iOS 控件設置圓角，避免離屏渲染。
離屏渲染：指GPU（圖形處理器）在當前屏幕緩衝區外新開闢一個渲染緩衝區進行工作。這會給我們帶來額外的性能損耗，如果這樣的操作達到一定數量，會觸發緩衝區的頻繁合併和上下文的的頻繁切換，會出現卡頓、掉幀現象。造成離屏渲染的原因有很多，如：shouldRasterize（光柵化）、mask（遮罩層）、shadows（陰影）、EdgeAnntialiasing（抗鋸齒）、cornerRadius（圓角）等等。
下面說一下什麼情況下設置圓角會造成離屏渲染：
//設置cornerRadius>0且masksToBounds爲YES
view.layer.cornerRadius = 10.f;
view.layer.masksToBounds = YES;
//設置cornerRadius>0且masksToBounds爲YES
view.layer.cornerRadius = 10.f;
view.clipToBounds = YES;
//像下面設置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;上面第一種應該是最常用的，是在設置了圓角及masksToBounds爲YES時，纔會觸發離屏渲染，而masksToBounds默認是NO，也就是說只要不設置這個屬性就能避免很多情況了，下面說一下如何切一個不觸發離屏渲染的圓角：
UIView、UITextField、UITextView等大部分控件都可以像下面這樣設置：
view.layer.cornerRadius = 10.f;
view.layer.masksToBounds = NO;有一些特殊情況，UILabel設置時，不要設置label.backgroundColor，應設置：
label.layer.cornerRadius = 10.f;
label.layer.backgroundColor = [UIColor whiteColor].CGColor;
有圖片的UIButton、UIIMageView，用drawInRect繪製UIImage圓角：
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();查看離屏渲染，模擬器可以選中“Debug - Color Off-screen Rendered”開啓調試，真機可以用Instruments檢測，“Instruments - Core Animation - Debug Options - Color Offscreen-Rendered Yellow”開啓調試，開啓後，有離屏渲染的圖層會變成高亮的黃色。
寫了個離屏渲染的樣例：
下面貼上代碼：
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];
//創建控件
[self creatControl];
}
- (void)creatControl
{
CGFloat margin = 20.f;
CGFloat controlW = (KMainW - margin * 3) * 0.5;
NSArray *titleArray = @[@"離屏渲染", @"非離屏渲染"];
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，設置view.layer.masksToBounds = YES;也不會造成離屏渲染
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) {
/*
這裏換了一種實現方法，用UIBezierPath賦值layer.mask，兩種方式都會造成離屏渲染
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設置圓角
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，設置imageView.layer.masksToBounds = YES;也不會造成離屏渲染
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設置陰影
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設置陰影+圓角
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];
}
}
@endUIImage分類，UIImage+HWCorner：
#import <UIKit/UIKit.h>
@interface UIImage (HWCorner)
//繪製圖片圓角
- (UIImage *)drawCornerInRect:(CGRect)rect cornerRadius:(CGFloat)cornerRadius;
@end
/*** ---------------分割線--------------- ***/
#import "UIImage+HWCorner.h"
@implementation UIImage (HWCorner)
//繪製圖片圓角
- (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最後說一下，只有在大量的離屏渲染情況時纔會出現卡頓、掉幀等現象，沒必要過分追求容不下一絲黃色。性能上Instruments工具還可以檢測很多。寫博客的初心是希望大家共同交流成長，博主水平有限難免有偏頗之處，歡迎批評指正。