Why I have to add IBOutlet identifier to my delegate when inside apple sdk there isn't any IBOutlets?

Question

I've created the following custom view:

@interface TNGalleryView : UIView

@property (assign, nonatomic) id <TNGalleryViewDelegate> delegate;
@property (assign, nonatomic) id <TNGalleryViewDataSource> dataSource;

@end

I can assign delegate of it in code, but when I want to assign delegate in IB, I can't. I have to change my code to:

@interface TNGalleryView : UIView

@property (assign, nonatomic) IBOutlet id <TNGalleryViewDelegate> delegate;
@property (assign, nonatomic) IBOutlet id <TNGalleryViewDataSource> dataSource;

@end

and then I can assign delegate and dataSource in IB.

Why if i would like to assign delegate and dataSource in IB I have to add IBOutlet identifier, but inside apple sdk there isn't any IBOutlets? For example this part of UICollectionView declaration:

NS_CLASS_AVAILABLE_IOS(6_0) @interface UICollectionView : UIScrollView

- (id)initWithFrame:(CGRect)frame collectionViewLayout:(UICollectionViewLayout *)layout; // the designated initializer

@property (nonatomic, retain) UICollectionViewLayout *collectionViewLayout;
@property (nonatomic, assign) id <UICollectionViewDelegate> delegate;
@property (nonatomic, assign) id <UICollectionViewDataSource> dataSource;

There isn't any IBOutlet, nevertheless I can assign dataSource and delegate to it in IB.

Answer 1

Quite a good question!

Open your storyboard/xib with text editor,search "TNGalleryView",you will found xml tag like this:

<view contentMode="scaleToFill" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="ke7-rk-01e" customClass="TNGalleryView">
    <rect key="frame" x="86" y="309" width="160" height="252"/>
    <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMaxY="YES"/>
    <color key="backgroundColor" white="1" alpha="1" colorSpace="custom" customColorSpace="calibratedWhite"/>
</view>

Note that: TNGalleryView was wrapped in a "view" tag with a property: customClass="TNGalleryView"

Now add a UICollectionView in storyboard/xib, save and reopne storyboard/xib with text editor,search "collectionView",you will found xml tag like this:

<collectionView opaque="NO" clipsSubviews="YES" multipleTouchEnabled="YES" contentMode="scaleToFill" fixedFrame="YES" minimumZoomScale="0.0" maximumZoomScale="0.0" dataMode="prototypes" translatesAutoresizingMaskIntoConstraints="NO" id="ouM-xa-T6a">
   <rect key="frame" x="93" y="72" width="160" height="252"/>
   <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
   <collectionViewFlowLayout key="collectionViewLayout" minimumLineSpacing="10" minimumInteritemSpacing="10" id="Cv0-O7-zGY">
   <size key="itemSize" width="50" height="50"/>
   <size key="headerReferenceSize" width="0.0" height="0.0"/>
   <size key="footerReferenceSize" width="0.0" height="0.0"/>
   <inset key="sectionInset" minX="0.0" minY="0.0" maxX="0.0" maxY="0.0"/>
    </collectionViewFlowLayout>

</collectionView>

Now,you will find the different ,collectionView is wrapped in a "collectionView" tag

I guess that was the reason why you can't assign custom delegate in IB without set the custom delegate to be a IBOutlet.

UPDATE

No enought reputation to comment other poeple's answer.So i say something here: read the question clearly,and have a test in xcode,you will realize what Sergey Demchenko's question is:

First,if TNGalleryViewDelegate is declare in this way:

@property (assign, nonatomic) id <TNGalleryViewDelegate> delegate;

The xcode screen shot:

Now if TNGalleryViewDelegate is declare in this way:

@property (assign, nonatomic) IBOutlet id <TNGalleryViewDelegate> delegate;

The xcode screen shot: