在iOS上present一个半透明的viewController

今天尝试着在一个ViewController上面调用：

- (void)presentModalViewController:(UIViewController *)modalViewController animated:(BOOL)animated

    UIViewController *vc = [[[UIViewController alloc] init] autorelease];
    vc.view.backgroundColor = [UIColor colorWithRed:0 green:0 blue:0 alpha:0.5];
    [self presentModalViewController:vc animated:YES];

为什么呢？搜索了一番得到一份比较合理的结论：

The “problem” is that iOS is very finicky about not wasting memory,

and since the modal view will completely cover the one beneath it,

it doesn’t make much sense to keep it loaded.

Therefore, iOS unloads the view that presents the modal one.

You may check this behavior by implementing -viewWillDisappear: and -viewDidDisappear:.

最终在SO上找到这么个问题，以及一份可行的方案：

viewController.view.backgroundColor = [UIColor clearColor];
rootViewController.modalPresentationStyle = UIModalPresentationCurrentContext;
[rootViewController presentModalViewController:viewController animated:YES];