用xib 建立一个 自定义view

- (instancetype)initWithCoder:(NSCoder *)aDecoder
{
     self = [super initWithCoder:aDecoder];

     if (self) {
         [[NSBundle mainBundle] loadNibNamed:@"CustomView" owner:self options:nil];
         self.contentView.frame = self.bounds;
         [self addSubview:self.contentView];
     }

     return self;
 }

load nib的时候，owner为self。
NOTE: load的时机，需要在initWithCoder:或者awakeFromNib里，或者在自己写的convenience initializer里。

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'invalid nib registered for identifier (ClientListCollectionViewCell) - nib must contain exactly one top level object which must be a UICollectionReusableView instance'