public class ControllerView extends View implements View.OnTouchListener {
  private Paint borderPaint = new Paint();//大圓的畫(huà)筆
  private Paint fingerPaint = new Paint();//小圓的畫(huà)筆
  private float radius = 160;//默認(rèn)大圓的半徑
  private float centerX = radius;//大圓中心點(diǎn)的位置cx
  private float centerY = radius;//大圓中心點(diǎn)的位置cy
  private float fingerX = centerX, fingerY = centerY;//小圓圓心的位置(ax,ay)
  private float lastX = fingerX, lastY = fingerY;//小圓自動(dòng)回歸中點(diǎn)動(dòng)畫(huà)中上一點(diǎn)的位置
  private float innerRadius = 30;//默認(rèn)小圓半徑
  private float radiusBorder = (radius - innerRadius);//大圓減去小圓的半徑
  private ValueAnimator positionAnimator;//自動(dòng)回中的動(dòng)畫(huà)
  private MoveListener moveListener;//移動(dòng)回調(diào)的接口

  public ControllerView(Context context) {
    super(context);
    init(context, null, 0);
  }

  public ControllerView(Context context,
      @Nullable AttributeSet attrs) {
    super(context, attrs);
    init(context, attrs, 0);
  }

  public ControllerView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
    init(context, attrs, defStyleAttr);
  }

  //初始化
  private void init(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
    if (attrs != null) {
      TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.ControllerView);
      int fingerColor = typedArray.getColor(R.styleable.ControllerView_fingerColor,
          Color.parseColor("#3fffffff"));
      int borderColor = typedArray.getColor(R.styleable.ControllerView_borderColor,
          Color.GRAY);
      radius = typedArray.getDimension(R.styleable.ControllerView_radius, 220);
      innerRadius = typedArray.getDimension(R.styleable.ControllerView_fingerSize, innerRadius);
      borderPaint.setColor(borderColor);
      fingerPaint.setColor(fingerColor);
      lastX = lastY = fingerX = fingerY = centerX = centerY = radius;
      radiusBorder = radius - innerRadius;
      typedArray.recycle();
    }
    setOnTouchListener(this);
    positionAnimator = ValueAnimator.ofFloat(1);
    positionAnimator.addUpdateListener(animation -> {
      Float aFloat = (Float) animation.getAnimatedValue();
      changeFingerPosition(lastX + (centerX - lastX) * aFloat, lastY + (centerY - lastY) * aFloat);
    });
  }

  @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(getActualSpec(widthMeasureSpec), getActualSpec(heightMeasureSpec));
  }


  //處理wrapcontent的測(cè)量
  //默認(rèn)wrapcontent,沒(méi)有做matchParent,指定大小的適配
  //view實(shí)際的大小是通過(guò)大圓半徑確定的
  public int getActualSpec(int spec) {
    int mode = MeasureSpec.getMode(spec);
    int len = MeasureSpec.getSize(spec);
    switch (mode) {
      case MeasureSpec.AT_MOST:
        len = (int) (radius * 2);
        break;
    }
    return MeasureSpec.makeMeasureSpec(len, mode);
  }

  //繪制
  @Override protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.drawCircle(centerX, centerY, radius, borderPaint);
    canvas.drawCircle(fingerX, fingerY, innerRadius, fingerPaint);
  }

  @Override public boolean onTouch(View v, MotionEvent event) {
    float evx = event.getX(), evy = event.getY();
    float deltaX = evx - centerX, deltaY = evy - centerY;
    switch (event.getAction()) {
      case MotionEvent.ACTION_DOWN:
        //圓外按壓不生效
        if (deltaX * deltaX + deltaY * deltaY > radius * radius) {
          break;
        }
      case MotionEvent.ACTION_MOVE:
        //如果觸摸點(diǎn)在圓外
        if (Math.abs(deltaX) > radiusBorder || Math.abs(deltaY) > radiusBorder) {
          float distance = (float) Math.sqrt(deltaX * deltaX + deltaY * deltaY);
          changeFingerPosition(centerX + (deltaX * radiusBorder / distance),
              centerY + (deltaY * radiusBorder / distance));
        } else { //如果觸摸點(diǎn)在圓內(nèi)
          changeFingerPosition(evx, evy);
        }
        positionAnimator.cancel();
        break;
      case MotionEvent.ACTION_UP:
        positionAnimator.setDuration(1000);
        positionAnimator.start();
        break;
    }
    return true;
  }

  /**
   * 改變位置的回調(diào)出來(lái)
   */
  private void changeFingerPosition(float fingerX, float fingerY) {
    this.fingerX = fingerX;
    this.fingerY = fingerY;
    if (moveListener != null) {
      float r = radius - innerRadius;
      if (r == 0) {
        invalidate();
        return;
      }
      moveListener.move((fingerX - centerX) / r, (fingerY - centerY) / r);
    }
    invalidate();
  }

  @Override protected void finalize() throws Throwable {
    super.finalize();
    positionAnimator.removeAllListeners();
  }

  public void setMoveListener(
      MoveListener moveListener) {
    this.moveListener = moveListener;
  }

  /**
    *回調(diào)事件的接口
    *
   **/
  public interface MoveListener {
    void move(float dx, float dy);
  }
}

<declare-styleable name="ControllerView">
  <attr name="fingerColor" format="color" />
  <attr name="borderColor" format="color" />
  <attr name="fingerSize" format="dimension" />
  <attr name="radius" format="dimension" />
</declare-styleable>