将 UIImageView 添加到屏幕中心

如果要将 UIImageView 添加到屏幕中心，宽度和高度为 100 像素，则需要将中心 x 约束和中心 y 约束设置为从 UIImageView 和宽度，高度约束到 UIIMageView 的超视图。这是代码。有可能在故事板中这样做，但我已经用代码编写，因为它可以理解文档中的这种情况。

UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 100.0, 100.0);
imageView.self.translatesAutoresizingMaskIntoConstraints = NO;

[imageView addConstraint:[NSLayoutConstraint constraintWithItem:imageView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:100.0]];
[imageView addConstraint:[NSLayoutConstraint constraintWithItem:imageView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:100.0]];
[superview addConstraint:[NSLayoutConstraint constraintWithItem:imageView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:superview attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0]];
[superview addConstraint:[NSLayoutConstraint constraintWithItem:imageView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:superview attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0]];