如何以编程方式添加标签并以编程方式使用自动布局定位

Question

我正在尝试为UIScrollView添加标签。我正在添加一个顶部约束和一个前导约束。 标签没有出现。

这是我的代码。

   self.label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 40, 40)];
    self.label.translatesAutoresizingMaskIntoConstraints = NO;
    self.label.backgroundColor = [UIColor clearColor];
    self.label.textAlignment = NSTextAlignmentCenter;
    self.label.textColor=[UIColor whiteColor];
    self.label.text = @"Hello";
    [self.imageCarouselScrollView addSubview:self.label];
    [self bringSubviewToFront:self.label];
    self.titleLabelTopConstraint = [NSLayoutConstraint
                                    constraintWithItem:self
                                    attribute:NSLayoutAttributeTop
                                    relatedBy:NSLayoutRelationEqual
                                    toItem:self.imageCarouselScrollView
                                    attribute:NSLayoutAttributeBottom
                                    multiplier:1.0
                                    constant:20];
    NSLayoutConstraint *titleLeadingConstraint = [NSLayoutConstraint
                                                  constraintWithItem:self
                                                  attribute:NSLayoutAttributeLeading
                                                  relatedBy:NSLayoutRelationEqual
                                                  toItem:self.imageCarouselScrollView
                                                  attribute:NSLayoutAttributeTrailing
                                                  multiplier:1.0
                                                  constant:20];
    [self addConstraints:@[self.titleLabelTopConstraint,titleLeadingConstraint]];

Answer 1

您必须在滚动视图的右侧（水平滚动）或底部（垂直滚动）中包含约束，以便它可以推断出内容大小。

Answer 2

我尝试评论你的代码：

    self.label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 40, 40)]; //if 'translatesAutoresizingMaskIntoConstraints=NO' frame not work work anymore. Use CGRectZero instead
    self.label.translatesAutoresizingMaskIntoConstraints = NO;
    self.label.backgroundColor = [UIColor clearColor];
    self.label.textAlignment = NSTextAlignmentCenter;
    self.label.textColor=[UIColor whiteColor];
    self.label.text = @"Hello";
    [self.imageCarouselScrollView addSubview:self.label]; //Your self.label added on 'self.imageCarouselScrollView' but your constraints added on self
    [self bringSubviewToFront:self.label]; // ??? Why?
    self.titleLabelTopConstraint = [NSLayoutConstraint
            constraintWithItem:self
                     attribute:NSLayoutAttributeTop
                     relatedBy:NSLayoutRelationEqual
                        toItem:self.imageCarouselScrollView
                     attribute:NSLayoutAttributeBottom
                    multiplier:1.0
                      constant:20]; //it constrain means: "self view top edge equal to self.imageCarouselScrollView bottom + offset 20 "
    NSLayoutConstraint *titleLeadingConstraint = [NSLayoutConstraint
            constraintWithItem:self
                     attribute:NSLayoutAttributeLeading
                     relatedBy:NSLayoutRelationEqual
                        toItem:self.imageCarouselScrollView
                     attribute:NSLayoutAttributeTrailing
                    multiplier:1.0
                      constant:20];//it constrain means: "self view Leading edge equal to self.imageCarouselScrollView Trailing + offset 20 "
    [self addConstraints:@[self.titleLabelTopConstraint,titleLeadingConstraint]];

现在我几乎没有问题：

1. 为什么要限制自我。标签？因为在translatesAutoresizingMaskIntoConstraints = no之后它的帧等于0;你再也看不到标签了。
2. 你有2个限制，其他边缘怎么样????
3. 毫无疑问，理事会，尝试使用约束与观点格式：选项：指标：观点：＆＃39;