circleToCircle method
圆形与圆形重叠
Implementation
bool circleToCircle(DFCircle other) {
if (!this.toRect().rectToRect(other.toRect())) return false;
final distance = this.radius + other.radius;
final w = this.center.x - other.center.x;
final h = this.center.y - other.center.y;
return sqrt(w * w + h * h) <= distance;
}