以编程方式添加的自动布局视图未显示

Question

I have a view controller with a table (defined through Interface Builder). I am trying to programmatically add a button 18px from the bottom of the table.

I have following in loadView:

- (void)loadView {
[super loadView];
UIButton *testButton = [UIButton buttonWithType:UIButtonTypeSystem];
testButton.titleLabel.font = [UIFont fontWithName:@"System" size:13];
[testButton setTitle:@"Logout" forState:UIControlStateNormal];
[testButton setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.view addSubview:testButton];
self.testButton = testButton;
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:testButton attribute:NSLayoutAttributeWidth 
                                                      relatedBy:NSLayoutRelationEqual 
                                                      toItem:self.twitterAuthButton 
                                                      attribute:NSLayoutAttributeWidth 
                                                      multiplier:1 constant:0]];
NSLayoutConstraint *centerXConstraint = [NSLayoutConstraint constraintWithItem:testButton 
                                                                     attribute:NSLayoutAttributeCenterX 
                                                                     relatedBy:NSLayoutRelationEqual 
                                                                     toItem:self.view 
                                                                     attribute:NSLayoutAttributeCenterX 
                                                                     multiplier:1.0 constant:0.0];
NSLayoutConstraint *alignToBottomOfTable = [NSLayoutConstraint constraintWithItem:testButton 
                                                                        attribute:NSLayoutAttributeTop 
                                                                        relatedBy:NSLayoutRelationEqual 
                                                                        toItem:availableStreamsTableView 
                                                                        attribute:NSLayoutAttributeBottom 
                                                                        multiplier:1 constant:18];
[self.view addConstraint:centerXConstraint];
[self.view addConstraint:alignToBottomOfTable];

}

But the button is not displaying. Not sure why. When I debug (in AppCode), it seems to be there and at the right x and y:

Any idea why the button is not showing?

Answer 1

Ah, I think I see it. You want a negative constant for your alignToBottomOfTable constraint. The positive 18 constant is pushing your button below the bottom of your table.

NSLayoutConstraint *alignToBottomOfTable = [NSLayoutConstraint constraintWithItem:testButton
                                                                        attribute:NSLayoutAttributeTop
                                                                        relatedBy:NSLayoutRelationEqual
                                                                           toItem:availableStreamsTableView
                                                                        attribute:NSLayoutAttributeBottom
                                                                       multiplier:1 constant:-18];