需要帮助使用自动布局在中心设置 UIButton

Question

我正在尝试使用 AutoLayouts 以编程方式在中心垂直和水平设置 UIButton 这是我的代码：

- (void)awakeFromNib {
UIView *av = [[UIView alloc]initWithFrame:CGRectMake(0, 0, self.bounds.size.width, 35.0)];

UIButton *doneButton = [[UIButton alloc]initWithFrame:CGRectMake(0, 0, 37, 30)];

//UIButton title, color, backgroung color, targer action ...

NSLayoutConstraint* cn3 = [NSLayoutConstraint constraintWithItem:doneButton
                                                          attribute:NSLayoutAttributeCenterX
                                                          relatedBy:NSLayoutRelationEqual
                                                             toItem:av
                                                          attribute:NSLayoutAttributeCenterX
                                                         multiplier:1.0
                                                           constant:0];
    [av addConstraint:cn3];

    NSLayoutConstraint* cn4 = [NSLayoutConstraint constraintWithItem:doneButton
                                      attribute:NSLayoutAttributeCenterY
                                      relatedBy:NSLayoutRelationEqual
                                         toItem:av
                                      attribute:NSLayoutAttributeCenterY
                                     multiplier:1.0
                                       constant:0];
    [av addConstraint:cn4];

[av addSubview:doneButton];
    self.theTextField.inputAccessoryView = av;
}

除了一件事，上面的代码工作正常，我在控制台中收到此警告：

The view hierarchy is not prepared for the constraint: NSLayoutConstraint:0x7fcfc494efb0 UIButton:0x7fcfc2624420'Done'.centerX == UIView:0x7fcfc2624230.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(UIConstraintBasedLayout) _viewHierarchyUnpreparedForConstraint:] to debug.

Y 得到同样的警告。

请告诉我出现此警告的原因是什么？

Answer 1

在 设置约束之前，将 doneButton 添加到 av 视图 。

[av addSubview:doneButton];

[av addConstraint:cn3];
[av addConstraint:cn4];