Need assistance setting UIButton in centre using Autolayouts - ios

Question

I am trying to programmatically set the UIButton vertically and horizontally in centre using AutoLayouts here is my code:
- (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;
}
The above code is working fine except one thing, I am getting this warning in console:
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.
And getting same warning for Y.
Please tell me what is going wrong why I am getting this warning?

Answer 1

Add doneButton to the av view before setting your constraints.
[av addSubview:doneButton];
[av addConstraint:cn3];
[av addConstraint:cn4];