Android开发之Fragment动态添加与管理

主界面布局资源

在activity_main.xml中，声明两个按钮备用，再加入一个帧布局，待会儿用来展示Fragment。

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/button1"
        android:text="@string/push"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/button2"
        android:text="@string/replace"/>

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/framelayout"
        android:background="@color/purple_200"/>

LinearLayout>

两个工具Fragment

用来展示的Fragment，随便找两个AS预设的即可，这里使用的是一个BlankFragment和一个ItemFragment。
BlankFragment：

<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=".BlankFragment1">

    
    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="@string/hello_blank_fragment" />

FrameLayout>

package com.example.dynamicfragment;

import android.os.Bundle;

import androidx.fragment.app.Fragment;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

/**
 * A simple {@link Fragment} subclass.
 * Use the {@link BlankFragment1#newInstance} factory method to
 * create an instance of this fragment.
 */
public class BlankFragment1 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;

    public BlankFragment1() {
        // 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 BlankFragment1.
     */
    // TODO: Rename and change types and number of parameters
    public static BlankFragment1 newInstance(String param1, String param2) {
        BlankFragment1 fragment = new BlankFragment1();
        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);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_blank1, container, false);
    }
}

ItemFragment：

<androidx.recyclerview.widget.RecyclerView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/list"
    android:name="com.example.dynamicfragment.ItemFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_marginLeft="16dp"
    android:layout_marginRight="16dp"
    app:layoutManager="LinearLayoutManager"
    tools:context=".ItemFragment"
    tools:listitem="@layout/fragment_item" />

package com.example.dynamicfragment;

import android.content.Context;
import android.os.Bundle;

import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.GridLayoutManager;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.example.dynamicfragment.placeholder.PlaceholderContent;

/**
 * A fragment representing a list of Items.
 */
public class ItemFragment extends Fragment {

    // TODO: Customize parameter argument names
    private static final String ARG_COLUMN_COUNT = "column-count";
    // TODO: Customize parameters
    private int mColumnCount = 1;

    /**
     * Mandatory empty constructor for the fragment manager to instantiate the
     * fragment (e.g. upon screen orientation changes).
     */
    public ItemFragment() {
    }

    // TODO: Customize parameter initialization
    @SuppressWarnings("unused")
    public static ItemFragment newInstance(int columnCount) {
        ItemFragment fragment = new ItemFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_COLUMN_COUNT, columnCount);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if (getArguments() != null) {
            mColumnCount = getArguments().getInt(ARG_COLUMN_COUNT);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_item_list, container, false);

        // Set the adapter
        if (view instanceof RecyclerView) {
            Context context = view.getContext();
            RecyclerView recyclerView = (RecyclerView) view;
            if (mColumnCount <= 1) {
                recyclerView.setLayoutManager(new LinearLayoutManager(context));
            } else {
                recyclerView.setLayoutManager(new GridLayoutManager(context, mColumnCount));
            }
            recyclerView.setAdapter(new MyItemRecyclerViewAdapter(PlaceholderContent.ITEMS));
        }
        return view;
    }
}

主程序

在主程序里，我们要实现点击按钮显示不同的Fragment。这里使用一种新的实现按钮方式，在声明MainActivity类的时候引用View.OnClickListener接口，然后在button1.setOnClickListener(this);中传入this，这样按钮被点击时就会自动调用后面写的OnClick函数。

在OnClick函数被调用时，我们判断是哪一个按钮被点击了，然后根据按钮ID将不同的Fragment展现在帧布局上。

package com.example.dynamicfragment;

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button button1 = findViewById(R.id.button1);
        Button button2 = findViewById(R.id.button2);
        button1.setOnClickListener(this);
        button2.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        switch (view.getId()){
            case R.id.button1:
                replaceFragment(new BlankFragment1());
                break;
            case R.id.button2:
                replaceFragment(new ItemFragment());
                break;
        }
    }

    private void replaceFragment(Fragment fragment) {
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction transaction = fragmentManager.beginTransaction();
        transaction.replace(R.id.framelayout, fragment);//创建replace事件
        transaction.addToBackStack(null);
        transaction.commit();//执行transaction中的事件
    }
}

还需要重点讲解一下的是replaceFragment函数中的栈，transaction.addToBackStack(null);中的null指代的是默认栈。加入该语句后，每次更新都会向栈中加入一个Fragment，且屏幕上显示的即是栈顶的Fragment。当我们点击返回按钮时，栈顶的Fragment被弹出，屏幕上显示下一个Fragment。

试验如下，交替点击两个按钮若干次，屏幕上会依次出现两个Fragment交替覆盖，而点击返回按钮后，最顶上Fragment则会被撤除。