ios - 如何制作一个 UIImageView，以编程方式在具有约束的容器中垂直居中

Question

我想在容器 View (self.view) 中居中 UIImageView (self.showImageView)

我试过下面的代码，但没有用:

NSLayoutConstraint *xCenterConstraint = [NSLayoutConstraint constraintWithItem:self.showImageView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0];
[self.view addConstraint:xCenterConstraint];

NSLayoutConstraint *yCenterConstraint = [NSLayoutConstraint constraintWithItem:self.showImageView attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0];
[self.view addConstraint:yCenterConstraint];

Answer 1

试试这个:

- (void)viewDidLoad
{
    [super viewDidLoad];
    UIView *redView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 200, 100)];
    redView.backgroundColor = [UIColor redColor];
    redView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:redView];
    [redView addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:0 multiplier:1 constant:100]];
    [redView addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:0 multiplier:1 constant:200]];

    [self.view addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1 constant:0]];
    [self.view addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1 constant:0]];
    [self.view addSubview:redView];
}