用Android Studio设计StopWatch APP。单击“Start”按钮，计时器开始工作，单击“Stop”按钮，计时器停止工作，单击“Reset”按钮，计时器复位。当计时器界面未获得焦点时暂停工作；当计时器重新获得焦点时继续工作的代码

以下是一个简单的 StopWatch APP 的代码，可以在 Android Studio 中使用：

public class MainActivity extends AppCompatActivity {

    private TextView mTextView;
    private Button mStartButton, mStopButton, mResetButton;
    private Handler mHandler = new Handler();
    private long mStartTime, mTimeInMillis;
    private boolean mIsRunning;

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

        mTextView = findViewById(R.id.text_view);
        mStartButton = findViewById(R.id.start_button);
        mStopButton = findViewById(R.id.stop_button);
        mResetButton = findViewById(R.id.reset_button);

        mStartButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startTimer();
            }
        });

        mStopButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                stopTimer();
            }
        });

        mResetButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                resetTimer();
            }
        });
    }

    private void startTimer() {
        if (!mIsRunning) {
            mStartTime = System.currentTimeMillis() - mTimeInMillis;
            mHandler.postDelayed(mRunnable, 0);
            mIsRunning = true;
        }
    }

    private void stopTimer() {
        if (mIsRunning) {
            mHandler.removeCallbacks(mRunnable);
            mTimeInMillis = System.currentTimeMillis() - mStartTime;
            mIsRunning = false;
        }
    }

    private void resetTimer() {
        mTimeInMillis = 0;
        updateTimerText();
    }

    private void updateTimerText() {
        int minutes = (int) (mTimeInMillis / 1000) / 60;
        int seconds = (int) (mTimeInMillis / 1000) % 60;
        int milliseconds = (int) (mTimeInMillis % 1000) / 10;

        String time = String.format(Locale.getDefault(), "%02d:%02d:%02d", minutes, seconds, milliseconds);
        mTextView.setText(time);
    }

    private Runnable mRunnable = new Runnable() {
        @Override
        public void run() {
            mTimeInMillis = System.currentTimeMillis() - mStartTime;
            updateTimerText();
            mHandler.postDelayed(this, 10);
        }
    };

    @Override
    protected void onPause() {
        super.onPause();
        if (mIsRunning) {
            mHandler.removeCallbacks(mRunnable);
        }
    }

    @Override
    protected void onResume() {
        super.onResume();
        if (mIsRunning) {
            mHandler.postDelayed(mRunnable, 0);
        }
    }
}

这个 APP 包含了三个按钮：Start、Stop 和 Reset。当用户单击 Start 按钮时，计时器开始工作；当用户单击 Stop 按钮时，计时器停止工作；当用户单击 Reset 按钮时，计时器复位。当计时器界面未获得焦点时暂停工作，当计时器重新获得焦点时继续工作。