在自动布局中将子视图的 X 居中会引发“未准备好约束”

我有一个自定义 UIView 子类，它是通过笔尖初始化的。

In -awakeFromNib，我正在创建一个子视图并尝试将其置于其超级视图的中心。

[self setInteralView: [[UIView alloc] init]];
[[self internalView] addConstraint: [NSLayoutConstraint constraintWithItem: [self internalView]
                                                                 attribute: NSLayoutAttributeCenterX
                                                                 relatedBy: NSLayoutRelationEqual
                                                                    toItem: self
                                                                 attribute: NSLayoutAttributeCenterX
                                                                multiplier: 1
                                                                  constant: 0]];

这会中断并导致以下输出：

2013-08-11 17:58:29.628 MyApp[32414:a0b] The view hierarchy is not prepared for the constraint: <NSLayoutConstraint:0xc1dcc80 UIView:0xc132a40.centerX == MyView:0xc1315a0.centerX>
    When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. Break on -[UIView _viewHierarchyUnpreparedForConstraint:] to debug.
2013-08-11 17:58:29.630 MyApp[32414:a0b] View hierarchy unprepared for constraint.
    Constraint: <NSLayoutConstraint:0xc1dcc80 UIView:0xc132a40.centerX == MyView:0xc1315a0.centerX>
    Container hierarchy: 
<UIView: 0xc132a40; frame = (0 0; 0 0); clipsToBounds = YES; layer = <CALayer: 0xc132bc0>>
    View not found in container hierarchy: <MyView: 0xc1315a0; frame = (-128 -118; 576 804); layer = <CALayer: 0xc131710>>
    That view's superview: <UIView: 0xc131a70; frame = (0 0; 320 568); autoresize = W+H; layer = <CALayer: 0xc131b50>>

对于来到这里的其他人：我收到此错误，因为我在将子视图添加到层​​次结构之前添加了约束。