为什么我无法使用“自动布局”使用渐变子图层来放置视图？ 没有出现

Question

I have the following code to create a view with a gradient background (using a sub layer) and then add it as a 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]];

However, when I run it, nothing shows up. If I set it up with a frame instead of Auto Layout, it does work. Why am I unable to do this with Auto Layout?

Answer 1

CALayer s don't follow Auto Layout, only UIKit views. Your gradient layer is begin added, but it probably just has a zero size frame. Best bet is subclass UIView and use +[UIView layerClass] to return a CAGradientLayer , then it will automatically match the size of the view under Auto Layout, otherwise you'll need to position your layers manually using something like layoutSubviews .

Answer 2

When configuring the height and width constraints, pass nil for the toItem: parameter and 0 for the 2nd attribute. I noticed that you set the constants to 0, which means your gradient view will have no size.

Also, the height and width constraints should be added to the gradientView , not the main view.