先抛出个人总结的有关圆角触发离屏渲染的一个结论背景和结论：

结论背景：

只考虑cornerRadius、masksToBounds、backgroundColor、layer.bacgroundColor、text、border这六个组合因素，不考虑阴影、光栅化等因素

结论：

masksToBounds=true和cornerRadius同时设置不一定会触发离屛渲染；masksToBounds=no一定不会触发离屛渲染（当然要结合结论背景）

masksToBounds和cornerRadius同时设置不一定会触发离屛渲染

首先打开模拟器的 Debug - Color Off-screen Rendered 工具来检测是否触发离屛渲染，控件标黄即为触发了离屏渲染；

iOS系统在定义圆角触发离屏渲染的时机是分不同控件的，以下是6种同时设置了masksToBounds和cornerRadius属性但并不会触发离屏渲染的case：

一、UIView无绘制元素

无绘制元素：

TestView *view = [[TestView alloc] initWithFrame:CGRectMake(100, 100, 30, 30)];
 view.layer.backgroundColor = UIColor.blueColor.CGColor;
 view.layer.borderWidth = 1.5;
 view.layer.masksToBounds = YES;
 view.layer.cornerRadius = 20;
 [self.view addSubview:view];

drawBefore.png

有绘制元素：
在TestView类中实现drawLayer: inContext:方法并调用[layer setNeedsDisplay]进行绘制后：

- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx {
    CGContextSetLineWidth(ctx, 10.f);
    CGContextSetStrokeColorWithColor(ctx, [UIColor blueColor].CGColor);
    CGContextStrokeEllipseInRect(ctx, CGRectMake(10, 10, 30, 30));
}

drawAfter.png

二、UILabel无text

无text时：

UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(100, 200, 200, 20)];
label.layer.backgroundColor = UIColor.cyanColor.CGColor;
label.layer.borderColor = [UIColor blackColor].CGColor;
label.layer.borderWidth = 1.5;
//    label.text = @"off screen render test";
label.textColor = [UIColor grayColor];
label.layer.masksToBounds = YES;
label.layer.cornerRadius = 20;
[self.view addSubview:label];

nullText

三、UILabel的backgroundColor=clearColor且无border

    UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(100, 200, 200, 20)];
//    label.layer.backgroundColor = UIColor.cyanColor.CGColor;
//    label.layer.borderColor = [UIColor blackColor].CGColor;
//    label.layer.borderWidth = 1.5;
    label.text = @"off screen render test";
    label.textColor = [UIColor grayColor];
    label.layer.masksToBounds = YES;
    label.layer.cornerRadius = 20;
    [self.view addSubview:label];

clearColorAndNullBorder

四、设置label.backgroundColor = 非clearColor ，且不设置 label.layer.backgroundColor，且无border

    UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(100, 200, 200, 20)];
    label.backgroundColor = UIColor.cyanColor;
//    label.layer.borderColor = [UIColor blackColor].CGColor;
//    label.layer.borderWidth = 1.5;
    label.text = @"off screen render test";
    label.textColor = [UIColor grayColor];
    label.layer.masksToBounds = YES;
    label.layer.cornerRadius = 20;
    [self.view addSubview:label];

屏幕快照 2019-08-20 17.51.19.png

五、UIImageview无backgroundColor且无border

UIImageView *imgView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"jiayou"]];
//    imgView.backgroundColor = [UIColor cyanColor];
//    imgView.layer.backgroundColor = [UIColor cyanColor].CGColor;
//    imgView.layer.borderWidth = 1.5;
    imgView.frame = CGRectMake(100, 300, 200, 100);
    imgView.contentMode = UIViewContentModeScaleAspectFit;
    imgView.layer.masksToBounds = YES;
    imgView.layer.cornerRadius = 20;
    [self.view addSubview:imgView];

屏幕快照 2019-08-20 17.52.56.png

六、UIButton未设置titleText且未设置image

    UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
//    [btn setTitle:@"button" forState:UIControlStateNormal];
//    [btn setImage:[UIImage imageNamed:@"jiayou"] forState:UIControlStateNormal];
    btn.layer.borderWidth = 1.5;
    btn.layer.borderColor = [UIColor redColor].CGColor;
    btn.backgroundColor = [UIColor cyanColor];
    btn.frame = CGRectMake(100, 100, 50, 50);
    btn.layer.masksToBounds = YES;
    btn.layer.cornerRadius = 20;
    [self.view addSubview:btn];

屏幕快照 2019-08-20 17.54.03.png

tips（要基于结论背景）

设置了圆角且能达到圆角效果的一些tips：

UILabel 可以不设置bgcolor且设置layer.backgroundColor = UIColor.xxxColor.CGColor 且 layer.masksToBounds = NO 避免离屛渲染
UILabel 可以不设置border 且不设置bgcolor和layerBgColor 且 layer.masksToBounds = YES 避免离屛渲染
UIImageView 设置 layer.masksToBounds = NO无法切除边角；可以不设置border 且不设置bgcolor和layerBgColor 且设置layer.masksToBounds = YES 避免离屛渲染
UIButton 设置 layer.masksToBounds = NO无法切除边角；并且设置label.layer.masksToBounds = YES 也不能避免离屛渲染

如有出入，欢迎各位指正~

iOS 圆角+masksToBounds一定触发离屛渲染吗？