以编程方式添加约束时，在UIView中启用AutoLayout

Question

I have a UIView. It is nothing fancy. The code is below. The problem is that when I run the app, updateConstraints is never called, so the constraints don't do anything.

I am used to doing everything by computing the coordinates manually. The containing view does exactly that. So I may be missing some piece I need to turn the constraints on. What could it be?

@implementation Subview

-(void) createFooButton {
    UIButton* foo = [[UIButton alloc]init];
    self.fooButton = foo;
    [foo setTitle:@"foo" forState:UIControlStateNormal];
    [foo sizeToFit];
    [self addSubview:foo];
    UIColor* green = [UIColor greenColor];
    foo.translatesAutoresizingMaskIntoConstraints = NO;
    [foo setBackgroundColor:green];
}

-(void) createBarButton {
    UIButton* bar = [[UIButton alloc]init];
    [bar sizeToFit];
    self.barButton = bar;
    [bar setTitle:@"bar" forState:UIControlStateNormal];
    bar.translatesAutoresizingMaskIntoConstraints = NO;
    [self addSubview:bar];
    UIColor* red = [UIColor redColor];
    [bar setBackgroundColor:red];

}

-(void) addLayoutConstraints {
    UIButton* foo = self.fooButton;
    UIButton* bar = self.barButton;
    NSLayoutConstraint* constraint = [NSLayoutConstraint constraintWithItem:foo attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self attribute:NSLayoutAttributeLeft multiplier:1 constant:100];
    NSLayoutConstraint* constraint2 = [NSLayoutConstraint constraintWithItem:bar attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:foo attribute:NSLayoutAttributeBottom multiplier:1 constant:20];
    NSArray* array = @[constraint, constraint2];
    [self addConstraints: array];
}

-(void) updateConstraints {
    [self addLayoutConstraints];
    [super updateConstraints];
}

- (id)initWithFrame:(CGRect)frame  {
    if ((self = [super initWithFrame:frame])) {
        self.backgroundColor = [UIColor cyanColor];
        [self createFooButton];
        [self createBarButton];
        self.autoresizesSubviews = false;
        self.translatesAutoresizingMaskIntoConstraints = NO;
    }
    return self;
}

@end

Answer 1

You can force the system to use auto layout by adding this to SubView :

+ (BOOL)requiresConstraintBasedLayout {
    return YES;
}

Notice that this is a class method, not an instance method.