更改NSLayoutConstraint在iOS7中不起作用

Question

I have this code in viewDidLoad

int height = 367 - self.vLayout.frame.size.height;
        int adjustHeight = height / 2;
        self.csPanelBottom.constant = adjustHeight - 2;
        if (height % 2 != 0) {
            adjustHeight += 1;
        }
        self.csPanelTop.constant = adjustHeight + 2;

which will adjust the panel to centre the screen. Everything works fine i get the correct adjustHeight value for both iOS6 and iOS7

but when i set the constant on iOS7 it is not updating. The layout still the same.

I don't know what will cause this because i have another pages which use the same piece of this code and that pages don't have a problem like this.

Any idea what can cause this problem and how can i solve it? Thanks.

Answer 1

Most probably, for the changes to populate you need to add

[self.view setNeedsLayout];
[self.view layoutIfNeeded];

However, it isn't the correct way to align the view to the centre. Correct way is adding a centring constraint, either in Interface Builder:

or in the code:

// Replace viewNeedingCenter and containerView with your actual views
[containerView addConstraint:
 [NSLayoutConstraint constraintWithItem:viewNeedingCenter
                              attribute:NSLayoutAttributeCenterY
                              relatedBy:NSLayoutRelationEqual
                                 toItem:containerView
                              attribute:NSLayoutAttributeCenterY
                             multiplier:1.0 constant:0]];