Autolayout in code ? height issue? - ios

Question

I have Two View say view1 and view2.
- view2 should be at 2 pixel distance from view1 (vertically) and
- Height of view2 should be such that bottom of view2 must be same as bottom of viewcontroller.
How to specify the constraint for this in code ?

Answer 1

Since I don't know what you already have in your view hierarchy I've set up a simple example. Look into this bit of code. It creates two new UIViews and position them using constraints.
- (void)viewDidLoad {
[super viewDidLoad];
UIView *view1 = [UIView new];
UIView *view2 = [UIView new];
view1.backgroundColor = [UIColor redColor];
view2.backgroundColor = [UIColor blueColor];
[self.view addSubview:view1];
[self.view addSubview:view2];
[view1 setTranslatesAutoresizingMaskIntoConstraints:NO];
[view2 setTranslatesAutoresizingMaskIntoConstraints:NO];
NSDictionary *views = NSDictionaryOfVariableBindings(view1, view2);
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"V:|-(100)-[view1(200)]-(2)-[view2]|" options:0 metrics:0 views:views]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"|[view1]|" options:0 metrics:0 views:views]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"|[view2]|" options:0 metrics:0 views:views]];
}
Here's the result:
Code is not that hard to understand. + constraintsWithVisualFormat: method creates an array of NSLayoutConstraints that you add to the superview of your custom views. Look here for more on visual format language. It's very effective at simple layout and saves a bunch of time compared to creating constraints one by one.
If you reeeaaally want to create constraints using constraintWithItem: method, here's your code:
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1 constant:100]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view2 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:view1 attribute:NSLayoutAttributeBottom multiplier:1 constant:2]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view2 attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1 constant:0]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeading multiplier:1 constant:0]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeTrailing relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTrailing multiplier:1 constant:0]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:view2 attribute:NSLayoutAttributeWidth multiplier:1 constant:0]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:200]];
It results in the same constraints, but takes up twice the space and is harder to read.So I strongly recommend you to use visual format instead. You should only use constraintWithItem: for more complicated layout which visual format can't always handle.