iOS 设置圆角阴影，避免离屏渲染 —— HERO博客

iOS 控件设置圆角，避免离屏渲染。

离屏渲染：指GPU（图形处理器）在当前屏幕缓冲区外新开辟一个渲染缓冲区进行工作。这会给我们带来额外的性能损耗，如果这样的操作达到一定数量，会触发缓冲区的频繁合并和上下文的的频繁切换，会出现卡顿、掉帧现象。造成离屏渲染的原因有很多，如：shouldRasterize（光栅化）、mask（遮罩层）、shadows（阴影）、EdgeAnntialiasing（抗锯齿）、cornerRadius（圆角）等等。

下面说一下什么情况下设置圆角会造成离屏渲染：

//设置cornerRadius>0且masksToBounds为YES
view.layer.cornerRadius = 10.f;
view.layer.masksToBounds = YES;

//设置cornerRadius>0且masksToBounds为YES
view.layer.cornerRadius = 10.f;
view.clipToBounds = YES;

//像下面设置view.layer.mask
UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:imgView.bounds byRoundingCorners:UIRectCornerAllCorners cornerRadii:CGSizeMake(10.f, 10.f)];
CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
maskLayer.frame = view.bounds;
maskLayer.path = path.CGPath;
view.layer.mask = maskLayer;

上面第一种应该是最常用的，是在设置了圆角及masksToBounds为YES时，才会触发离屏渲染，而masksToBounds默认是NO，也就是说只要不设置这个属性就能避免很多情况了，下面说一下如何切一个不触发离屏渲染的圆角：

UIView、UITextField、UITextView等大部分控件都可以像下面这样设置：

view.layer.cornerRadius = 10.f;
view.layer.masksToBounds = NO;

有一些特殊情况，UILabel设置时，不要设置label.backgroundColor，应设置：

label.layer.cornerRadius = 10.f;
label.layer.backgroundColor = [UIColor whiteColor].CGColor;

有图片的UIButton、UIIMageView，用drawInRect绘制UIImage圆角：

UIBezierPath *bezierPath = [UIBezierPath bezierPathWithRoundedRect:rect cornerRadius:cornerRadius];
UIGraphicsBeginImageContextWithOptions(rect.size, false, [UIScreen mainScreen].scale);
CGContextAddPath(UIGraphicsGetCurrentContext(), bezierPath.CGPath);
CGContextClip(UIGraphicsGetCurrentContext());
[self drawInRect:rect];
    
CGContextDrawPath(UIGraphicsGetCurrentContext(), kCGPathFillStroke);
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

查看离屏渲染，模拟器可以选中“Debug - Color Off-screen Rendered”开启调试，真机可以用Instruments检测，“Instruments - Core Animation - Debug Options - Color Offscreen-Rendered Yellow”开启调试，开启后，有离屏渲染的图层会变成高亮的黄色。

写了个离屏渲染的样例：

下面贴上代码：

控制器ViewController：

#import "ViewController.h"
#import "UIImage+HWCorner.h"

#define KMainW [UIScreen mainScreen].bounds.size.width

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor whiteColor];

    //创建控件
    [self creatControl];
}

- (void)creatControl
{
    CGFloat margin = 20.f;
    CGFloat controlW = (KMainW - margin * 3) * 0.5;
    
    NSArray *titleArray = @[@"离屏渲染", @"非离屏渲染"];
    for (int i = 0; i < titleArray.count; i++) {
        CGFloat controlX = margin + (controlW + margin) * i;
        //UILabel
        UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(controlX, 30, controlW, 40)];
        label.layer.backgroundColor = [[UIColor grayColor] CGColor];
        label.text = titleArray[i];
        label.textAlignment = NSTextAlignmentCenter;
        label.layer.cornerRadius = label.bounds.size.height * 0.5;
        label.layer.masksToBounds = i == 0 ? YES : NO;
        [self.view addSubview:label];

        //UIView
        UIView *view = [[UIView alloc] initWithFrame:CGRectMake(controlX, CGRectGetMaxY(label.frame) + margin, controlW, 40)];
        view.backgroundColor = [UIColor grayColor];
        view.layer.cornerRadius = view.bounds.size.height * 0.5;
        view.layer.masksToBounds = i == 0 ? YES : NO;
        [self.view addSubview:view];

        //UIView若未添加子控件，设置view.layer.masksToBounds = YES;也不会造成离屏渲染
        UIView *subView = [[UIView alloc] initWithFrame:CGRectMake(30, 0, controlW - 60, 40)];
        subView.backgroundColor = [UIColor redColor];
        [view addSubview:subView];

        //UITextView
        UITextView *textView = [[UITextView alloc] initWithFrame:CGRectMake(controlX, CGRectGetMaxY(view.frame) + margin, controlW, 40)];
        textView.userInteractionEnabled = NO;
        textView.backgroundColor = [UIColor grayColor];
        if (i == 0) {
            /*
             这里换了一种实现方法，用UIBezierPath赋值layer.mask，两种方式都会造成离屏渲染
             textView.layer.cornerRadius = textView.bounds.size.height * 0.5;
             textView.layer.masksToBounds = YES;
             */
            UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:textView.bounds byRoundingCorners:UIRectCornerTopLeft | UIRectCornerTopRight cornerRadii:CGSizeMake(textView.bounds.size.height * 0.5, textView.bounds.size.height * 0.5)];
            CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
            maskLayer.frame = textView.bounds;
            maskLayer.path = path.CGPath;
            textView.layer.mask = maskLayer;
        }else {
            textView.layer.cornerRadius = textView.bounds.size.height * 0.5;
            textView.layer.masksToBounds = NO;
        }
        [self.view addSubview:textView];
        
        //UIButton
        UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(controlX, CGRectGetMaxY(textView.frame) + margin, controlW, controlW)];
        if (i == 0) {
            [button setImage:[UIImage imageNamed:@"hero_1"] forState:UIControlStateNormal];
            button.layer.cornerRadius = button.bounds.size.width * 0.5;
            button.layer.masksToBounds = YES;
        }else {
            [button setImage:[[UIImage imageNamed:@"hero_1"] drawCornerInRect:button.bounds cornerRadius:button.bounds.size.width * 0.5] forState:UIControlStateNormal];
        }
        [self.view addSubview:button];
        
        //UIImageView设置圆角
        UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(controlX, CGRectGetMaxY(button.frame) + margin, controlW, controlW)];
        if (i == 0) {
            [imageView setImage:[UIImage imageNamed:@"hero_1"]];
            imageView.layer.cornerRadius = imageView.bounds.size.width * 0.5;
            imageView.layer.masksToBounds = YES;
        }else {
            [imageView setImage:[[UIImage imageNamed:@"hero_1"] drawCornerInRect:imageView.bounds cornerRadius:imageView.bounds.size.width * 0.5]];
        }
        [self.view addSubview:imageView];
        
        //UIImageView若未添加子控件，设置imageView.layer.masksToBounds = YES;也不会造成离屏渲染
        UIView *subImageView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, controlW, 40)];
        subImageView.backgroundColor = [UIColor redColor];
        subImageView.layer.cornerRadius = imageView.bounds.size.width * 0.5;
        [imageView addSubview:subImageView];
        
        //UIImageView设置阴影
        CGFloat imgW = 70.f;
        CGFloat imgPadding = (KMainW - imgW * 4 - margin * 2) / 3;
        UIImageView *shadowImgView = [[UIImageView alloc] initWithFrame:CGRectMake(margin + (imgW + imgPadding) * 2 * i, CGRectGetMaxY(imageView.frame) + margin, imgW, imgW)];
        [shadowImgView setImage:[UIImage imageNamed:@"hero_1"]];
        shadowImgView.layer.shadowColor = [UIColor redColor].CGColor;
        shadowImgView.layer.shadowOpacity = 0.8f;
        shadowImgView.layer.shadowOffset = CGSizeMake(5, 5);
        shadowImgView.layer.shadowRadius = 5.f;
        if (i == 1) {
            UIBezierPath *path = [UIBezierPath bezierPathWithRect:shadowImgView.bounds];
            shadowImgView.layer.shadowPath = path.CGPath;
        }
        [self.view addSubview:shadowImgView];
        
        //UIImageView设置阴影+圆角
        UIImageView *shadowCorImgView = [[UIImageView alloc] initWithFrame:CGRectMake(margin + imgW + imgPadding + (imgW + imgPadding) * 2 * i, CGRectGetMinY(shadowImgView.frame), imgW, imgW)];
        [shadowCorImgView setImage:[[UIImage imageNamed:@"hero_1"] drawCornerInRect:imageView.bounds cornerRadius:imageView.bounds.size.width * 0.5]];
        shadowCorImgView.layer.shadowColor = [UIColor redColor].CGColor;
        shadowCorImgView.layer.shadowOpacity = 0.8f;
        shadowCorImgView.layer.shadowOffset = CGSizeMake(0, 0);
        shadowCorImgView.layer.shadowRadius = 5.f;
        if (i == 1) {
            UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:shadowCorImgView.bounds cornerRadius:shadowCorImgView.bounds.size.height * 0.5];
            shadowCorImgView.layer.shadowPath = path.CGPath;
        }
        [self.view addSubview:shadowCorImgView];
    }
}

@end

UIImage分类，UIImage+HWCorner：

#import 

@interface UIImage (HWCorner)

//绘制图片圆角
- (UIImage *)drawCornerInRect:(CGRect)rect cornerRadius:(CGFloat)cornerRadius;

@end

/*** ---------------分割线--------------- ***/  

#import "UIImage+HWCorner.h"

@implementation UIImage (HWCorner)

//绘制图片圆角
- (UIImage *)drawCornerInRect:(CGRect)rect cornerRadius:(CGFloat)cornerRadius
{
    UIBezierPath *bezierPath = [UIBezierPath bezierPathWithRoundedRect:rect cornerRadius:cornerRadius];
    UIGraphicsBeginImageContextWithOptions(rect.size, false, [UIScreen mainScreen].scale);
    CGContextAddPath(UIGraphicsGetCurrentContext(), bezierPath.CGPath);
    CGContextClip(UIGraphicsGetCurrentContext());
    [self drawInRect:rect];
    
    CGContextDrawPath(UIGraphicsGetCurrentContext(), kCGPathFillStroke);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return image;
}

@end

最后说一下，只有在大量的离屏渲染情况时才会出现卡顿、掉帧等现象，没必要过分追求容不下一丝黄色。性能上Instruments工具还可以检测很多。写博客的初心是希望大家共同交流成长，博主水平有限难免有偏颇之处，欢迎批评指正。