Android Activity、Fragment间通信

Activity中获取Fragment

// 用于静态创建的Fragment
getSupportFragmentManager().findFragmentById(int id);
// 用于动态创建的Fragment
getSupportFragmentManager().findFragmentByTag(String tag);
// 用于动态创建的Fragment
getSupportFragmentManager().getFragments().get(int index);

Fragment中获取Activity

getActivity();

Fragment中获取另一个Fragment

// 用于静态创建的Fragment
getFragmentManager().findFragmentById(int id);
// 用于动态创建的Fragment
getFragmentManager().findFragmentByTag(String tag);
// 用于动态创建的Fragment
getFragmentManager().getFragments().get(int index);

举个完整的例子：

先看看MainActivity3

package com.gui.gui;

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

import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;

public class MainActivity3 extends BaseActivity {
    EditText mEditText;
    Button mButton;
    int mIndex = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main3);
        mEditText = findViewById(R.id.edittext);
        mButton = findViewById(R.id.btn);
        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                switch (mIndex) {
                    case 0:
                        LeftFragment fragment0 = (LeftFragment) getSupportFragmentManager().getFragments().get(0);
                        String value0 = fragment0.mEditText.getText().toString();
                        mEditText.setText(value0);
                        break;
                    case 1:
                        LeftFragment fragment1 = (LeftFragment) getSupportFragmentManager().getFragments().get(0);
                        String value1 = mEditText.getText().toString();
                        fragment1.mEditText.setText(value1);
                        break;
                    case 2:
                        RightFragment fragment2 = (RightFragment) getSupportFragmentManager().getFragments().get(1);
                        String value2 = fragment2.mEditText.getText().toString();
                        mEditText.setText(value2);
                        break;
                    case 3:
                        RightFragment fragment3 = (RightFragment) getSupportFragmentManager().getFragments().get(1);
                        String value3 = mEditText.getText().toString();
                        fragment3.mEditText.setText(value3);
                        break;
                    default:
                        break;
                }
                mIndex = (++mIndex) % 4;
            }
        });

        FragmentManager manager = getSupportFragmentManager();
        FragmentTransaction transaction = manager.beginTransaction();
        transaction.replace(R.id.left_framelayout, new LeftFragment());
        transaction.replace(R.id.right_framelayout, new RightFragment());
        transaction.commit();
    }


}

<?xml version="1.0" encoding="utf-8"?>
<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=".MainActivity3">

    <EditText
        android:id="@+id/edittext"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="在此输入内容" />

    <Button
        android:id="@+id/btn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="操作按钮" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="horizontal">

        <FrameLayout
            android:id="@+id/left_framelayout"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1" />

        <FrameLayout
            android:id="@+id/right_framelayout"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1" />

    </LinearLayout>
</LinearLayout>

LeftFragment代码

package com.gui.gui;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

public class LeftFragment extends Fragment {
    EditText mEditText;
    Button mButton;
    int mIndex;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.left_fragment, container, false);
        mEditText = view.findViewById(R.id.edittext);
        mButton = view.findViewById(R.id.btn);
        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                switch (mIndex) {
                    case 0:
                        MainActivity3 activity0 = (MainActivity3) getActivity();
                        String value0 = activity0.mEditText.getText().toString();
                        mEditText.setText(value0);
                        break;
                    case 1:
                        MainActivity3 activity1 = (MainActivity3) getActivity();
                        String value1 = mEditText.getText().toString();
                        activity1.mEditText.setText(value1);
                        break;
                    case 2:
                        RightFragment fragment2 = (RightFragment) getFragmentManager().getFragments().get(1);
                        String value2 = fragment2.mEditText.getText().toString();
                        mEditText.setText(value2);
                        break;
                    case 3:
                        RightFragment fragment3 = (RightFragment) getFragmentManager().getFragments().get(1);
                        String value3 = mEditText.getText().toString();
                        fragment3.mEditText.setText(value3);
                        break;
                    default:
                        break;
                }
                mIndex = (++mIndex) % 4;
            }
        });
        return view;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <EditText
        android:id="@+id/edittext"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="left fragment"/>

    <Button
        android:id="@+id/btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="left操作"
        android:textAllCaps="false"/>

</LinearLayout>

RightFragment

package com.gui.gui;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

public class RightFragment extends Fragment {
    EditText mEditText;
    Button mButton;
    int mIndex = 0;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.right_fragment, container, false);
        mEditText = view.findViewById(R.id.edittext);
        mButton = view.findViewById(R.id.btn);
        return view;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <EditText
        android:id="@+id/edittext"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="right fragment"/>

    <Button
        android:id="@+id/btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="right操作"
        android:textAllCaps="false"/>

</LinearLayout>