转载自：《How To Present An Alert View Using UIAlertController When You Don't Have A View Controller》

Over the summer I updated a number of apps to iOS 9. A common tasks for me was replacing the deprecated UIAlertView with new UIAlertController class. Generally this is easy to do, but there is one scenario that UIAlertView handles well that UIAlertController doesn’t support at all: displaying an alert view from parts of your code without a reference to the current view controller, for instance, displaying an alert view from the app delegate.

Trying to display an alert view using UIAlertController from a non-UI specific class such as the app delegate is a common problem, and developers have found different workarounds. One particular solution that caught my eye was this one from agilityvision, who writes:

At WWDC I stopped in at one of the labs and asked an Apple Engineer this same question: “What was the best practice for displaying a UIAlertController?” And he said they had been getting this question a lot and we joked that they should have had a session on it. He said that internally Apple is creating a UIWindow with a transparent UIViewController and then presenting the UIAlertController on it.

This got me thinking about how I would solve the problem for apps I work on. Liking the approach mentioned by the Apple Engineer, I decided to give that a try. But unlike some of the other solutions that use a category, I decided to create a new class, WPSAlertController, that is a subclass of UIAlertController, and provide additional convenience methods making it possible to display an alert view with a single call…like this:

NSString *title = ...
NSString *message = ...
[WPSAlertController presentOkayAlertWithTitle:title message:message];

Or display the localizedDescription from an NSError object:

[WPSAlertController presentOkayAlertWithError:error];

And of course, since WPSAlertController is a subclass of UIAlertController, you can create your alert the same way as you do using UIAlertController. The only difference is you call show to present the alert when you don’t have a view controller. For example:

NSString *title = ...
NSString *message = ...
WPSAlertController *alertController = [WPSAlertController alertControllerWithTitle:title message:message preferredStyle:UIAlertControllerStyleAlert];
[alertController addAction:[UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
  // Your code.
}]];

[alertController show];

You can grab the source code for WPSAlertController from my github repository and include it in your project, or use the entire WPSKit.framework in your project if you want to use WPSAlertController plus other handy classes that I use when creating iOS apps.