Autolayout when adding subview to UIWindow - objective-c

Question

I am creating a customalertview that needs to be added as a subview to UIWindow. I've setup the constraints for it's subviews correctly but I'm confused as to when/how to set the constraints on the view itself in relation to the window.
I would like the width of the alertview to be 70% of the width of the screen. The height is already in relation to it's subviews.
- (void)show {
UIWindow *window = [[UIApplication sharedApplication] keyWindow];
[window addSubview:self];
[self addConstraint:[NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:[UIApplication sharedApplication].keyWindow attribute:NSLayoutAttributeWidth multiplier:0.7 constant:0]];
[self addConstraint:[NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:_messageLabel attribute:NSLayoutAttributeBottom multiplier:1.0 constant:10]];
[self addConstraint:[NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:(_imageView.image ? _imageView : _titleLabel) attribute:NSLayoutAttributeTop multiplier:1.0 constant:-10]];
}
What am I doing wrong? I am getting the following error message:
NSLayoutConstraint:0x181cd7c0 WFSoftAlertView:0x16e384c0.width == 0.7*UIWindow:0x16e7c8c0.width>
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.

Answer 1

WFSoftAlertView.translatesAutoresizingMaskIntoConstraints=NO;
NSDictionary *views = NSDictionaryOfVariableBindings(WFSoftAlertView);
float width30 = self.view.frame.size.width*.3;
width30 = width30/2; // divided by 2 for padding of its left/rigth
NSString *str =[NSString stringWithFormat:#"%f",width30];
NSDictionary *metrics = #{#"width":str};
[self.view addSubview:WFSoftAlertView];
//add the WFSoftAlertView in center(x) of superview with a padding of "width"
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"|-width-[WFSoftAlertView]-width-|" options:NSLayoutFormatAlignAllTop | NSLayoutFormatAlignAllBottom metrics:metrics views:views]];
// if you want center(y)
NSLayoutConstraint *centerY = [NSLayoutConstraint
constraintWithItem:WFSoftAlertView
attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeCenterY
multiplier:1.0f
constant:0.f];

Answer 2

Try changing your constraintWithItem from
self
to
self.view