OC-UIImageView优化圆角，防止离屏渲染

先使用Core Graphics自己画出了一个圆角矩形图片，再设置边框宽度和颜色，也不会触发离屏渲染

iconImage = [UIImage createRoundedRectImage:self.iconImage size:CGSizeMake(100, 50) radius:25];
iconView = [[UIImageView alloc] initWithImage:iconImage];
iconView.layer.borderWidth = 2.0;
iconView.layer.borderColor = UIColor.redColor.CGColor;
iconView.frame = CGRectMake(20, 5, 100, 50);
iconView.layer.cornerRadius = 25;
iconView.layer.masksToBounds = YES;
[self.view addSubview:iconView];

#import <UIKit/UIKit.h>

@interface UIImage (DSRoundImage)

+ (id)createRoundedRectImage:(UIImage*)image size:(CGSize)size radius:(int)radius;

@end

#import "UIImage+DSRoundImage.h"

@implementation UIImage (DSRoundImage)

static void addRoundedRectToPath(CGContextRef context, CGRect rect, float ovalWidth, float ovalHeight)
{
    float fw, fh;
    
    if (ovalWidth == 0 || ovalHeight == 0)
    {
        CGContextAddRect(context, rect);
        return;
    }
    
    CGContextSaveGState(context);
    CGContextTranslateCTM(context, CGRectGetMinX(rect), CGRectGetMinY(rect));
    CGContextScaleCTM(context, ovalWidth, ovalHeight);
    fw = CGRectGetWidth(rect) / ovalWidth;
    fh = CGRectGetHeight(rect) / ovalHeight;
    
    CGContextMoveToPoint(context, fw, fh/2);  // Start at lower right corner
    CGContextAddArcToPoint(context, fw, fh, fw/2, fh, 1);  // Top right corner
    CGContextAddArcToPoint(context, 0, fh, 0, fh/2, 1); // Top left corner
    CGContextAddArcToPoint(context, 0, 0, fw/2, 0, 1); // Lower left corner
    CGContextAddArcToPoint(context, fw, 0, fw, fh/2, 1); // Back to lower right
    
    CGContextClosePath(context);
    CGContextRestoreGState(context);
}

+ (id)createRoundedRectImage:(UIImage *)image size:(CGSize)size radius:(int)radius{
    
    size = CGSizeMake(size.width*2, size.height*2);
    radius = radius*2;
    
    UIImage * img = image;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(NULL, size.width, size.height, 8, 4 * size.width, colorSpace, kCGImageAlphaPremultipliedFirst);
    CGRect rect = CGRectMake(0, 0, size.width, size.height);
    
    CGContextBeginPath(context);
    addRoundedRectToPath(context, rect, radius, radius);
    CGContextClosePath(context);
    CGContextClip(context);
    CGContextDrawImage(context, CGRectMake(0, 0, size.width, size.height), img.CGImage);
    CGImageRef imageMasked = CGBitmapContextCreateImage(context);
    img = [UIImage imageWithCGImage:imageMasked];
    
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    CGImageRelease(imageMasked);
    return img;
}

@end