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
  }
  /**
   * 设置扇形颜色
   * UIThred
   */
  public void setColor(int color) {
    this.mColor = color;
    paint.setColor(mColor);
    //调用onDraw重绘
    invalidate();
  }
  /**
   * 设置扇形的区域0-360
   * UIThred
   */
  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);
  }
  /**
   * 绘制的宽
   * 其实里面的内容我不懂,好惭愧...我以后会弄懂的.....0.0
   */
  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;
  }
  /**
   * 绘制的高
   * 这里面的内容我也不懂,好惭愧...我以后会弄懂的.....0.0
   */
  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);
  }
}

<&#63;xml version="1.0" encoding="utf-8"&#63;>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:id="@+id/activity_main"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  tools:context="com.example.meijun.myapplication.MainActivity">
  <RelativeLayout
    android:layout_centerInParent="true"
    android:layout_width="20dp"
    android:background="#00f"
    android:layout_height="20dp">
    <com.example.meijun.myapplication.SweepView
      android:layout_width="wrap_content"
      android:id="@+id/sweepView"
      android:layout_height="wrap_content" />
  </RelativeLayout>
</RelativeLayout>

public class MainActivity extends AppCompatActivity {
  private SweepView sweepView;
  float angle = 0;//绘制的角度
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    sweepView = (SweepView) findViewById(R.id.sweepView);
    sweepView.setColor(Color.WHITE);  //设置画笔颜色
    sweepView.setSweep(0); //初始绘制0度
    new Thread(new Runnable() {
      @Override
      public void run() {
        while (angle <= 360) { //这里相当于绘制一个完整的圆,结合下面的3.6及50,也就是5秒钟的倒计时
          angle += 3.6;
          runOnUiThread(new Runnable() {
            @Override
            public void run() {
              sweepView.setSweep(angle);
            }
          });
          try {
            Thread.sleep(50);
          } catch (InterruptedException e) {
            e.printStackTrace();
          }
        }
      }
    }).start();
  }
}