adding NSLayoutConstraint makes views disappear - ios

Question

I have a simple view (controller) with two subviews:
- (void)viewDidLoad {
[super viewDidLoad];
UIView *viewA = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 50, 50)];
viewA.translatesAutoresizingMaskIntoConstraints = NO;
viewA.backgroundColor = [UIColor redColor];
[self.view addSubview:viewA];
UIView *viewB = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 50, 50)];
viewB.translatesAutoresizingMaskIntoConstraints = NO;
viewB.backgroundColor = [UIColor grayColor];
[self.view addSubview:viewB];
}
Of course they are overlapping at this point (i can only see viewB). So I add a constraint to make viewB go below viewA, like this:
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:viewB
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:viewA
attribute:NSLayoutAttributeBottom
multiplier:1.f
constant:0]];
This makes both views disappear. What am I doing wrong?
Thanks!

Answer 1

When using constraints, you should do everything with constraints (no setting of frames), so adding one constraint isn't enough to define you positions and sizes. I'm not sure why adding the one constraint negated the sizes you set for the views, but it seems that it does. You should do something like this to fully describe the views,
- (void)viewDidLoad {
[super viewDidLoad];
UIView *viewA = [UIView new];
viewA.translatesAutoresizingMaskIntoConstraints = NO;
viewA.backgroundColor = [UIColor redColor];
[self.view addSubview:viewA];
UIView *viewB = [UIView new];
viewB.translatesAutoresizingMaskIntoConstraints = NO;
viewB.backgroundColor = [UIColor grayColor];
[self.view addSubview:viewB];
NSDictionary *dict = NSDictionaryOfVariableBindings(viewA,viewB);
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"|[viewA(==50)]" options:0 metrics:nil views:dict]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"[viewB(==50)]" options:0 metrics:nil views:dict]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"V:|[viewA(==50)][viewB(==50)]" options:NSLayoutFormatAlignAllLeft metrics:nil views:dict]];
}