presentedViewController是指 ** presented by this view controller(modally) **
presentingViewController是指 ** presented this view controller(modally) **
所以通常viewControllerA ---(modally)--> viewControllerB, 设置A = B.delegate, 想要在A中使得B消失，则用
[self.presentedViewController dismissViewControllerAnimated:YES completion:nil];
另外，push对应的是pop。比如：
[self.navigationController popViewControllerAnimated:YES];
而modal对应的是dismiss,如第一个例子。
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一句话解释：
A presentViewController B 后，a.presentedViewController就是b，b.presentingViewController就是a，over。