#import <Masonry/Masonry.h>


@implementation ViewController {
    UIView * _v;
    UILabel * _label;
    
    bool _constraintsAdded;
}

- (void)viewDidLoad {
    [super viewDidLoad];

    UILabel *l = [UILabel new];
    l.text = @"我能优先显示吗？";
    l.tintColor = [UIColor magentaColor];
    l.frame = CGRectMake(100, 50, 160, 20);
    [self.view addSubview:l];

    _v = UIView.new;
    _v.backgroundColor = [UIColor greenColor];
    [self.view addSubview:_v];

    _label = UILabel.new;
    _label.tintColor = [UIColor redColor];
    _label.text = @"hey";
    [_v addSubview:_label];

    // Do NOT add any constraint at here, or else `updateViewController` will be called twice.
    
    // IMPORTANT: 否则updateViewConstraints不被执行
    self.view.translatesAutoresizingMaskIntoConstraints = false;
    //[self.view setNeedsUpdateConstraints]; // another method, but it trigger -[UIViewController updateViewConstraints] executed twice
    
    //[self.view setNeedsLayout]; // useless code
}

- (void)updateViewConstraints {
    if (!_constraintsAdded) {
        _constraintsAdded = true;
        
        [_v mas_makeConstraints:^(MASConstraintMaker *make) {
            make.edges.equalTo(self.view).insets(UIEdgeInsetsMake(100, 50, 200, 30));
        }];

        [_label mas_makeConstraints:^(MASConstraintMaker *make) {
            make.center.equalTo(_v);
        }];
    }

    NSLog(@"update constraint.");
    [super updateViewConstraints];
}


@end