当视图位于视图层次结构的顶部时，使用自动布局以编程方式在屏幕上居中放置UIView

Question

I have implemented a popup UIView which I add to the topmost window via [[[[UIApplication sharedApplication] windows] lastObject] addSubview:popupView] , so it will appear on top of everything even the keyboard. I need to ensure this popup view that I programmatically created will always remain centered on screen. I was attempting to add auto layout constraints, but it doesn't like the fact I'm trying to align with the topmost window. Could you let me know how I could accomplish this? Thank you.

This is what I have implemented, which will generate a (nicely detailed) error that states 'The view hierarchy is not prepared for the constraint ... the constraint's items must be descendants of that view':

    [popupView setTranslatesAutoresizingMaskIntoConstraints:NO];
    NSLayoutConstraint *cn = nil;
    cn = [NSLayoutConstraint constraintWithItem:popupView
                                      attribute:NSLayoutAttributeCenterX
                                      relatedBy:NSLayoutRelationEqual
                                         toItem:[[[UIApplication sharedApplication] windows] lastObject]
                                      attribute:NSLayoutAttributeCenterX
                                     multiplier:1.0
                                       constant:0];
    [popupView addConstraint:cn];
    cn = [NSLayoutConstraint constraintWithItem:popupView
                                      attribute:NSLayoutAttributeCenterY
                                      relatedBy:NSLayoutRelationEqual
                                         toItem:[[[UIApplication sharedApplication] windows] lastObject]
                                      attribute:NSLayoutAttributeCenterY
                                     multiplier:1.0
                                       constant:0];
    [popupView addConstraint:cn];
    cn = [NSLayoutConstraint constraintWithItem:popupView
                                      attribute:NSLayoutAttributeHeight
                                      relatedBy:NSLayoutRelationEqual
                                         toItem:nil
                                      attribute:NSLayoutAttributeNotAnAttribute
                                     multiplier:1
                                       constant:blockSize.height];
    [popupView addConstraint:cn];
    cn = [NSLayoutConstraint constraintWithItem:popupView
                                      attribute:NSLayoutAttributeWidth
                                      relatedBy:NSLayoutRelationEqual
                                         toItem:nil
                                      attribute:NSLayoutAttributeNotAnAttribute
                                     multiplier:1
                                       constant:blockSize.width];
    [popupView addConstraint: cn];

Answer 1

Don't go sticking your view into a window that wasn't set up specifically to hold your views. You don't know what the real owner of that window might do, now or in a future version of iOS.

Create your own window and set its windowLevel high enough to be above the keyboard. Give the new window its own root view controller (so it will handle different orientations properly) and center your popup view inside that root view controller's view.

You will find lots of useful information in the answers to this question .

Answer 2

将高度和宽度约束添加到popupView是可以的，但是由于popupView是窗口的子视图，因此应该将居中约束添加到窗口，而不是popupView（并且您需要先将popupView作为子视图添加，然后再添加添加约束）。