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1.iPad上使用presentModalViewController，custion size not work on ios8 SDK。使用如下方法解决：

In iOS 8 you can also use UIPresentationController which gives you more customization options.

#define IsiOS8Later ([[[UIDevice currentDevice] systemVersion] doubleValue]>=8.0)?YES:NO

This will Work:

AboutViewController * _aboutViewController = [[AboutViewController alloc] init];
_aboutViewController.modalPresentationStyle = UIModalPresentationFormSheet;
if(IS_IOS8)
{
   _aboutViewController.preferredContentSize = CGSizeMake(300, 300);
}
[self presentViewController:_aboutViewController animated:YES completion:nil];

In AboutViewController.m

- (void)viewWillLayoutSubviews{
    [super viewWillLayoutSubviews];

    if(!IS_IOS8)
    {
        self.view.superview.bounds = CGRectMake(0, 0, 300, 300);
    }
}