添加NSLayoutConstraint使视图消失

0xSina 发表于 Dev

0x新浪

我有一个带有两个子视图的简单视图（控制器）：

- (void)viewDidLoad {
    [super viewDidLoad];

    UIView *viewA = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 50, 50)];
    viewA.translatesAutoresizingMaskIntoConstraints = NO;
    viewA.backgroundColor = [UIColor redColor];
    [self.view addSubview:viewA];


    UIView *viewB = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 50, 50)];
    viewB.translatesAutoresizingMaskIntoConstraints = NO;
    viewB.backgroundColor = [UIColor grayColor];
    [self.view addSubview:viewB];
}

当然，它们在这一点上是重叠的（我只能看到viewB）。因此，我添加了一个约束以使viewB低于viewA，如下所示：

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:viewB
                                                      attribute:NSLayoutAttributeTop
                                                      relatedBy:NSLayoutRelationEqual
                                                         toItem:viewA
                                                      attribute:NSLayoutAttributeBottom
                                                     multiplier:1.f
                                                       constant:0]];

这使两个视图都消失了。我究竟做错了什么？

谢谢！

德尔玛

使用约束时，应该对约束进行所有操作（不设置框架），因此仅添加一个约束不足以定义位置和尺寸。我不确定为什么添加一个约束会否定您为视图设置的大小，但似乎确实如此。您应该执行以下操作来充分描述视图，

- (void)viewDidLoad {
    [super viewDidLoad];

    UIView *viewA = [UIView new];
    viewA.translatesAutoresizingMaskIntoConstraints = NO;
    viewA.backgroundColor = [UIColor redColor];
    [self.view addSubview:viewA];


    UIView *viewB = [UIView new];
    viewB.translatesAutoresizingMaskIntoConstraints = NO;
    viewB.backgroundColor = [UIColor grayColor];
    [self.view addSubview:viewB];
    NSDictionary *dict = NSDictionaryOfVariableBindings(viewA,viewB);

    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"|[viewA(==50)]" options:0 metrics:nil views:dict]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"[viewB(==50)]" options:0 metrics:nil views:dict]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[viewA(==50)][viewB(==50)]" options:NSLayoutFormatAlignAllLeft metrics:nil views:dict]];
}