ios - 为什么我无法使用带有自动布局的渐变子层来定位我的 View ？它没有出现

Question

我有以下代码来创建一个具有渐变背景的 View (使用子图层)，然后将其添加为 subview :

// Add a gradient overlay to make caption more legible (add a little extra to the sides to make it continue over the spacing between view controllers)
UIView *gradientView = [[UIView alloc] init];
gradientView.translatesAutoresizingMaskIntoConstraints = NO;
gradientView.userInteractionEnabled = NO;

// Add the gradient effect
CAGradientLayer *gradient = [CAGradientLayer layer];
gradient.frame = gradientView.bounds;
gradient.colors = @[(id)[UIColor colorWithWhite:0.0 alpha:0.0].CGColor,
                    (id)[UIColor colorWithWhite:0.0 alpha:0.65].CGColor,
                    (id)[UIColor colorWithWhite:0.0 alpha:0.8].CGColor];
gradient.locations = @[@0.0, @0.77, @1.0];

[gradientView.layer insertSublayer:gradient atIndex:0];

[self.view addSubview:gradientView];

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:gradientView attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:0.0]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:gradientView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeWidth multiplier:1.0 constant:0.0]];

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:gradientView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeHeight multiplier:1.0 constant:0.0]];

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:gradientView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0 constant:0.0]];

但是，当我运行它时，什么也没有显示。如果我用框架而不是自动布局来设置它，它就可以工作。为什么我无法使用自动布局来执行此操作？

Answer 1

CALayer 不遵循自动布局，仅遵循 UIKit View 。您的渐变图层已开始添加，但它可能只有一个零尺寸的框架。最好的办法是子类 UIView 并使用 +[UIView layerClass] 返回一个 CAGradientLayer，然后它会自动匹配 Auto 下 View 的大小布局，否则您需要使用 layoutSubviews 之类的东西手动定位图层。