UICollectionView must be initialized with a non-nil layout parameter
UICollectonView必须传入不为空的layout参数来初始化
解决方法:
重写init方法
-(id)init{
UICollectionViewFlowLayout *layout=[[UICollectionViewFlowLayoutalloc]init];
if (self=[superinitWithCollectionViewLayout:layout]) {
}
return self;
}