如何为UICollectionView添加整体的header v

self.collectionView  =  [[UICollectionView alloc] initWithFrame:CGRectMake(0, 0, 320, self.view.frame.size.height) collectionViewLayout:flowlayout];  
self.collectionView.contentInset = UIEdgeInsetsMake(50, 0, 0, 0);  
UIImageView *imagev = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"015.png"]];  
imagev.frame = CGRectMake(0, -50, 320, 50);  
[self.collectionView addSubview: imagev];  
[self.view addSubview: _collectionView];