Adding View Programatically With Auto Layout Gives 'NSGenericException', reason: 'Unable to install constraint on view

Question

I am adding a view as a subview using [self.view addSubview:myView]. This works fine in portrait mode. However, it doesn\'t work at all in landscape. How do I add l

Answer 1

A couple of observations:

1. Your constraint references a toItem of self.view.superview. I assume you meant self.view.

2. You're adding the constraint to _preView, but you should add it to self.view (if you make the above change; if not, you'd use self.view.superview). You always add the constraint to the nearest shared parent.

3. For the views you're creating programmatically, make sure to set translatesAutoresizingMaskIntoConstraints to NO.

   Thus:

   _preView.translatesAutoresizingMaskIntoConstraints = NO;
   [self.view addSubview:_preView];
   NSLayoutConstraint *myConstraint = [NSLayoutConstraint constraintWithItem:_preView
                                                                   attribute:NSLayoutAttributeBottom
                                                                   relatedBy:NSLayoutRelationEqual
                                                                      toItem:self.view
                                                                   attribute:NSLayoutAttributeBottom
                                                                  multiplier:1.0
                                                                    constant:-239];
   [self.view addConstraint:myConstraint];
   

---

Chatting to you offline, two final observations:

1. Your constraints were ambiguous. In the future, you can identify that by running the app in your debugger, hitting the pause button while the app is running () and then at the (lldb) prompt, you can enter

   po [[UIWindow keyWindow] _autolayoutTrace]

   

   If you see AMBIGUOUS LAYOUT, then your constraints are not fully qualified (and thus you'll get unpredictable behavior). If you add the missing constraints, you should be able to eliminate this warning.

2. If you want to animate constraint based views, you animate the changing of constant properties of the constraints, not by changing frame properties yourself. For example:

       // create subview
   
       UIView *subview = [[UIView alloc] init];
       subview.backgroundColor = [UIColor lightGrayColor];
       subview.translatesAutoresizingMaskIntoConstraints = NO;
       [self.view addSubview:subview];
   
       // create dictionary for VFL commands
   
       NSDictionary *views = @{@"subview" : subview, @"superview" : self.view};
   
       // add horizontal constraints
   
       [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[subview]|" options:0 metrics:nil views:views]];
   
       // set the height of the offscreen subview to be the same as its superview
   
       [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[subview(==superview)]" options:0 metrics:nil views:views]];
   
       // set the location of the subview to be just off screen below the current view
   
       NSLayoutConstraint *constraint = [NSLayoutConstraint constraintWithItem:subview attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:self.view.bounds.size.height];
       [self.view addConstraint:constraint];
   
       // then in two seconds, animate this subview back on-screen (i.e. change the top constraint `constant` to zero)
   
       double delayInSeconds = 2.0;
       dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
       dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
           constraint.constant = 0.0;
           [UIView animateWithDuration:1.0
                            animations:^{
                                [self.view layoutIfNeeded];
                            }];
       });