Android组件之Fragment（一）---基础知识与运用
一、Fragment是什么？为什么要引入它？
是什么？
Fragment是Android3.0后引入的一个新的API，他出现的初衷是为了适应大屏幕的平板电脑， 当然现在他仍然是平板APP UI设计的宠儿，而且我们普通手机开发也会加入这个Fragment， 我们可以把他看成一个小型的Activity，又称Activity片段！想想，如果一个很大的界面，我们 就一个布局，写起界面来会有多麻烦，而且如果组件多的话是管理起来也很麻烦！而使用Fragment 我们可以把屏幕划分成几块，然后进行分组，进行一个模块化的管理！从而可以更加方便的在 运行过程中动态地更新Activity的用户界面！另外Fragment并不能单独使用，他需要嵌套在Activity 中使用，尽管他拥有自己的生命周期，但是还是会受到宿主Activity的生命周期的影响，比如Activity 被destory销毁了，他也会跟着销毁！
为什么引入它？
引用官方的一张图片，其实已经说明问题了，就是为了更好的适配大屏，在大屏的时候，不需要去在一个activity内部通过复杂的布局和界面去实现，只需要去在一个activity内部，通过多个fragment来做界面布局实现即可，而且针对于多个fragment来说， 每个fragment有单独的生命周期，
二、使用方法
Demo样例，我们在一个界面中，有上下两个fragment，如图所示：
1.动态加载
代码如下（示例）：
Step 1: activity布局文件，两个FrameLayout
task_test.xml
<?xml version="1.0" encoding="utf-8"?><LinearLayout 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" android:background="@color/white" android:orientation="vertical"> <FrameLayout android:id="@+id/fragment1" android:layout_width="match_parent" android:layout_height="0dp" android:layout_weight="1"/> <FrameLayout android:id="@+id/fragment2" android:layout_width="match_parent" android:layout_height="0dp" android:layout_weight="1"/></LinearLayout>Step 2: Fragment创建，视图加载，数据赋值
BlankFragment .java
package com.itbird.fragment;import android.content.Context;import android.os.Bundle;import androidx.annotation.NonNull;import androidx.annotation.Nullable;import androidx.fragment.app.Fragment;import android.util.Log;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import android.widget.TextView;import com.itbird.R;/ * A simple {@link Fragment} subclass. * Use the {@link BlankFragment#newInstance} factory method to * create an instance of this fragment. */public class BlankFragment extends Fragment { // TODO: Rename parameter arguments, choose names that match // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER private static final String ARG_PARAM1 = "param1"; private static final String ARG_PARAM2 = "param2"; // TODO: Rename and change types of parameters private String mParam1; private String mParam2; private String TAG = BlankFragment.class.getSimpleName(); public BlankFragment() { // Required empty public constructor } / * Use this factory method to create a new instance of * this fragment using the provided parameters. * * @param param1 Parameter 1. * @param param2 Parameter 2. * @return A new instance of fragment BlankFragment. */ // TODO: Rename and change types and number of parameters public static BlankFragment newInstance(String param1, String param2) { BlankFragment fragment = new BlankFragment(); Bundle args = new Bundle(); args.putString(ARG_PARAM1, param1); args.putString(ARG_PARAM2, param2); fragment.setArguments(args); return fragment; } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.d(TAG, "onCreate"); if (getArguments() != null) { mParam1 = getArguments().getString(ARG_PARAM1); mParam2 = getArguments().getString(ARG_PARAM2); } } @Override public void onStart() { Log.d(TAG, "onStart"); super.onStart(); } @Override public void onStop() { Log.d(TAG, "onStop"); super.onStop(); } @Override public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) { Log.d(TAG, "onViewCreated"); super.onViewCreated(view, savedInstanceState); } @Override public void onActivityCreated(@Nullable Bundle savedInstanceState) { Log.d(TAG, "onActivityCreated"); super.onActivityCreated(savedInstanceState); } @Override public void onViewStateRestored(@Nullable Bundle savedInstanceState) { super.onViewStateRestored(savedInstanceState); } @Override public void onDestroy() { Log.d(TAG, "onDestroy"); super.onDestroy(); } @Override public void onDestroyView() { Log.d(TAG, "onDestroyView"); super.onDestroyView(); } @Override public void onDetach() { Log.d(TAG, "onDetach"); super.onDetach(); } @Override public void onAttach(@NonNull Context context) { Log.d(TAG, "onAttach"); super.onAttach(context); } @Override public void onHiddenChanged(boolean hidden) { Log.d(TAG, "onHiddenChanged"); super.onHiddenChanged(hidden); } @Override public void onResume() { Log.d(TAG, "onResume"); super.onResume(); } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { // Inflate the layout for this fragment Log.d(TAG, "onCreateView"); View view = inflater.inflate(R.layout.fragment_blank, container, false); textView = view.findViewById(R.id.textview); textView.setText(mParam1 + "" + mParam2); return view; } TextView textView;}Step 3: Activity在onCreate( )方法中调用setContentView()之后调用FragmentTransaction 进行事务提交
FragmentTestActivity.java
package com.itbird.fragment;import android.os.Bundle;import android.util.Log;import androidx.fragment.app.FragmentActivity;import androidx.fragment.app.FragmentTransaction;import com.itbird.R;public class FragmentTestActivity extends FragmentActivity { private static final String TAG = FragmentTestActivity.class.getSimpleName(); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.task_test); Log.e(TAG, TAG + " onCreate"); FragmentTransaction transaction = getSupportFragmentManager().beginTransaction(); transaction.add(R.id.fragment1, BlankFragment.newInstance("name1", "age1"), "top"); transaction.add(R.id.fragment2, BlankFragment.newInstance("name2", "age2"), "bootom"); transaction.commit(); } @Override public void onStart() { Log.d(TAG, "onStart"); super.onStart(); } @Override public void onStop() { Log.d(TAG, "onStop"); super.onStop(); } @Override public void onResume() { Log.d(TAG, "onResume"); super.onResume(); } @Override protected void onPause() { Log.d(TAG, "onPause"); super.onPause(); } @Override protected void onDestroy() { Log.e(TAG, TAG + " onDestroy"); super.onDestroy(); }}2.静态加载
在xml中声明两个fragment，指定为具体的fragment
Step 1:定义Fragment的布局，就是fragment显示内容的
<?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=".fragment.BlankFragment"> <TextView android:id="@+id/textview" android:layout_width="match_parent" android:layout_height="match_parent" android:text="@string/hello_blank_fragment" /></FrameLayout>Step 2:自定义一个Fragment类,需要继承Fragment或者他的子类,重写onCreateView()方法 在该方法中调用:inflater.inflate()方法加载Fragment的布局文件,接着返回加载的view对象
BlankFragment.java
package com.itbird.fragment;import android.content.Context;import android.os.Bundle;import androidx.annotation.NonNull;import androidx.annotation.Nullable;import androidx.fragment.app.Fragment;import android.util.Log;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import android.widget.TextView;import com.itbird.R;/ * A simple {@link Fragment} subclass. * Use the {@link BlankFragment#newInstance} factory method to * create an instance of this fragment. */public class BlankFragment extends Fragment { // TODO: Rename parameter arguments, choose names that match // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER private static final String ARG_PARAM1 = "param1"; private static final String ARG_PARAM2 = "param2"; // TODO: Rename and change types of parameters private String mParam1; private String mParam2; private String TAG = BlankFragment.class.getSimpleName(); public BlankFragment() { // Required empty public constructor } / * Use this factory method to create a new instance of * this fragment using the provided parameters. * * @param param1 Parameter 1. * @param param2 Parameter 2. * @return A new instance of fragment BlankFragment. */ // TODO: Rename and change types and number of parameters public static BlankFragment newInstance(String param1, String param2) { BlankFragment fragment = new BlankFragment(); Bundle args = new Bundle(); args.putString(ARG_PARAM1, param1); args.putString(ARG_PARAM2, param2); fragment.setArguments(args); return fragment; } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.d(TAG, "onCreate"); if (getArguments() != null) { mParam1 = getArguments().getString(ARG_PARAM1); mParam2 = getArguments().getString(ARG_PARAM2); } } @Override public void onStart() { Log.d(TAG, "onStart"); super.onStart(); } @Override public void onStop() { Log.d(TAG, "onStop"); super.onStop(); } @Override public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) { Log.d(TAG, "onViewCreated"); super.onViewCreated(view, savedInstanceState); } @Override public void onActivityCreated(@Nullable Bundle savedInstanceState) { Log.d(TAG, "onActivityCreated"); super.onActivityCreated(savedInstanceState); } @Override public void onViewStateRestored(@Nullable Bundle savedInstanceState) { super.onViewStateRestored(savedInstanceState); } @Override public void onDestroy() { Log.d(TAG, "onDestroy"); super.onDestroy(); } @Override public void onDestroyView() { Log.d(TAG, "onDestroyView"); super.onDestroyView(); } @Override public void onDetach() { Log.d(TAG, "onDetach"); super.onDetach(); } @Override public void onAttach(@NonNull Context context) { Log.d(TAG, "onAttach"); super.onAttach(context); } @Override public void onHiddenChanged(boolean hidden) { Log.d(TAG, "onHiddenChanged"); super.onHiddenChanged(hidden); } @Override public void onResume() { Log.d(TAG, "onResume"); super.onResume(); } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { // Inflate the layout for this fragment Log.d(TAG, "onCreateView"); View view = inflater.inflate(R.layout.fragment_blank, container, false); textView = view.findViewById(R.id.textview); textView.setText(mParam1 + "" + mParam2); return view; } TextView textView;}Step 3:在需要加载Fragment的Activity对应的布局文件中添加fragment的标签， 记住，name属性是全限定类名哦，就是要包含Fragment的包名，如:
<?xml version="1.0" encoding="utf-8"?><LinearLayout 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" android:background="@color/white" android:orientation="vertical"> <androidx.fragment.app.FragmentContainerView android:name="com.itbird.fragment.BlankFragment" android:layout_width="match_parent" android:layout_height="0dp" android:layout_weight="1" /> <androidx.fragment.app.FragmentContainerView android:name="com.itbird.fragment.BlankFragment" android:layout_width="match_parent" android:layout_height="0dp" android:layout_weight="1" /></LinearLayout>Step 4: Activity在onCreate( )方法中调用setContentView()加载布局文件即可!
package com.itbird.fragment;import android.os.Bundle;import android.util.Log;import com.itbird.R;public class FragmentTestActivity extends FragmentActivity { private static final String TAG = FragmentTestActivity.class.getSimpleName(); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.task_test); Log.e(TAG, TAG + " onCreate"); }}三、生命周期
几种情况下，fragment生命周期：
①Activity加载Fragment的时候,依次调用下面的方法: onAttach -> onCreate -> onCreateView -> onActivityCreated -> onStart ->onResume
②当我们弄出一个悬浮的对话框风格的Activity,或者其他,就是让Fragment所在的Activity可见,但不获得焦点 onPause
③当对话框关闭,Activity又获得了焦点: onResume
④当我们替换Fragment,并调用addToBackStack()将他添加到Back栈中 onPause -> onStop -> onDestoryView ！！注意,此时的Fragment还没有被销毁哦!!!
⑤当我们按下键盘的回退键，Fragment会再次显示出来: onCreateView -> onActivityCreated -> onStart -> onResume
⑥如果我们替换后,在事务commit之前没有调用addToBackStack()方法将 Fragment添加到back栈中的话;又或者退出了Activity的话,那么Fragment将会被完全结束, Fragment会进入销毁状态 onPause -> onStop -> onDestoryView -> onDestory -> onDetach
四、主要方法
针对在一个Activity中的某个Layout中切换Fragment,无非两种方法：
1）replace
我们自己看一下方法注释
/ * Replace an existing fragment that was added to a container. This is * essentially the same as calling {@link #remove(Fragment)} for all * currently added fragments that were added with the same containerViewId * and then {@link #add(int, Fragment, String)} with the same arguments * given here. * * @param containerViewId Identifier of the container whose fragment(s) are * to be replaced. * @param fragment The new fragment to place in the container. * @param tag Optional tag name for the fragment, to later retrieve the * fragment with {@link FragmentManager#findFragmentByTag(String) * FragmentManager.findFragmentByTag(String)}. * * @return Returns the same FragmentTransaction instance. */ @NonNull public FragmentTransaction replace(@IdRes int containerViewId, @NonNull Fragment fragment, @Nullable String tag) { if (containerViewId == 0) { throw new IllegalArgumentException("Must use non-zero containerViewId"); } doAddOp(containerViewId, fragment, tag, OP_REPLACE); return this; }源码方法注释里面说的很明白，这个方法会移除所有的fragment，然后添加当前的fragment。
这时分为两种情况，一种是fragment已有并且在前台展示，一种是未有或者在后台，针对于前者，此时replace，生命周期不会发生变化，针对后者，生命周期会重新走
2022-03-18 15:51:53.408 1141-1141/com.itbird D/BlankFragment1: onAttach2022-03-18 15:51:53.408 1141-1141/com.itbird D/BlankFragment1: onCreate2022-03-18 15:51:53.411 1141-1141/com.itbird D/BlankFragment1: onCreateView2022-03-18 15:51:53.414 1141-1141/com.itbird D/BlankFragment1: onViewCreated2022-03-18 15:51:53.414 1141-1141/com.itbird D/BlankFragment1: onActivityCreated2022-03-18 15:51:53.416 1141-1141/com.itbird D/BlankFragment1: onStart2022-03-18 15:51:53.418 1141-1141/com.itbird D/BlankFragment2: onDestroyView2022-03-18 15:51:53.419 1141-1141/com.itbird D/BlankFragment2: onDestroy2022-03-18 15:51:53.419 1141-1141/com.itbird D/BlankFragment2: onDetach2022-03-18 15:51:53.420 1141-1141/com.itbird D/BlankFragment1: onResume2）add+hide+show
分为两种情况，一种fragment已存在，一种未存在，针对于前者，生命周期无变化，但是会回调onHiddenChanged方法两次，针对于后者，生命周期会创建一次。
2022-03-18 16:02:30.520 2410-2410/com.itbird D/FragmentTestActivity: fragment1 = BlankFragment1{21ae25c} (4a8180fe-e646-4cca-89a5-0f617e61eb45)2022-03-18 16:02:30.546 2410-2410/com.itbird D/BlankFragment1: onAttach2022-03-18 16:02:30.547 2410-2410/com.itbird D/BlankFragment1: onCreate2022-03-18 16:02:30.548 2410-2410/com.itbird D/BlankFragment1: onCreateView2022-03-18 16:02:30.580 2410-2410/com.itbird D/BlankFragment1: onViewCreated2022-03-18 16:02:30.582 2410-2410/com.itbird D/BlankFragment1: onActivityCreated2022-03-18 16:02:30.585 2410-2410/com.itbird D/BlankFragment1: onStart2022-03-18 16:02:30.590 2410-2410/com.itbird D/BlankFragment1: onResume2022-03-18 16:02:35.302 2410-2410/com.itbird D/FragmentTestActivity: fragment1 = BlankFragment1{21ae25c} (4a8180fe-e646-4cca-89a5-0f617e61eb45 id=0x7f0801b7)2022-03-18 16:02:35.308 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged2022-03-18 16:02:35.312 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged2022-03-18 16:02:37.212 2410-2410/com.itbird D/FragmentTestActivity: fragment1 = BlankFragment1{21ae25c} (4a8180fe-e646-4cca-89a5-0f617e61eb45 id=0x7f0801b7)2022-03-18 16:02:37.219 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged2022-03-18 16:02:37.220 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged2022-03-18 16:02:39.500 2410-2410/com.itbird D/FragmentTestActivity: fragment1 = BlankFragment1{21ae25c} (4a8180fe-e646-4cca-89a5-0f617e61eb45 id=0x7f0801b7)2022-03-18 16:02:39.509 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged2022-03-18 16:02:39.510 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged2022-03-18 16:02:41.125 2410-2410/com.itbird D/FragmentTestActivity: fragment1 = BlankFragment1{21ae25c} (4a8180fe-e646-4cca-89a5-0f617e61eb45 id=0x7f0801b7)2022-03-18 16:02:41.126 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged2022-03-18 16:02:41.130 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged2022-03-18 16:02:50.987 2410-2410/com.itbird D/FragmentTestActivity: fragment2 = BlankFragment2{8536d4a} (05586a4a-4abd-4d28-8ad4-2630c17a50c5)2022-03-18 16:02:50.999 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged2022-03-18 16:02:51.000 2410-2410/com.itbird D/BlankFragment2: onAttach2022-03-18 16:02:51.000 2410-2410/com.itbird D/BlankFragment2: onCreate2022-03-18 16:02:51.002 2410-2410/com.itbird D/BlankFragment2: onCreateView2022-03-18 16:02:51.004 2410-2410/com.itbird D/BlankFragment2: onViewCreated2022-03-18 16:02:51.005 2410-2410/com.itbird D/BlankFragment2: onActivityCreated2022-03-18 16:02:51.005 2410-2410/com.itbird D/BlankFragment2: onStart2022-03-18 16:02:51.007 2410-2410/com.itbird D/BlankFragment2: onResume2022-03-18 16:02:55.315 2410-2410/com.itbird D/FragmentTestActivity: fragment2 = BlankFragment2{8536d4a} (05586a4a-4abd-4d28-8ad4-2630c17a50c5 id=0x7f0801b7)2022-03-18 16:02:55.316 2410-2410/com.itbird D/BlankFragment2: onHiddenChanged2022-03-18 16:02:55.317 2410-2410/com.itbird D/BlankFragment2: onHiddenChanged2022-03-18 16:02:57.005 2410-2410/com.itbird D/FragmentTestActivity: fragment1 = BlankFragment1{21ae25c} (4a8180fe-e646-4cca-89a5-0f617e61eb45 id=0x7f0801b7)2022-03-18 16:02:57.024 2410-2410/com.itbird D/BlankFragment2: onHiddenChanged2022-03-18 16:02:57.025 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged总结
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