How to create a UIAppearance proxy for an inherited view in MonoTouch?

Question

I created a subclass of UIButton which acts as a custom Back button inside UIBarButtonItem:

 - - - - - - - - - - - - - - - - - 
|         UIBarButtonItem         |

|     ------------------------    |
     /                       |
|    \ BackButton : UIButton |    |
      ------------------------
|  _  _  _  _  _  _  _  _  _  _  _|

As I was switching my code to using UIAppearance, I noticed that BackButton.Appearance is, unsuprisingly, inherited from UIButton.Appearance, and therefore changing it will change all UIButtons throughout the application and not just my custom buttons.

I know I can use AppearanceWhenContainedIn but, because UIBarButtonItem is not an appearance container, I'd have to insert another view between them to recognize back buttons, which I don't want to do.

How do I provide a UIAppearance proxy for my custom UIButton subclass that has all the methods available in UIButton.UIButtonAppearance? I noticed that its constructor is internal.

Answer 1

Although there may be a simpler answer, I ended up inheriting from UIAppearance directly and copy-pasting relevant piece of UIButton and UIButton+UIButtonAppearance binding implementation from MonoDevelop C# disassembler and fixing internal fields here and there.

Inside BackButton:

static readonly IntPtr class_ptr = Class.GetHandle(typeof(BackButton));

public new static BackButtonAppearance Appearance
{
    get {
        return new BackButtonAppearance (Messaging.IntPtr_objc_msgSend (class_ptr, UIAppearance.SelectorAppearance));
    }
}

public new static BackButtonAppearance AppearanceWhenContainedIn (params Type[] containers)
{
    return new BackButtonAppearance (UIAppearance.GetAppearance (class_ptr, containers));
}

Nested BackButtonAppearance:

public class BackButtonAppearance : UIAppearance {
    public BackButtonAppearance(IntPtr handle) : base(handle) { }

    // These are copied from UIButton disassembly:

    static readonly IntPtr selSetBackgroundImageForState_ = Selector.GetHandle ("setBackgroundImage:forState:");
    static readonly IntPtr selSetTitleShadowColorForState_ = Selector.GetHandle ("setTitleShadowColor:forState:");            
    static readonly IntPtr selSetTitleColorForState_ = Selector.GetHandle ("setTitleColor:forState:");

    // These are copied from UIButton+UIButtonAppearance disassembly,
    // with internal UIButton.selSet* fields replaced by fields declared above.

    [Export ("setBackgroundImage:forState:"), CompilerGenerated]
    public virtual void SetBackgroundImage (UIImage image, UIControlState forState)
    {
        UIApplication.EnsureUIThread ();
        if (this.IsDirectBinding) {
            Messaging.void_objc_msgSend_IntPtr_UInt32 (base.Handle, selSetBackgroundImageForState_, (image != null) ? image.Handle : IntPtr.Zero, (uint)forState);
        } else {
            Messaging.void_objc_msgSendSuper_IntPtr_UInt32 (base.SuperHandle, selSetBackgroundImageForState_, (image != null) ? image.Handle : IntPtr.Zero, (uint)forState);
        }
    }


    [Export ("setTitleColor:forState:"), CompilerGenerated]
    public virtual void SetTitleColor (UIColor color, UIControlState forState)
    {
        UIApplication.EnsureUIThread ();
        if (this.IsDirectBinding) {
            Messaging.void_objc_msgSend_IntPtr_UInt32 (base.Handle, selSetTitleColorForState_, (color != null) ? color.Handle : IntPtr.Zero, (uint)forState);
        } else {
            Messaging.void_objc_msgSendSuper_IntPtr_UInt32 (base.SuperHandle, selSetTitleColorForState_, (color != null) ? color.Handle : IntPtr.Zero, (uint)forState);
        }
    }

    [Export ("setTitleShadowColor:forState:"), CompilerGenerated]
    public virtual void SetTitleShadowColor (UIColor color, UIControlState forState)
    {
        UIApplication.EnsureUIThread ();
        if (this.IsDirectBinding) {
            Messaging.void_objc_msgSend_IntPtr_UInt32 (base.Handle, selSetTitleShadowColorForState_, (color != null) ? color.Handle : IntPtr.Zero, (uint)forState);
        } else {
            Messaging.void_objc_msgSendSuper_IntPtr_UInt32 (base.SuperHandle, selSetTitleShadowColorForState_, (color != null) ? color.Handle : IntPtr.Zero, (uint)forState);
        }
    }
}

This allows me to bring my custom view to support UIAppearance-style API.