iOS Tip: Add member into Object dynamically with Extensions

You might know that Extensions can define only computed properties using already defined members of the object.

extension UIView {
    var borderLeftWidth: Float = 1
}

error: playground.playground:5:9: error: extensions must not contain stored properties
    var borderLeftWidth: Float = 1
        ^

However I wanted to make new stored properties to draw a left line easily with Extension.

extension UIView {
  @IBInspectable
  public var borderLeftWidth : CGFloat{
      get{
          let borderLineView = ... //get from UIView
          return borderLineView?.frame.width ?? 0.0;
      }
  
      set(value){
          var borderLineView : UIView! = //get from UIView
  
          if borderLineView == nil{
              borderLineView = self._createNewBorderLineView();
          }
  
          borderLineView.backgroundColor = self.borderUIColor;
          borderLineView.widthConstraints.first?.constant = value;
          self.bringSubviewToFront(borderLineView);
      }
  }
}

How can I make variable to store the line view for each views?

objc_setAssociatedObject

There is a function to insert new member into the object. (Maybe it is saving it another space according to the descriptions)

Apple provides objc_setAssociatedObject.

func objc_setAssociatedObject(
    _ object: Any,
    _ key: UnsafeRawPointer,
    _ value: Any?,
    _ policy: objc_AssociationPolicy
)

First argument is an object to contain a new variable.

objc_setAssociatedObject(self,

Second is a key address to identify the new variable.

&UIView.BorderPropertyKeys.left,

/**
  private struct BorderPropertyKeys{
      static var left = "uiview.border.left";
  }
*/

Third is a value to be stored as a new variable.

borderLineView,

Last is a property policy

.OBJC_ASSOCIATION_RETAIN

//like @property (nonatomic, retain) UIView *borderLIneView;

And I can add a new UIView into the view and store it dynamically like this.

objc_setAssociatedObject(self, 
    &UIView.BorderPropertyKeys.left, 
    borderLineView, 
    .OBJC_ASSOCIATION_RETAIN);

So.. how can I get the stored value again??

objc_getAssociatedObject

Of source, there is also a function to get stored variable. Just replace ‘set’ with ‘get’. Right! It is objc_getAssociatedObject.

func objc_getAssociatedObject(
    _ object: Any,
    _ key: UnsafeRawPointer
) -> Any?

I won’t explain how to use this here. (You can already know by argument names 😄)

Property

With those functions I created getter and setter to manage a border line view.

get

get{
  let borderLineView = objc_getAssociatedObject(self, &UIView.BorderPropertyKeys.left) as? UIView;
  return borderLineView?.frame.width ?? 0.0;
}

set

set(value){
    var borderLineView : UIView! = objc_getAssociatedObject(self, &UIView.BorderPropertyKeys.left) as? UIView;

    if borderLineView == nil{
        borderLineView = self._createNewBorderLineView();
        
        borderLineView?.widthAnchor.constraint(equalToConstant: value).isActive = true;
        borderLineView?.leftAnchor.constraint(equalTo: self.leftAnchor).isActive = true
        borderLineView?.topAnchor.constraint(equalTo: self.topAnchor).isActive = true
        borderLineView?.bottomAnchor.constraint(equalTo: self.bottomAnchor).isActive = true
        
        self.setObjcProperty(&UIView.BorderPropertyKeys.left, value: borderLineView, policy: .OBJC_ASSOCIATION_RETAIN);
        borderLineView = self.objcProperty(&UIView.BorderPropertyKeys.left) as? UIView;
    }

    borderLineView.backgroundColor = self.borderUIColor;
    borderLineView.widthConstraints.first?.constant = value;
    self.bringSubviewToFront(borderLineView);
}

Advanced

I don’t like such a function names, therefore I created the shortcuts of those methods.

objcProperty

/**
    Wrapper for objc_getAssociatedObject();
    - parameter key: key for associated object
*/
public func objcProperty(_ key: UnsafeRawPointer) -> Any?{
    return objc_getAssociatedObject(self, key);
}

setObjcProperty

/**
    Wrapper for objc_setAssociatedObject()
    - parameter key: key for associated object
    - parameter value: value for associated object
    - parameter policy: Release policy for associated object
 */
public func setObjcProperty(_ key: UnsafeRawPointer, value: Any?, policy: objc_AssociationPolicy) -> Void{
    objc_setAssociatedObject(self, key, value, policy);
}

Final

This is my full source of the property.

Source

@IBInspectable
public var borderLeftWidth : CGFloat{
    get{
        let borderLineView = self.objcProperty(&UIView.BorderPropertyKeys.left) as? UIView;
        return borderLineView?.frame.width ?? 0.0;
    }

    set(value){
        var borderLineView : UIView! = self.objcProperty(&UIView.BorderPropertyKeys.left) as? UIView;

        if borderLineView == nil{
            borderLineView = self._createNewBorderLineView();
            
            borderLineView?.widthAnchor.constraint(equalToConstant: value).isActive = true;
            borderLineView?.leftAnchor.constraint(equalTo: self.leftAnchor).isActive = true
            borderLineView?.topAnchor.constraint(equalTo: self.topAnchor).isActive = true
            borderLineView?.bottomAnchor.constraint(equalTo: self.bottomAnchor).isActive = true
            
            self.setObjcProperty(&UIView.BorderPropertyKeys.left, value: borderLineView, policy: .OBJC_ASSOCIATION_RETAIN);
            borderLineView = self.objcProperty(&UIView.BorderPropertyKeys.left) as? UIView;
        }

        borderLineView.backgroundColor = self.borderUIColor;
        borderLineView.widthConstraints.first?.constant = value;
        self.bringSubviewToFront(borderLineView);
    }
}       

Storyboard

And I could use it in the storyboard attributes inspector!

There is also objc_removeAssociatedObjects, however I have never used it yet.

You can see original source of the property at this link.

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References

objc_setAssociatedObject(_:_:_:_:) | Apple Developer Documentation