ios – UICollectionView布局更新 – 项目大小不变

我遇到了collectionView项目大小的问题.我想在纵向模式中显示3个项目,在横向模式下显示6个项目.我已经设置了像这样的-layoutSubviews：

- (void)layoutSubviews {
    [super layoutSubviews];

    if ([[UIApplication sharedApplication] statusBarOrientation] == UIInterfaceOrientationPorTrait || ([[UIApplication sharedApplication] statusBarOrientation] == UIInterfaceOrientationPorTraitUpsideDown)) {
        //PorTrait orientation
        self.flowLayoutPorTrait.itemSize = CGSizeMake(106,106);

    } else if ([[UIApplication sharedApplication] statusBarOrientation] == UIInterfaceOrientationLandscapeLeft || ([[UIApplication sharedApplication] statusBarOrientation] == UIInterfaceOrientationLandscapeRight)) {
        //Landscape orientation
        self.flowLayoutPorTrait.itemSize = CGSizeMake(93,93);
    }
    [self.collectionView.collectionViewLayout invalidateLayout];
}

但是单元格大小不会更新,它对于两个方向始终保持不变.
如果我创建2个flowLayouts并使用：

[self.collectionView setCollectionViewLayout:self.flowLayoutLandscape];

一切正常,但我真的不喜欢它们是如何改变的.动画真的很糟糕.
而且因为itemSize是唯一需要更新的属性,所以使用1布局似乎是最佳选择.

如何告诉collectionView更新布局？

- (CGSizE)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemATindexPath:(NSIndexPath *)indexPath
{
    if (_isLandscapE)
        return CGSizeMake(yourLandscapeWidth,yourLandscapeHeight);
    else
        return CGSizeMake(yourNonLandscapeWidth,yourNonLandscapeHeight);
}