试试这个：

  NSLayoutConstraint * xCenterConstraint = [NSLayoutConstraint constraintWithItem：厂景属性：NSLayoutAttributeCenterX relatedBy：NSLayoutRelationEqual toItem：视图2属性：NSLayoutAttributeCenterX乘数：1.0常数：0];
[上海华addConstraint：xCenterConstraint];NSLayoutConstraint * yCenterConstraint = [NSLayoutConstraint constraintWithItem：厂景属性：NSLayoutAttributeCenterY relatedBy：NSLayoutRelationEqual toItem：视图2属性：NSLayoutAttributeCenterY乘数：1.0常数：0];
[上海华addConstraint：yCenterConstraint];
 

更新了斯威夫特：

  NSLayoutConstraint（项目：视图1，属性：.CenterX，relatedBy：.Equal，toItem：视图2，属性：.CenterX，事半​​功倍：1，恒：0）。主动=真NSLayoutConstraint（项目：视图1，属性：.CenterY，relatedBy：.Equal，toItem：视图2，属性：.CenterY，事半功倍：1，恒：0）。主动=真
 

In case of a successful Foursquare checkin, my iPhone app shows adds a view on top of the view which is shown.

I want the view to be centered on the X and Y and for it to have a definite width and height using autolayout, but I'm not sure which constraints to add programmatically. I know how to do it in Storyboard, but I'm not sure what exactly to type in code to do the same as this view is added later in response to a successful action in the app.

I can't get it working properly and the UIView has a nib which it loads with loadNibNamed:.

SuccessView *successfulCheckInView = [[SuccessView alloc] initWithFrame:CGRectZero];
successfulCheckInView.placeNameLabel.text = properVenue.name;
successfulCheckInView.placeAddressLabel.text = properVenue.address;
successfulCheckInView.delegate = self;

[self.ownerVC.view addSubview:successfulCheckInView];

Try this:

NSLayoutConstraint *xCenterConstraint = [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:view2 attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0];
[superview addConstraint:xCenterConstraint];

NSLayoutConstraint *yCenterConstraint = [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:view2 attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0];
[superview addConstraint:yCenterConstraint];

Updated for Swift:

NSLayoutConstraint(item: view1, attribute: .CenterX, relatedBy: .Equal, toItem: view2, attribute: .CenterX, multiplier: 1, constant: 0).active = true

NSLayoutConstraint(item: view1, attribute: .CenterY, relatedBy: .Equal, toItem: view2, attribute: .CenterY, multiplier: 1, constant: 0).active = true