Android组件之Fragment（一）---基础知识与运用

作者: itbird01 | 来源:发表于2022-03-24 07:10 被阅读0次

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: onAttach
2022-03-18 15:51:53.408 1141-1141/com.itbird D/BlankFragment1: onCreate
2022-03-18 15:51:53.411 1141-1141/com.itbird D/BlankFragment1: onCreateView
2022-03-18 15:51:53.414 1141-1141/com.itbird D/BlankFragment1: onViewCreated
2022-03-18 15:51:53.414 1141-1141/com.itbird D/BlankFragment1: onActivityCreated
2022-03-18 15:51:53.416 1141-1141/com.itbird D/BlankFragment1: onStart
2022-03-18 15:51:53.418 1141-1141/com.itbird D/BlankFragment2: onDestroyView
2022-03-18 15:51:53.419 1141-1141/com.itbird D/BlankFragment2: onDestroy
2022-03-18 15:51:53.419 1141-1141/com.itbird D/BlankFragment2: onDetach
2022-03-18 15:51:53.420 1141-1141/com.itbird D/BlankFragment1: onResume

2）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，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: onAttach
2022-03-18 16:02:30.547 2410-2410/com.itbird D/BlankFragment1: onCreate
2022-03-18 16:02:30.548 2410-2410/com.itbird D/BlankFragment1: onCreateView
2022-03-18 16:02:30.580 2410-2410/com.itbird D/BlankFragment1: onViewCreated
2022-03-18 16:02:30.582 2410-2410/com.itbird D/BlankFragment1: onActivityCreated
2022-03-18 16:02:30.585 2410-2410/com.itbird D/BlankFragment1: onStart
2022-03-18 16:02:30.590 2410-2410/com.itbird D/BlankFragment1: onResume
2022-03-18 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: onHiddenChanged
2022-03-18 16:02:35.312 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged
2022-03-18 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: onHiddenChanged
2022-03-18 16:02:37.220 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged
2022-03-18 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: onHiddenChanged
2022-03-18 16:02:39.510 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged
2022-03-18 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: onHiddenChanged
2022-03-18 16:02:41.130 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged
2022-03-18 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: onHiddenChanged
2022-03-18 16:02:51.000 2410-2410/com.itbird D/BlankFragment2: onAttach
2022-03-18 16:02:51.000 2410-2410/com.itbird D/BlankFragment2: onCreate
2022-03-18 16:02:51.002 2410-2410/com.itbird D/BlankFragment2: onCreateView
2022-03-18 16:02:51.004 2410-2410/com.itbird D/BlankFragment2: onViewCreated
2022-03-18 16:02:51.005 2410-2410/com.itbird D/BlankFragment2: onActivityCreated
2022-03-18 16:02:51.005 2410-2410/com.itbird D/BlankFragment2: onStart
2022-03-18 16:02:51.007 2410-2410/com.itbird D/BlankFragment2: onResume

2022-03-18 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: onHiddenChanged
2022-03-18 16:02:55.317 2410-2410/com.itbird D/BlankFragment2: onHiddenChanged
2022-03-18 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: onHiddenChanged
2022-03-18 16:02:57.025 2410-2410/com.itbird D/BlankFragment1: onHiddenChanged

总结

1.Fragment是Google官方引入的一个为了适配大屏、多页面的一个组件。您可以理解为它就是一个类而已，只不过里面包含了View，并且与activity的生命周期进行了关联。
2.动态加载与静态加载相对来说，建议使用动态加载，静态加载固定在了xml文件中，永远不变。
3.replace的fragment如果不在前台，会执行所有生命周期，反之不会执行任何生命周期方法；hide+show生命周期并不会发生变化，但是会回调onHiddenChanged方法，在实际开发中，建议add之后，使用hide+show来操作fragment，一方面减少资源的重复加载和创建，另外一方面提升用户体验感。
4.fragment的生命周期大体上和activity一致，但是前期和后期多了一些东西，因为fragment内部有view，那么这个view需要进行创建、然后添加到activity内部，所以相应的在onCreate与onStart之间就多了几个方法-onCreateView、onViewCreated。相同的道理，fragment的view与activity解绑，也相应的在onStop与onDestory之间多个方法-onDestroyView。onAttach与onDetach可以理解为视图与activity产生关联和接触关联，是最开始和最后的步骤。

Demo地址

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？

为什么引入它？

二、使用方法

1.动态加载

2.静态加载

三、生命周期

四、主要方法

1）replace

2）add+hide+show

总结

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