1.前言

之前需求有個刮一刮出答案的功能，做個記錄，希望對需要的同學有幫助。

實現(xiàn)原理

給展示的view添加CAShapeLayer的mask，當我們滑動的時候，把軌跡添加到path上，添加的地方就可以顯示出來。
不太懂mask的同學可以看ios CALayer之mask使用。

2.實現(xiàn)方法

實現(xiàn)方法如下

2.1初始化

@interface LayerScrapeView()
@property (nonatomic, strong) UIImageView *scrapeMaskView;
@property (nonatomic, strong) UIView *scrapeContentView;
@property (nonatomic, strong) CAShapeLayer *maskLayer;
@property (nonatomic, strong) UIBezierPath *maskPath;
@end

- (instancetype)initWithFrame:(CGRect)frame
                  contentView:(UIView *)contentView
                     maskView:(UIImageView *)maskView{
    self = [super initWithFrame:frame];
    if(self){
        self.scrapeMaskView = maskView;
        [self addSubview:maskView];
        self.scrapeContentView = contentView;
        [self addSubview:contentView];
        
        self.maskLayer = [CAShapeLayer layer];
        self.maskLayer.strokeColor = [UIColor whiteColor].CGColor;
//        self.maskLayer.backgroundColor = [UIColor colorWithWhite:1 alpha:0].CGColor;
        self.maskLayer.lineWidth = 20;
        self.maskLayer.frame = contentView.bounds;
        self.maskLayer.lineCap = kCALineCapRound;
        self.scrapeContentView.layer.mask = self.maskLayer;
        self.maskPath = [UIBezierPath bezierPath];
    }
    return self;
}

2.2移動添加path

- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    UITouch *touch = touches.anyObject;
    CGPoint point = [touch locationInView:self.scrapeContentView];
    [self.maskPath addLineToPoint:point];
    [self.maskPath moveToPoint:point];
    self.maskLayer.path = self.maskPath.CGPath;
    
}

- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    UIGraphicsBeginImageContextWithOptions(self.scrapeContentView.bounds.size, NO, [UIScreen mainScreen].scale);
    CGContextRef context = UIGraphicsGetCurrentContext();
    [self.scrapeContentView.layer renderInContext:context];
    UIImage *image= UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    CGFloat percent = 1 - [self pixelPercentWithImage:image];
    if(percent < 0.75) return ;//超過0.75則不用滑動
    [self showContentView];
    self.userInteractionEnabled = NO;
}

2.3計算透明占比

截圖然后計算圖片透明像素占比

- (CGFloat)pixelPercentWithImage:(UIImage *)image{
    NSInteger alphaCount = 0;
    CGImageRef imageRef = [image CGImage];
    size_t imageWidth = CGImageGetWidth(imageRef);
    size_t imageHeight = CGImageGetHeight(imageRef);
    size_t bytesPerRow = imageWidth * 4;
    uint32_t* rgbImageBuf = (uint32_t*)malloc(bytesPerRow * imageHeight);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(rgbImageBuf, imageWidth, imageHeight, 8, bytesPerRow, colorSpace,kCGImageAlphaPremultipliedLast);
    CGContextDrawImage(context, CGRectMake(0, 0, imageWidth, imageHeight), imageRef);
    size_t pixelNum = imageWidth * imageHeight;
    uint32_t* pCurPtr = rgbImageBuf;
    for (int i = 0; i < pixelNum; i++, pCurPtr++){
        uint8_t *ptr = (uint8_t *)pCurPtr;
        if(ptr[1] == 0 && ptr[2] == 0 && ptr[3] == 0)alphaCount++; //透明顏色
    }
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    free(rgbImageBuf);
    return alphaCount*1.0/pixelNum;
}

3.補充

這里也有一種比較生硬的方式

-(void)touchesMoved:(NSSet<UITouch *>* )touches withEvent:(UIEvent* )event {
    UITouch * touch = touches.anyObject;
    CGPoint contentPoint = [touch locationInView:self.scrapeMaskView];
    CGRect rect = CGRectMake(contentPoint.x, contentPoint.y, 30, 30);
    
    UIGraphicsBeginImageContextWithOptions(self.scrapeMaskView.bounds.size, NO, [UIScreen mainScreen].scale);
    CGContextRef ref = UIGraphicsGetCurrentContext();
    [self.scrapeMaskView.layer renderInContext:ref];
    CGContextClearRect(ref, rect);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    self.scrapeMaskView.image = image;
}

- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
//    UIImageWriteToSavedPhotosAlbum(self.scrapeMaskView.image, self, @selector(savedPhotoImage:savingWithError:contextInfo:), nil);
    CGFloat percent = [self pixelPercentWithImage:self.scrapeMaskView.image];
    NSLog(@"-----------%f",percent);
}

這種也可以實現(xiàn)，但是流暢度比較差，另外計算像素透明度會有很大的偏差。

4.demo

demo鏈接

參考資料

ios CALayer之mask使用
一個萬能的刮刮樂控件