怎么在Android应用中添加一个倒计时功能

public class SweepView extends View {  private static final int DEFAULT_WIDTH = 100;  private static final int DEFAULT_HEIGHT = 100;  private int mWidth;   //这里并没卵用  private int mHeight;  //这个也没卵用  private RectF rectF;  private Paint paint;  private int mColor = Color.RED;//默认颜色为红色  private float mSweep = 0;  //扇形角度  public SweepView(Context context) {    super(context);    init();  }  public SweepView(Context context, AttributeSet attrs) {    super(context, attrs);    init();  }  public SweepView(Context context, AttributeSet attrs, int defStyleAttr) {    super(context, attrs, defStyleAttr);    init();  }  private void init() {    paint = new Paint();    paint.setColor(mColor); //画笔颜色    paint.setStyle(Paint.Style.FILL);  //填充    paint.setAntiAlias(true);  //是否抗锯齿  }    public void setColor(int color) {    this.mColor = color;    paint.setColor(mColor);    //调用onDraw重绘    invalidate();  }    public void setSweep(float mSweep) {    this.mSweep = mSweep;    //调用onDraw重绘    invalidate();  }  @Override  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {    int resultWidth = measureWidth(widthMeasureSpec);    int resultHeight = measureHeight(heightMeasureSpec);    setMeasuredDimension(resultWidth, resultHeight);  }    private int measureWidth(int widthMeasureSpec) {    int size = MeasureSpec.getSize(widthMeasureSpec);    int mode = MeasureSpec.getMode(widthMeasureSpec);    int result;    if (mode == MeasureSpec.EXACTLY) {      result = size;    } else {      result = DEFAULT_WIDTH;      if (mode == MeasureSpec.AT_MOST) {        result = Math.min(size, DEFAULT_WIDTH);      }    }    return result;  }    private int measureHeight(int heightMeasureSpec) {    int size = MeasureSpec.getSize(heightMeasureSpec);    int mode = MeasureSpec.getMode(heightMeasureSpec);    int result;    if (mode == MeasureSpec.EXACTLY) {      result = size;    } else {      result = DEFAULT_HEIGHT;      if (mode == MeasureSpec.AT_MOST) {        result = Math.min(size, DEFAULT_HEIGHT);      }    }    return result;  }  @Override  protected void onSizeChanged(int w, int h, int oldw, int oldh) {    this.mHeight = h;    this.mWidth = w;    rectF = new RectF(0, 0, w, h);    super.onSizeChanged(w, h, oldw, oldh);  }  @Override  protected void onDraw(Canvas canvas) {    //画扇形    canvas.drawArc(rectF, -90, mSweep, true, paint);  }}