iOS设置圆角阴影 避免离屏渲染

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 <UIKit/UIKit.h>

@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。

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