//AZMetaBallCanvas.h
@interface AZMetaBallCanvas : UIView
@property(nonatomic,strong) Circle *centerCircle;
@property(nonatomic,strong) Circle *touchCircle;
@end

//AZMetaBallCanvas.m
#define RADIUS 40.0
@interface AZMetaBallCanvas() {
UIBezierPath *_path;
CGPoint _touchPoint;
}
@end
@implementation AZMetaBallCanvas
- (instancetype)initWithCoder:(NSCoder *)aDecoder {
self = [super initWithCoder:aDecoder];
NSLog(@"initWithCorder");
if (self) {
[self initData];
}
return self;
}
- (void)initData {
_touchCircle = [Circle initWithcenterPoint:self.center radius:RADIUS];
_centerCircle = [Circle initWithcenterPoint:self.center radius:RADIUS];
_touchPoint = self.center;
NSLog(@"self.center (%f, %f)", self.center.x, self.center.y);
}
@end

- (void)drawRect:(CGRect)rect {
_path = [[UIBezierPath alloc] init];

[self drawCenterCircle];

[self drawTouchCircle:_touchPoint];

[self drawBezierCurveWithCircle1:_centerCircle Circle2:_touchCircle];
}

#pragma mark draw circle --- 画圆
- (void) drawCenterCircle {
[self drawCircle:_path circle:_centerCircle];
}
- (void) drawTouchCircle:(CGPoint)center {
_touchCircle.centerPoint = center;
[self drawCircle:_path circle:_touchCircle];
}
- (void)drawCircle:(UIBezierPath *)path circle:(Circle *)circle {
[_path addArcWithCenter:circle.centerPoint radius:circle.radius startAngle:0 endAngle:360 clockwise:true];
[_path fill];
[_path stroke];
[_path removeAllPoints];
}

#pragma mark draw curve --- 画贝塞尔曲线
- (void)drawBezierCurveWithCircle1:(Circle *)circle1 Circle2:(Circle *)circle2 {
float circle1_x = circle1.centerPoint.x;
float circle1_y = circle1.centerPoint.y;
float circle2_x = circle2.centerPoint.x;
float circle2_y = circle2.centerPoint.y;
//连心线的长度
float d = sqrt(powf(circle1_x - circle2_x, 2) + powf(circle1_y - circle2_y, 2));
//连心线x轴的夹角
float angle1 = atan((circle2_y - circle1_y) / (circle1_x - circle2_x));
//连心线和公切线的夹角
float angle2 = asin((circle1.radius - circle2.radius) / d);
//切点到圆心和x轴的夹角
float angle3 = M_PI_2 - angle1 - angle2;
float angle4 = M_PI_2 - angle1 + angle2;
float offset1_X = cos(angle3) * circle1.radius;
float offset1_Y = sin(angle3) * circle1.radius;
float offset2_X = cos(angle3) * circle2.radius;
float offset2_Y = sin(angle3) * circle2.radius;
float offset3_X = cos(angle4) * circle1.radius;
float offset3_Y = sin(angle4) * circle1.radius;
float offset4_X = cos(angle4) * circle2.radius;
float offset4_Y = sin(angle4) * circle2.radius;
float p1_x = circle1_x - offset1_X;
float p1_y = circle1_y - offset1_Y;
float p2_x = circle2_x - offset2_X;
float p2_y = circle2_y - offset2_Y;
float p3_x = circle1_x + offset3_X;
float p3_y = circle1_y + offset3_Y;
float p4_x = circle2_x + offset4_X;
float p4_y = circle2_y + offset4_Y;
CGPoint p1 = CGPointMake(p1_x, p1_y);
CGPoint p2 = CGPointMake(p2_x, p2_y);
CGPoint p3 = CGPointMake(p3_x, p3_y);
CGPoint p4 = CGPointMake(p4_x, p4_y);
CGPoint p1_center_p4 = CGPointMake((p1_x + p4_x) / 2, (p1_y + p4_y) / 2);
CGPoint p2_center_p3 = CGPointMake((p2_x + p3_x) / 2, (p2_y + p3_y) / 2);
[self drawBezierCurveStartAt:p1 EndAt:p2 controlPoint:p2_center_p3];
[self drawLineStartAt:p2 EndAt:p4];
[self drawBezierCurveStartAt:p4 EndAt:p3 controlPoint:p1_center_p4];
[self drawLineStartAt:p3 EndAt:p1];
[_path moveToPoint:p1];
[_path closePath];
[_path stroke];
}

#pragma mark touch event
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
UITouch *touch = [touches anyObject];
_touchPoint = [touch locationInView:self];
[self setNeedsDisplay];
}
- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
UITouch *touch = [touches anyObject];
_touchPoint = [touch locationInView:self];
[self setNeedsDisplay];
}

#pragma 改变半径
-(void)changeCenterCircleRadiusTo:(float)radius {
_centerCircle.radius = radius;
[self setNeedsDisplay];
}
-(void)changeTouchCircleRadiusTo:(float)radius {
_touchCircle.radius = radius;
[self setNeedsDisplay];
}

- (void)attach:(UIView *)item {
UIPanGestureRecognizer *drag = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(drag:)];
item.userInteractionEnabled = YES;
[item addGestureRecognizer:drag];
}
- (void)drag:(UIPanGestureRecognizer *)recognizer {
//得到触摸点
_touchPoint = [recognizer locationInView:self];
//得到触摸的view
UIView *touchView = recognizer.view;
switch (recognizer.state) {
case UIGestureRecognizerStateBegan:{
//touch开始：在画布上绘制一个touchView的副本
//...此部分参看源码
break;
}
case UIGestureRecognizerStateChanged:{
//移动中：记录触摸位置，更改touchView和touchCircle的坐标位置
[self resetTouchCenter:_touchPoint];
break;
}
case UIGestureRecognizerStateEnded: {
//touch结束：根据连心线长度判断是执行爆炸动画还是弹簧动画
//...此部分参看源码
break;
}
default:
break;
}
[self setNeedsDisplay]; //重绘
}