Android：无法停止屏幕记录

Question

我尝试为android编写一个简单的屏幕录像机。录音有效，但我无法阻止它。由于“--time-limit”选项，它会在30秒后自动停止，但如果按下停止按钮则不会。 这是我的代码：

import android.os.AsyncTask;
import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.File;
import java.io.IOException;
import java.io.InputStreamReader;
import java.text.SimpleDateFormat;
import java.util.Calendar;

public class MainActivity extends AppCompatActivity {

    private Button buttonStart;
    private Button buttonStop;

    private Process process;
    private DataOutputStream stream;
    private int pid = -1;

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

        buttonStart = (Button) findViewById(R.id.buttonStart);
        buttonStart.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                record("/storage/sdcard1/screencast-" + getFilename() + ".mp4");
            }
        });

        buttonStop = (Button) findViewById(R.id.buttonStop);
        buttonStop.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                showToast("Stopping ...");
                try {
                    stop();
                    showToast("Stopped.");
                } catch (Exception e) {
                    showToast("Failed to stop recording.");
                }
            }
        });
    }

    private void record(final String path) {
        new AsyncTask<Void, Void, Void>() {
            @Override
            protected Void doInBackground(Void... params) {
                try{
                    process = Runtime.getRuntime().exec("su");
                    stream = new DataOutputStream(process.getOutputStream());

                    stream.writeBytes("echo $$\n");
                    stream.flush();

                    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(process.getInputStream()));
                    pid = Integer.parseInt(bufferedReader.readLine());
                    bufferedReader.close();

                    // just using "--time-limit 30" because i don´t want to restart to stop recording
                    stream.writeBytes("screenrecord --time-limit 30 " + path + "\n");
                    stream.flush();
                    stream.writeBytes("exit\n");
                    stream.close();
                    process.waitFor();
                }catch(Exception e){
                    e.printStackTrace();
                }
                return null;
            }
        }.execute();
    }

    private void stop() throws IOException {
        if(pid != -1) {

            // both following lines don't work
            android.os.Process.killProcess(pid);
            android.os.Process.sendSignal(pid, android.os.Process.SIGNAL_QUIT);

            if(process != null) {
                process.destroy();
            }
        }
    }

    private String getFilename() {
        SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd-HH-mm-ss");
        Calendar now = Calendar.getInstance();
        return formatter.format(now.getTime());
    }

    private void showToast(String msg) {
        Toast.makeText(getApplicationContext(), msg, Toast.LENGTH_SHORT).show();
    }

}

我的布局：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="me.bleuzen.screenrecorder.MainActivity">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Start"
        android:id="@+id/buttonStart"
        android:layout_above="@+id/buttonStop"
        android:layout_centerHorizontal="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Stop"
        android:id="@+id/buttonStop"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true" />
</RelativeLayout>