使用代码给View添加约束

    _scrollContentView = [[UIView alloc] init];
    _scrollContentView.backgroundColor = [UIColor redColor];
    [_scrollView addSubview:_scrollContentView];
    
    _scrollContentView.translatesAutoresizingMaskIntoConstraints = NO;
    
    NSLayoutConstraint * scrollContentLeftCT = [NSLayoutConstraint constraintWithItem:_scrollContentView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:_scrollContentView.superview attribute:NSLayoutAttributeLeft multiplier:1.0 constant:0];
    
    NSLayoutConstraint * scrollContentTopCT = [NSLayoutConstraint constraintWithItem:_scrollContentView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:_scrollContentView.superview attribute:NSLayoutAttributeTop multiplier:1.0 constant:0];
    
    NSLayoutConstraint * scrollContentRightCT = [NSLayoutConstraint constraintWithItem:_scrollContentView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:_scrollContentView.superview attribute:NSLayoutAttributeRight multiplier:1.0 constant:0];
    
    NSLayoutConstraint * scrollContentbottomCT = [NSLayoutConstraint constraintWithItem:_scrollContentView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:_scrollContentView.superview attribute:NSLayoutAttributeBottom multiplier:1.0 constant:0];
    
    NSLayoutConstraint * scrollContentVerticallCT = [NSLayoutConstraint constraintWithItem:_scrollContentView attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:_scrollContentView.superview attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0];
    
    NSLayoutConstraint * scrollContentWidthCT = [NSLayoutConstraint constraintWithItem:_scrollContentView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeWidth multiplier:1.0 constant:WIDTH];
    
    [_scrollContentView.superview addConstraints:@[scrollContentLeftCT,scrollContentTopCT,scrollContentRightCT,scrollContentbottomCT,scrollContentVerticallCT]];
    [_scrollContentView.superview.superview addConstraint:scrollContentWidthCT];

 用代码写约束很麻烦，而且很容易出错，而且容易显示不出来，原因可能是少了一句话

    _scrollContentView.translatesAutoresizingMaskIntoConstraints = NO;

 如果写了这句话还不能显示出来，就把superView也添加以下这个属性

关于height，和Width这两个属性，第二个item应该为空，该约束应该添加到自己身上，而不是superView身上

关于bottom这个约束，如果想要与superView的底部有一点空隙，constant需要写成负数