Android应用增加计时器

textView = (TextView) findViewById(R.id.time_text);
timer = (Chronometer) findViewById(R.id.chronometer);
timer.setBase(SystemClock.elapsedRealtime()+timeWhenStopped);
timer.start();

pauseButton = (Button) findViewById(R.id.pause);
pauseButton.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        pauseOrStart++;
        if (pauseOrStart % 2 == 1) {
            Toast.makeText(StartActivity.this, "Pause", Toast.LENGTH_SHORT).show();
            timeWhenStopped = timer.getBase() - SystemClock.elapsedRealtime();
            timer.stop();
        } else {
            Toast.makeText(StartActivity.this, "Start", Toast.LENGTH_SHORT).show();
            timer.setBase(SystemClock.elapsedRealtime()+timeWhenStopped);
            timer.start();
        }
    }
});

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:paddingTop="5dp"
    android:orientation="horizontal">

    <TextView
        android:id="@+id/time_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Time:"
        android:textSize="30sp"
        android:textColor="@color/green"
        android:layout_weight="2"/>

    <Chronometer
        android:id ="@+id/chronometer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:format="%s"
        android:textSize="30sp"
        android:textColor="@color/green"
        android:layout_weight="25"/>

    <Button
        android:id="@+id/pause"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Pause"
        android:textColor="@color/green"
        android:background="@drawable/shape2"
        android:layout_weight="1"/>

</LinearLayout>

class PausableChronometer extends Chronometer {

    private long timeWhenStopped = 0;

    public PausableChronometer(Context context) {
        super(context);
    }

    public PausableChronometer(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public PausableChronometer(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    public void start() {
        setBase(SystemClock.elapsedRealtime()+timeWhenStopped);
        super.start();
    }

    @Override
    public void stop() {
        super.stop();
        timeWhenStopped = getBase() - SystemClock.elapsedRealtime();
    }

    public void reset() {
        stop();
        setBase(SystemClock.elapsedRealtime());
        timeWhenStopped = 0;
    }

    public long getCurrentTime() {
        return timeWhenStopped;
    }

    public void setCurrentTime(long time) {
        timeWhenStopped = time;
        setBase(SystemClock.elapsedRealtime()+timeWhenStopped);
    }
}