StackOverflow2013

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copied!<p>A couple of observations:</p> <ol> <li><p>Your constraint references a <code>toItem</code> of <code>self.view.superview</code>. I assume you meant <code>self.view</code>.</p></li> <li><p>You're adding the constraint to <code>_preView</code>, but you should add it to <code>self.view</code> (if you make the above change; if not, you'd use <code>self.view.superview</code>). You always add the constraint to the nearest shared parent.</p></li> <li><p>For the views you're creating programmatically, make sure to set <code>translatesAutoresizingMaskIntoConstraints</code> to <code>NO</code>.</p> <p>Thus:</p> <pre><code>_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]; </code></pre></li> </ol> <hr> <p>Chatting to you offline, two final observations:</p> <ol> <li><p>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 (<img src="https://i.stack.imgur.com/MZmky.png" alt="enter image description here">) and then at the <code>(lldb)</code> prompt, you can enter</p> <p><code>po [[UIWindow keyWindow] _autolayoutTrace]</code></p> <p><img src="https://i.stack.imgur.com/HrfIE.png" alt="ambiguous layout"></p> <p>If you see <code>AMBIGUOUS LAYOUT</code>, 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.</p></li> <li><p>If you want to animate constraint based views, you animate the changing of <code>constant</code> properties of the <code>constraints</code>, not by changing <code>frame</code> properties yourself. For example:</p> <pre><code> // 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]; }]; }); </code></pre></li> </ol>

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