMainLooper())override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main)addLl = findViewById(R.id.add_child_ll) as? LinearLayoutreplaceLl = findViewById(R.id.replace_child_ll) as? LinearLayoutmMainHandler.postDelayed({val firstFragment1 = MyFragmentA()val firstFragment2 = MyFragmentA()addLl?.let {replaceFragment(firstFragment1, it)}replaceLl?.let {replaceFragment(firstFragment2, it)}}, 3000)mMainHandler.postDelayed({val secondFragment1 = MyFragmentB()val secondFragment2 = MyFragmentB()addLl?.let {addFragment(secondFragment1, it)}replaceLl?.let {replaceFragment(secondFragment2, it)}}, 6000)}private fun addFragment(fragment: Fragment, targetView: View) {val transaction = supportFragmentManager.beginTransaction()transaction?.add(targetView.id, fragment)?.commitNowAllowingStateLoss()}private fun replaceFragment(fragment: Fragment, targetView: View) {val transaction = supportFragmentManager.beginTransaction()transaction?.replace(targetView.id, fragment)?.commitNowAllowingStateLoss()}
}

3s后通过replace()方法在两个LinearLayout中以居中方式添加蓝色背景的MyFragmentA，MyFragmentA的background不会覆盖掉原有Activity的background
6s后通过add()方法在R.id.add_child_ll的LinearLayout中以居中方式添加橘色背景的MyFragmentB，add()方法添加并不会移除覆盖掉第一次添加的MyFragmentA的background。因为不会移除，所以橘色背景的MyFragmentB在第一次添加的MyFragmentA的background下方继续布局绘制
6s后通过replace()方法在R.id.replace_child_ll的LinearLayout中以居中方式添加橘色背景的MyFragmentB，add()方法添加会移除覆盖掉第一次添加的MyFragmentA的background。因为会移除，所以橘色背景的MyFragmentB直接在原有R.id.replace_child_ll的LinearLayout中直接居中布局绘制
如果只是将新的Fragment添加到Activity，而不关心或者不需要移除已存在的Fragment，使用add()方法。但是如果希望替换container中已存在的Fragment，使用replace()方法

获取当前`Fragment`的数量

获取Activity中Fragment的数量

    override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)Log.i(TAG, "onCreate")setContentView(R.layout.activity_main)addLl = findViewById(R.id.add_child_ll) as? LinearLayoutreplaceLl = findViewById(R.id.replace_child_ll) as? LinearLayoutmMainHandler.postDelayed({val firstFragment1 = MyFragmentA()val firstFragment2 = MyFragmentA()addLl?.let {replaceFragment(firstFragment1, it)}replaceLl?.let {replaceFragment(firstFragment2, it)}Log.i(TAG, "3000ms Number of fragments: ${supportFragmentManager.fragments.size}")Log.i(TAG, "addLl active fragment count = ${addLl?.let { getActiveFragmentCount(it) }} ")Log.i(TAG, "replaceLl active fragment count = ${replaceLl?.let { getActiveFragmentCount(it) }} ")}, 3000)mMainHandler.postDelayed({val secondFragment1 = MyFragmentB()val secondFragment2 = MyFragmentB()addLl?.let {addFragment(secondFragment1, it)}replaceLl?.let {replaceFragment(secondFragment2, it)}Log.i(TAG, "6000ms Number of fragments: ${supportFragmentManager.fragments.size}")Log.i(TAG, "addLl active fragment count = ${addLl?.let { getActiveFragmentCount(it) }} ")Log.i(TAG, "replaceLl active fragment count = ${replaceLl?.let { getActiveFragmentCount(it) }} ")}, 6000)}// log result
// 3s添加了两个MyFragmentA，数量为2
// 6s添加两个MyFragmentB，移除一个MyFragmentA，数量为3
2024-06-06 11:11:31.427 24538-24538 Yang       I  3000ms Number of fragments: 2
2024-06-06 11:11:34.423 24538-24538 Yang       I  6000ms Number of fragments: 3

获取View中Fragment的数量

fun getActiveFragmentCount(view : View): Int {var count = 0supportFragmentManager.fragments.forEach {if(it.view?.parent == view){count++}}return count
}// log result
// 3s添加了两个LinearLayout各自添加一个Fragment
2024-06-06 12:57:55.747 24983-24983 Yang        I  3000ms Number of fragments: 2
2024-06-06 12:57:55.747 24983-24983 Yang        I  addLl active fragment count = 1 
2024-06-06 12:57:55.747 24983-24983 Yang        I  replaceLl active fragment count = 1 
// 6s第一个LinearLayout通过add方式添加，
2024-06-06 12:57:58.739 24983-24983 Yang        I  6000ms Number of fragments: 3
2024-06-06 12:57:58.740 24983-24983 Yang        I  addLl active fragment count = 2 
2024-06-06 12:57:58.740 24983-24983 Yang        I  replaceLl active fragment count = 1