为什么以编程方式创建的视图会忽略其约束？

我有一个视图，可以以编程方式创建一个子视图并为其添加一些约束。由于某些原因，这些约束已被完全忽略。没有引发任何错误。约束根本不会生效。子视图与父视图具有相同的框架。我希望它是400x400，并以其超级视图为中心。我在这里做错了什么？这是我在超级视图中的initWithFrame：

- (id) initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame])
    {
        instructionsView = [[UIView alloc] initWithFrame:frame];
        [self addSubview:instructionsView];
        [self bringSubviewToFront:instructionsView];

        [self addConstraint:[NSLayoutConstraint constraintWithItem:instructionsView
                                                         attribute:NSLayoutAttributeCenterX
                                                         relatedBy:NSLayoutRelationEqual
                                                            toItem:self
                                                         attribute:NSLayoutAttributeCenterX
                                                        multiplier:1.
                                                          constant:0]];

        [self addConstraint:[NSLayoutConstraint constraintWithItem:instructionsView
                                                         attribute:NSLayoutAttributeCenterY
                                                         relatedBy:NSLayoutRelationEqual
                                                            toItem:self
                                                         attribute:NSLayoutAttributeCenterY
                                                        multiplier:1.
                                                          constant:0]];

        [instructionsView addConstraint:[NSLayoutConstraint constraintWithItem:instructionsView
                                                                     attribute:NSLayoutAttributeWidth
                                                                     relatedBy:NSLayoutRelationEqual
                                                                        toItem:nil
                                                                     attribute:NSLayoutAttributeNotAnAttribute
                                                                    multiplier:1.
                                                                      constant:400]];

        [instructionsView addConstraint:[NSLayoutConstraint constraintWithItem:instructionsView
                                                                     attribute:NSLayoutAttributeHeight
                                                                     relatedBy:NSLayoutRelationEqual
                                                                        toItem:nil
                                                                     attribute:NSLayoutAttributeNotAnAttribute
                                                                    multiplier:1.
                                                                      constant:400]];
    }
    return self;
}

我尝试改变

instructionsView = [[UIView alloc] initWithFrame:frame];

至

instructionsView = [[UIView alloc] initWithFrame:CGRectZero];

没有成功。