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,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,controlW - 60,40)];
subView.backgroundColor = [UIColor redColor];
[view addSubview:subView];
//UITextView
UITextView *textView = [[UITextView alloc] initWithFrame:CGRectMake(controlX,CGRectGetMaxY(view.frame) + margin,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)];
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)];
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(0,40)];
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)];
[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,[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。
