最近项目中的需求，拿出来记录一下，先看效果图:

效果图.gif

分析：这个动画其实只有两端弧L1和L2，动画过程就是L1的起点到L2的终点，L1的终点到L2的起点，L2的起点到L1的终点，L2的终点到L1的起点，L1和L2增量是一样的，这样就简单了。

然后写一个继承自CALayer的类，然后写两个参数，使用这两个参数来控制L1和L2的起点和终点，然后我们对着两个参数做动画，然后动画就可以实现了

思路说完了，开始上代码，这是layer里边的全部代码：

@interface N_refreshLayer : CALayer
// 开始绘制点增量 取值范围 0 - 1
@property (nonatomic, assign) CGFloat startPross;
// 结束绘制点增量 取值范围 0 - 1
@property (nonatomic, assign) CGFloat endPross;
// 这个用来控制layer的透明度（项目需求，没有需求可以不要）
@property (nonatomic, assign) CGFloat lineAlpha;

@end

@implementation N_refreshLayer

// 重写系统方法，当key为startPross或者endPross的时候进行重新绘制
+ (BOOL)needsDisplayForKey:(NSString *)key {
    if ([key isEqualToString:@"startPross"] || [key isEqualToString:@"endPross"]) {
        return YES;
    }
    return [super needsDisplayForKey:key];
}

// 重写系统方法，绘制圆弧
- (void)drawInContext:(CGContextRef)ctx {
    
    // 这里边 - 1 是减去线宽的一半， - 6 是弧距离边界的距离
    CGFloat radius = MIN(CGRectGetWidth(self.bounds), CGRectGetHeight(self.bounds)) / 2 - 1 - 6;
    CGPoint center = CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds));
    // L1
    UIBezierPath *path1 = [UIBezierPath bezierPath];
    // 这里说一下，顺时针角度为正，逆时针角度为负，我在-90°处开始绘制，后边M_PI*0.05是两条弧之间的间隔，最后边2*M_PI*参数，是控制增量的（M_PI是180°）
    CGFloat startAngle1= -M_PI_2 + M_PI*0.05- 2*M_PI*self.startPross;
    CGFloat endAngle1 = -M_PI_2 - M_PI*0.05 - 2*M_PI*self.endPross;
    [path1 addArcWithCenter:center radius:radius startAngle:startAngle1 endAngle:endAngle1 clockwise:NO];
    // L1    
    UIBezierPath *path2 = [UIBezierPath bezierPath];
    CGFloat startAngle2 = -M_PI_2 - M_PI*0.25 - 2*M_PI*self.endPross;
    CGFloat endAngle2 = -M_PI_2 + M_PI*0.25 - 2*M_PI*self.startPross;
    [path2 addArcWithCenter:center radius:radius startAngle:startAngle2 endAngle:endAngle2 clockwise:NO];
    
    CGContextAddPath(ctx, path1.CGPath);
    CGContextAddPath(ctx, path2.CGPath);
    CGContextSetLineWidth(ctx, 2);
    CGContextSetLineCap(ctx, kCGLineCapRound);
    CGContextSetStrokeColorWithColor(ctx, kRGBAColor(255, 0, 0, 1.0).CGColor);
    CGContextStrokePath(ctx);
}

- (void)setLineAlpha:(CGFloat)lineAlpha {
    self.opacity = lineAlpha;
}

然后在封装一个view将layer添加到view的layer上，在view里边控制layer的参数，view中的全部代码：

// .h文件
@interface N_RefreshAnimView : UIView
// 提供几个控制动画的方法
// 开始执行动画
- (void)startAnimation;
// 停止动画
- (void)stopAnimation;
// 设置增量（可以根据scrollView的偏移量来设置进度）
- (void)setProgressWith:(CGFloat)progress;
// 设置layer的透明度
- (void)setLineAlphaWith:(CGFloat)lineAlpha;

@end

// .m文件
@interface N_RefreshAnimView ()

@property (nonatomic, strong) N_refreshLayer *animLayer;
@property (nonatomic, strong) CABasicAnimation *anim1;
@property (nonatomic, strong) CABasicAnimation *anim2;

@end

@implementation N_RefreshAnimView

// 初始化方法
- (instancetype)init {
    if (self = [super init]) {
        [self.layer addSublayer:self.animLayer];
    }
    return self;
}

- (instancetype)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
        [self.layer addSublayer:self.animLayer];
    }
    return self;
}

- (instancetype)initWithCoder:(NSCoder *)aDecoder {
    if (self = [super initWithCoder:aDecoder]) {
        [self.layer addSublayer:self.animLayer];
    }
    return self;
}

- (void)startAnimation {
    CABasicAnimation *anim1 = (CABasicAnimation *)[self.animLayer animationForKey:@"anim1"];
    CABasicAnimation *anim2 = (CABasicAnimation *)[self.animLayer animationForKey:@"anim2"];
    // 如果layer上不存在动画，则进行添加动画
    if (!anim1) [self.animLayer addAnimation:self.anim1 forKey:@"anim1"];
    if (!anim2) [self.animLayer addAnimation:self.anim2 forKey:@"anim2"];
    // 如果存在动画则不继续执行
    if (anim1 && anim2 && self.animLayer.speed == 1) return;
    CFTimeInterval pauseTime = self.animLayer.timeOffset;
    CFTimeInterval begin = CACurrentMediaTime() - pauseTime;
    [self.animLayer setTimeOffset:0];
    [self.animLayer setBeginTime:begin];
    self.animLayer.speed = 1;
}

- (void)stopAnimation {
    // 设置layer的动画速度为0则停止动画
    CFTimeInterval pausedTime = [self.animLayer convertTime:CACurrentMediaTime() fromLayer:nil];
    self.animLayer.speed = 0.0;
    // 记录动画暂停的时间，在恢复动画的时候使用
    self.animLayer.timeOffset = pausedTime;
}

- (void)setProgressWith:(CGFloat)progress {
    [self.animLayer removeAllAnimations];
    self.animLayer.speed = 1.0;
    self.animLayer.startPross = progress * 0.15;
    self.animLayer.endPross = progress * 0.85;
    [self.animLayer setNeedsDisplay];
}

- (void)setLineAlphaWith:(CGFloat)lineAlpha {
    self.animLayer.lineAlpha = lineAlpha;
}

// 懒加载layer
- (N_refreshLayer *)animLayer {
    if (!_animLayer) {
        _animLayer = [[N_refreshLayer alloc] init];
        _animLayer.startPross = 0.0;
        _animLayer.endPross = 0.0;
        [_animLayer setNeedsDisplay];
    }
    return _animLayer;
}

// 懒加载动画
- (CABasicAnimation *)anim1 {
    if (!_anim1) {
        _anim1 = [CABasicAnimation animationWithKeyPath:@"startPross"];
        _anim1.removedOnCompletion = NO;
        _anim1.fillMode = kCAFillModeForwards;
        _anim1.repeatCount = MAXFLOAT;
        _anim1.duration = 0.8;
       // 动画1的增量加上动画2的增量为360°，增量与1之间差值的和为L1加间距的弧度 
        _anim1.toValue = [NSNumber numberWithFloat:0.15];
        _anim1.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseOut];
    }
    return _anim1;
}

- (CABasicAnimation *)anim2 {
    if (!_anim2) {
        _anim2 = [CABasicAnimation animationWithKeyPath:@"endPross"];
        _anim2.removedOnCompletion = NO;
        _anim2.fillMode = kCAFillModeForwards;
        _anim2.repeatCount = MAXFLOAT;
        _anim2.duration = 0.8;
       // 动画1的增量加上动画2的增量为360°，增量与1之间差值的和为L1加间距的弧度
        _anim2.toValue = [NSNumber numberWithFloat:0.85];
        _anim2.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseOut];
    }
    return _anim2;
}

- (void)layoutSubviews {
    [super layoutSubviews];
    self.animLayer.frame = self.bounds;
}

到此为止这个动画就可以使用了，细心的小伙伴会发现，这样画出来的线条会有锯齿，解决办法设置 _animLayer.contentsScale = [[UIScreen mainScreen] scale]; 就可以了，为了这个问题，浪费了我一下午时间，最后就TM一行代码

下边是配合MJRefresh使用做下拉刷新，那么自定义MJRefresh，代码如下：

.h文件
#import <MJRefresh/MJRefresh.h>
#import "N_RefreshAnimView.h"

@interface N_RefreshHeader : MJRefreshHeader

@end

.m文件
@interface N_RefreshHeader ()

@property (nonatomic, strong) N_RefreshAnimView *animView;

@end


@implementation N_RefreshHeader

- (void)prepare {
    [super prepare];
    [self addSubview:self.animView];
}

- (void)placeSubviews {
    [super placeSubviews];
    // 设置子控件位置
    self.animView.frame = CGRectMake(0, 0, 35, 35);
    self.animView.center = CGPointMake(self.mj_w/2.0, self.mj_h/2.0);
}

- (void)setState:(MJRefreshState)state {
    
    MJRefreshCheckState;
    
    switch (state) {
        case MJRefreshStateIdle:
            [self.animView stopAnimation];
            break;
        case MJRefreshStatePulling:
            break;
        case MJRefreshStateRefreshing:
            [self.animView startAnimation];
            break;
        default:
            break;
    }
}

- (void)setPullingPercent:(CGFloat)pullingPercent {
    // 控制layer的动画进度和透明度
    if (pullingPercent > 0) [self.animView setLineAlphaWith:pullingPercent];
    CGFloat progress = pullingPercent - (int)pullingPercent;
    [self.animView setProgressWith:progress];
}

- (void)endRefreshing {
    [super endRefreshing];
}

- (N_RefreshAnimView *)animView {
    if (!_animView) {
        _animView = [[N_RefreshAnimView alloc] init];
        _animView.backgroundColor = [UIColor whiteColor];
        // 这里我的需求需要圆形和阴影效果
        _animView.layer.cornerRadius = 17.5;
        _animView.layer.shadowColor = [UIColor blackColor].CGColor;
        _animView.layer.shadowOpacity = 0.1;
        _animView.layer.shadowOffset = CGSizeMake(3, 3);
        _animView.layer.shadowRadius = 5.0;
    }
    return _animView;
}

至此这个动画就完成了！有意见或者建议的小伙伴欢迎下边评论!