看效果图：

用例如以下代码，想弹出一个模态窗体，设置它的背景透明度为0.5，却发觉prsent后的背景色变为黑色的。

ShareVC *share = [[ShareVC alloc] init];
[self presentViewController:share animated:YES completion:nil];

起初还以为是设置透明度或者是[UIColor clearColor]出的问题，鼓捣几次之后发现不是这个问题。

google之后，在stackOverflow上找到几个比較靠谱的答案~

Why Does presentModalViewController:animated: Turn The Background Black?

Display clearColor UIViewController over UIViewController

终于结论为：

NavigationController and the View Controllers are designed in such a way that only one view controller may show at a time. When a new view controller is pushed/presented the previous view controller will be hidden by the system. So when you reduce the modal view’s alpha you will possibly see the window’s backgroundColor (the black color you see now).

If you want a translucent view to slide-in over the main view, you can add the view as the subView of main view and animate it using UIView Animations.

所以还是改用为
UIView Animations实现：

if (!_isShareViewOpen) {
 
_isShareViewOpen = YES;
 
ShareVC *shareVC = [[ShareVC alloc] initWithNibName:@"ShareVC" bundle:nil];
 
shareVC.shareVC = shareVC;
 
shareVC.awardList = self;
 
[self.view addSubview:shareVC.view];
 
[UIView animateWithDuration:0.75f animations:^{
 
shareVC.view.frame = CGRectMake(0, 640, self.view.frame.size.width, self.view.frame.size.height);
 
shareVC.view.frame = CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height);
 
} completion:^(BOOL finished) {
 
}];
 
}

————————————————————————–2014.09.26 再次编辑 ——————————————————————

假设你不想用动画来实现 present 的效果，能够设置 presentViewController 的属性,一样能够达到以上效果：

presentingViewController.modalPresentationStyle = <span style="font-family: Arial, Helvetica, sans-serif;">UIModalPresentationCurrentContext</span>;

————————————————————————–2015.01.22 再次编辑 ——————————————————————

以上代码仅仅是在 IOS7 中起作用。到 IOS8 就没用了，IOS8 能够用下面代码 fix：

_imagePickerCtrl.modalPresentationStyle = UIModalPresentationOverCurrentContext;
_imagePickerCtrl.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
        
[appViews.rootViewController presentViewController:_imagePickerCtrl animated:YES completion:nil];

參考链接：

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