title: How to get constraints from UIView Programmatically
body: <p>I want get UILabel constraints from UIView but I can't get any constraints.
  I set the constraints in CustomView.m like this:</p>
  
  <pre><code>- (id)initWithFrame:(CGRect)frame {
       self = [super initWithFrame:frame];
       if (self) {
           _titleLabel = [UILabel new];
           [self addSubview:_titleLabel];
       }
       return self;
  }
  
  - (void)layoutSubviews {
       [super layoutSubviews];
       _titleLabel.translatesAutoresizingMaskIntoConstraints = NO;
       NSLayoutConstraint *titleLabelBottom = [NSLayoutConstraint constraintWithItem:_titleLabel
                                                attribute:NSLayoutAttributeBottom
                                                relatedBy:NSLayoutRelationEqual
                                                   toItem:self
                                                attribute:NSLayoutAttributeCenterY
                                               multiplier:1
                                                 constant:0];
       [self addConstraints:@[titleLabelBottom]];
  
       ...more code
  
  }
  </code></pre>
  
  <p>in ViewController.m</p>
  
  <pre><code> CustomView *view = [CustomView alloc] initWithFrame:viewFrame];
   NSLog(@"%@",view.titleLabel.constraints); // nil
  </code></pre>