UICollectionView can't show all content - ios

Question

I init the collectionview with this(with code not the storyboard):
-(id) init{
self = [super init];
self.view = [[UIView alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
self.page = 0;
self.step = 20;
self.subCateItems = [[NSMutableArray alloc] init];
self.scaleFactor = [[UIScreen mainScreen] bounds].size.width / 375.f;
UICollectionViewFlowLayout *layout=[[UICollectionViewFlowLayout alloc] init];
layout.sectionInset = UIEdgeInsetsMake(9.f*self.scaleFactor, 6.f*self.scaleFactor, 0, 6.f*self.scaleFactor);
_collectionView=[[UICollectionView alloc] initWithFrame:self.view.frame collectionViewLayout:layout];
_collectionView.mj_header = [MJRefreshNormalHeader headerWithRefreshingBlock:^{
self.page = 0;
[self updateData];
}];
[_collectionView setDataSource:self];
[_collectionView setDelegate:self];
[_collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:#"cellIdentifier"];
[_collectionView setBackgroundColor:[UIColor whiteColor]];
[self.view addSubview:_collectionView];
return self;
}
but the collection view seems can't show all the collection cells:
How to fix this?
as comments have suggested, I have caputured the view hierarchy of the views

Answer 1

Judging from your scroll view indicators, the bottom inset of the scroll view is set incorrectly, as it underlaps UITabBar. The code is in Swift but you should get the idea ;-)
I usually do this to make my scroll views appear on top:
_collectionView.scrollIndicatorInsets = UIEdgeInsetsMake(0, 0, -50, 0)
_collectionView.contentInset = UIEdgeInsetsMake(0, 0, -50, 0)
Also, make sure to make this:
self.automaticallyAdjustsScrollViewInsets = false

Answer 2

The first problem is here
self.view = [[UIView alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
Must be like this
self.view = [[UIView alloc] initWithFrame:self.view.bounds];
Then, who is the self ? Is it tab of UITabBarController?
If yes, all is good. If no then you will set correct inset ofUICollectionView