建立一个android工程，修改main.xml文件并编写Activity代码，实现三个按钮分别可以发送广播、代码注册广播和代码取消注册广播，点击发送广播按钮可以发送无序广播，若广播接收者已经注册广播，则吐司提示“收到广播”，否则无吐司提示

好的，我会根据你的要求来回答你的问题。首先，这里是一个简单的实现代码：

main.xml文件：

<?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"
   android:gravity="center">

   <Button
       android:id="@+id/btn_send_broadcast"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:layout_marginBottom="20dp"
       android:text="发送广播"/>

   <Button
       android:id="@+id/btn_register_broadcast"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:layout_marginBottom="20dp"
       android:text="代码注册广播"/>

   <Button
       android:id="@+id/btn_unregister_broadcast"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:text="代码取消注册广播"/>

</LinearLayout>

MainActivity.java文件：

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.Bundle;
import android.widget.Button;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

   private BroadcastReceiver broadcastReceiver;
   private IntentFilter intentFilter;

   private Button btnSendBroadcast;
   private Button btnRegisterBroadcast;
   private Button btnUnregisterBroadcast;

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

       initView();
       initListener();
   }

   private void initView() {
       btnSendBroadcast = findViewById(R.id.btn_send_broadcast);
       btnRegisterBroadcast = findViewById(R.id.btn_register_broadcast);
       btnUnregisterBroadcast = findViewById(R.id.btn_unregister_broadcast);
   }

   private void initListener() {
       btnSendBroadcast.setOnClickListener(v -> {
           Intent intent = new Intent();
           intent.setAction("com.example.broadcast.MY_BROADCAST");
           sendBroadcast(intent);
       });

       btnRegisterBroadcast.setOnClickListener(v -> {
           if (broadcastReceiver == null) {
               broadcastReceiver = new BroadcastReceiver() {
                   @Override
                   public void onReceive(Context context, Intent intent) {
                       Toast.makeText(context, "收到广播", Toast.LENGTH_SHORT).show();
                   }
               };
               intentFilter = new IntentFilter();
               intentFilter.addAction("com.example.broadcast.MY_BROADCAST");
               registerReceiver(broadcastReceiver, intentFilter);
           }
       });

       btnUnregisterBroadcast.setOnClickListener(v -> {
           if (broadcastReceiver != null) {
               unregisterReceiver(broadcastReceiver);
               broadcastReceiver = null;
           }
       });
   }

   @Override
   protected void onDestroy() {
       super.onDestroy();
       if (broadcastReceiver != null) {
           unregisterReceiver(broadcastReceiver);
           broadcastReceiver = null;
       }
   }
}

这段代码实现了在Activity中发送广播以及代码注册和取消注册广播的功能。在这里，我们定义了一个BroadcastReceiver，用于接收广播。在“代码注册广播”按钮的监听器中，我们判断了broadcastReceiver是否为空，如果为空则创建一个新的broadcastReceiver实例并注册，否则不重复注册。在“代码取消注册广播”按钮的监听器中，我们判断了broadcastReceiver是否为空，如果不为空则取消注册并将broadcastReceiver置为空。同时，在onDestroy()方法中，我们也判断了broadcastReceiver是否为空，如果不为空则取消注册并将其置为空。