<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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"
    tools:context=".HeadActivity">

    <GridView
        android:id="@+id/gridView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginTop="10dp"
        android:horizontalSpacing="3dp"
        android:verticalSpacing="3dp"
        android:numColumns="2"
        >
    </GridView>

</RelativeLayout>

<?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"
    tools:context=".MainActivity"
    android:orientation="vertical"
    >

    <ImageView
        android:id="@+id/image"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/b"
        >
    </ImageView>
    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="更换图片"
        />



</LinearLayout>

package com.example.myactivityiii;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.media.Image;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.BaseAdapter;
import android.widget.GridView;
import android.widget.ImageView;

public class HeadActivity extends AppCompatActivity {

    private int[] imageId = new int[] {
            R.drawable.a,R.drawable.b,R.drawable.c,R.drawable.d
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_head);

        //给图像视图创建适配器
        GridView gridView = (GridView)findViewById(R.id.gridView);
        BaseAdapter adapter = new BaseAdapter() {
            @Override
            public int getCount() {
                return imageId.length;
            }
            @Override
            public Object getItem(int position) {
                return null;
            }
            @Override
            public long getItemId(int position) {
                return 0;
            }
            @Override//返回一个ImageView
            public View getView(int position, View convertView, ViewGroup parent) {
                ImageView imageView;
                if(convertView == null){
                    imageView = new ImageView(HeadActivity.this);
                 }
                else {
                    imageView = (ImageView)convertView;
                }
                imageView.setImageResource(imageId[position]);
                return imageView;
            }
        };
        gridView.setAdapter(adapter);

        //选项被点击的监听器
        gridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override//获取选择头像的id，保存到数据包中，传回到Main中
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Intent intent = getIntent();
                Bundle bundle = new Bundle();
                bundle.putInt("imageId",imageId[position]);
                intent.putExtras(bundle);
                //设置返回码,要和请求码一样
                setResult(0x11,intent);
                finish();
            }
        });

    }
}

package com.example.myactivityiii;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;

/*
* 使用startActivityForResult，调用一个Activity并返回结果
* */
public class MainActivity extends AppCompatActivity {

    //判断请求码与返回码是否与之前设置的相同，如果相同则获取传递的数据包
    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        //1.判断请求码与返回码是否与之前设置的相同
        if(requestCode == 0x11 && resultCode == 0x11){
            //2.获取在Head活动中传递的数据
            Bundle bundle = data.getExtras();
            int imageId = bundle.getInt("imageId");
            //3.在Main中的ImageView中更新
            ImageView imageView = (ImageView)findViewById(R.id.image);
            imageView.setImageResource(imageId);
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button button = (Button)findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(
                        MainActivity.this,HeadActivity.class
                );
                startActivityForResult(intent,0x11);
            }
        });


    }
}