Programmatically Add CenterX CenterY Constraints

Question

I have a UITableViewController that doesn t display any sections if there is nothing to show  I ve added a label to indicate to the user that there is nothing to display with this code    label   UILabel frame  CGRectMake 20  20  250  100   label text    Nothing to show  self tableView addSubview label    But now  I want it to be centered Horizontally and Vertically  Normally  I would choose the two options highlighted  plus ones for height and width  in the screenshot      I ve tried the following code to add the constraints but the app crashes with error    label   UILabel frame  CGRectMake 20  20  250  100   label text    Nothing to show   let xConstraint   NSLayoutConstraint item  label  attribute   CenterX  relatedBy   Equal  toItem  self tableView  attribute   CenterX  multiplier  1  constant  0  let yConstraint   NSLayoutConstraint item  label  attribute   CenterY  relatedBy   Equal  toItem  self tableView  attribute   CenterY  multiplier  1  constant  0   label addConstraint xConstraint  label addConstraint yConstraint    error    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  2014-12-23 08 17 36 755  982 227877      Assertion failure in - UILabel  layoutEngine didAddLayoutConstraint roundingAdjustment mutuallyExclusiveConstraints     SourceCache UIKit UIKit-3318 16 21 NSLayoutConstraint UIKitAdditions m 560   The label should always center horizontally and vertically because the app supports rotation of the device    What am I doing wrong  How do I add these constraints successfully   Thanks

User · Answer

The ObjectiveC equivalent is       myView translatesAutoresizingMaskIntoConstraints   NO         myView centerXAnchor constraintEqualToAnchor self view centerXAnchor  setActive YES          myView centerYAnchor constraintEqualToAnchor self view centerYAnchor  setActive YES

User · Answer

Center in container    The code below does the same thing as centering in the Interface Builder   override func viewDidLoad         super viewDidLoad           set up the view     let myView   UIView       myView backgroundColor   UIColor blue     myView translatesAutoresizingMaskIntoConstraints   false     view addSubview myView          Add code for one of the constraint methods below                Method 1  Anchor Style  myView centerXAnchor constraint equalTo  view centerXAnchor  isActive   true myView centerYAnchor constraint equalTo  view centerYAnchor  isActive   true   Method 2  NSLayoutConstraint Style  NSLayoutConstraint item  myView  attribute  NSLayoutConstraint Attribute centerX  relatedBy  NSLayoutConstraint Relation equal  toItem  view  attribute  NSLayoutConstraint Attribute centerX  multiplier  1  constant  0  isActive   true NSLayoutConstraint item  myView  attribute  NSLayoutConstraint Attribute centerY  relatedBy  NSLayoutConstraint Relation equal  toItem  view  attribute  NSLayoutConstraint Attribute centerY  multiplier  1  constant  0  isActive   true   Notes   Anchor style is the preferred method over NSLayoutConstraint Style  however it is only available from iOS 9  so if you are supporting iOS 8 then you should still use NSLayoutConstraint Style  You will also need to add length and width constraints  My full answer is here

User · Answer

A solution for me was to create a UILabel and add it to the UIButton as a subview  Finally I added a constraint to center it within the button   UILabel   myTextLabel     UILabel alloc  initWithFrame CGRectMake 0  0  75  75    myTextLabel text     Some Text   myTextLabel translatesAutoresizingMaskIntoConstraints   false    myButton addSubView myTextLabel       Add Constraints   myTextLabel centerYAnchor  constraintEqualToAnchor myButton centerYAnchor  active   true    myTextLabel centerXAnchor  constraintEqualToAnchor myButton centerXAnchor  active   true

User · Answer

Update for Swift 3 Swift 4   As of iOS 8  you can and should activate your constraints by setting their isActive property to true   This enables the constraints to add themselves to the proper views   You can activate multiple constraints at once by passing an array containing the constraints to NSLayoutConstraint activate    let label   UILabel frame  CGRect zero  label text    Nothing to show  label textAlignment    center label backgroundColor    red     Set background color to see if label is centered label translatesAutoresizingMaskIntoConstraints   false self tableView addSubview label   let widthConstraint   NSLayoutConstraint item  label  attribute   width  relatedBy   equal                                           toItem  nil  attribute   notAnAttribute  multiplier  1 0  constant  250   let heightConstraint   NSLayoutConstraint item  label  attribute   height  relatedBy   equal                                            toItem  nil  attribute   notAnAttribute  multiplier  1 0  constant  100   let xConstraint   NSLayoutConstraint item  label  attribute   centerX  relatedBy   equal  toItem  self tableView  attribute   centerX  multiplier  1  constant  0   let yConstraint   NSLayoutConstraint item  label  attribute   centerY  relatedBy   equal  toItem  self tableView  attribute   centerY  multiplier  1  constant  0   NSLayoutConstraint activate  widthConstraint  heightConstraint  xConstraint  yConstraint     Better Solution   Since this question was originally answered  layout anchors were introduced making it much easier to create the constraints   In this example I create the constraints and immediately activate them   label widthAnchor constraint equalToConstant  250  isActive   true label heightAnchor constraint equalToConstant  100  isActive   true label centerXAnchor constraint equalTo  self tableView centerXAnchor  isActive   true label centerYAnchor constraint equalTo  self tableView centerYAnchor  isActive   true   or the same using NSLayoutConstraint activate     NSLayoutConstraint activate       label widthAnchor constraint equalToConstant  250       label heightAnchor constraint equalToConstant  100       label centerXAnchor constraint equalTo  self tableView centerXAnchor       label centerYAnchor constraint equalTo  self tableView centerYAnchor       Note   Always add your subviews to the view hierarchy before creating and activating the constraints     Original Answer   The constraints make reference to self tableView   Since you are adding the label as a subview of self tableView  the constraints need to be added to the  common ancestor       self tableView addConstraint xConstraint     self tableView addConstraint yConstraint      As  mustafa and  kcstricks pointed out in the comments  you need to set  label translatesAutoresizingMaskIntoConstraints to false   When you do this  you also need to specify the width and height of the label with constraints because the frame no longer is used   Finally  you also should set the textAlignment to  Center so that your text is centered in your label       var  label   UILabel frame  CGRectZero      label text    Nothing to show      label textAlignment    Center     label backgroundColor   UIColor redColor       Set background color to see if label is centered     label translatesAutoresizingMaskIntoConstraints   false     self tableView addSubview label       let widthConstraint   NSLayoutConstraint item  label  attribute   Width  relatedBy   Equal          toItem  nil  attribute   NotAnAttribute  multiplier  1 0  constant  250      label addConstraint widthConstraint       let heightConstraint   NSLayoutConstraint item  label  attribute   Height  relatedBy   Equal          toItem  nil  attribute   NotAnAttribute  multiplier  1 0  constant  100      label addConstraint heightConstraint       let xConstraint   NSLayoutConstraint item  label  attribute   CenterX  relatedBy   Equal  toItem  self tableView  attribute   CenterX  multiplier  1  constant  0       let yConstraint   NSLayoutConstraint item  label  attribute   CenterY  relatedBy   Equal  toItem  self tableView  attribute   CenterY  multiplier  1  constant  0       self tableView addConstraint xConstraint      self tableView addConstraint yConstraint

User · Answer

If you don t care about this question being specifically about a tableview  and you d just like to center one view on top of another view here s to do it       let horizontalConstraint   NSLayoutConstraint item  newView  attribute  NSLayoutAttribute CenterX  relatedBy  NSLayoutRelation Equal  toItem  parentView  attribute  NSLayoutAttribute CenterX  multiplier  1  constant  0      parentView addConstraint horizontalConstraint       let verticalConstraint   NSLayoutConstraint item  newView  attribute  NSLayoutAttribute CenterY  relatedBy  NSLayoutRelation Equal  toItem  parentView  attribute  NSLayoutAttribute CenterY  multiplier  1  constant  0      parentView addConstraint verticalConstraint

User · Answer

In Swift 5 it looks like this   label translatesAutoresizingMaskIntoConstraints   false label centerXAnchor constraint equalTo  vc view centerXAnchor  isActive   true label centerYAnchor constraint equalTo  vc view centerYAnchor  isActive   true

User · Answer

Programmatically you can do it by adding the following constraints   NSLayoutConstraint  constraintHorizontal    NSLayoutConstraint constraintWithItem self                                                                         attribute NSLayoutAttributeCenterX                                                                        relatedBy NSLayoutRelationEqual                                                                           toItem self superview                                                                        attribute attribute                                                                       multiplier 1 0f                                                                         constant 0 0f    NSLayoutConstraint  constraintVertical    NSLayoutConstraint constraintWithItem self                                                                         attribute NSLayoutAttributeCenterY                                                                          relatedBy NSLayoutRelationEqual                                                                            toItem self superview                                                                          attribute attribute                                                                         multiplier 1 0f                                                                          constant 0 0f

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