场景:用户点击视图控制器上的按钮。视图控制器是导航堆栈中最顶层的(很明显)。tap调用在另一个类上调用的实用程序类方法。这里发生了不好的事情我想在控件返回到视图控制器之前在那里显示一个警告。
+ (void)myUtilityMethod {
// do stuff
// something bad happened, display an alert.
}
这是可能的UIAlertView(但可能不太合适)。
在这种情况下，你如何在myUtilityMethod中呈现UIAlertController ?
当前回答
你可以用Swift 2.2做以下事情:
let alertController: UIAlertController = ...
UIApplication.sharedApplication().keyWindow?.rootViewController?.presentViewController(alertController, animated: true, completion: nil)
Swift 3.0:
let alertController: UIAlertController = ...
UIApplication.shared.keyWindow?.rootViewController?.present(alertController, animated: true, completion: nil)
其他回答
补充Zev的回答(并切换回Objective-C)，你可能会遇到这样的情况，你的根视图控制器通过segue或其他东西呈现其他VC。在根VC上调用presenttedviewcontroller会处理这个:
[[UIApplication sharedApplication].keyWindow.rootViewController.presentedViewController presentViewController:alertController animated:YES completion:^{}];
这解决了一个问题，我有根VC已经segue到另一个VC，而不是显示警报控制器，像上面报告的警告发出:
Warning: Attempt to present <UIAlertController: 0x145bfa30> on <UINavigationController: 0x1458e450> whose view is not in the window hierarchy!
我还没有测试它，但如果你的根VC恰好是一个导航控制器，这可能也是必要的。
在调用类方法之前注册通知。
斯威夫特代码:
NSNotificationCenter.defaultCenter().addObserver(self, selector: "displayAlert", name: "ErrorOccured", object: nil)
在displayAlert实例方法中，您可以显示警报。
创建扩展像在Aviel Gross回答。这里有Objective-C扩展。
这里有头文件*.h
// UIAlertController+Showable.h
#import <UIKit/UIKit.h>
@interface UIAlertController (Showable)
- (void)show;
- (void)presentAnimated:(BOOL)animated
completion:(void (^)(void))completion;
- (void)presentFromController:(UIViewController *)viewController
animated:(BOOL)animated
completion:(void (^)(void))completion;
@end
和实现:*.m
// UIAlertController+Showable.m
#import "UIAlertController+Showable.h"
@implementation UIAlertController (Showable)
- (void)show
{
[self presentAnimated:YES completion:nil];
}
- (void)presentAnimated:(BOOL)animated
completion:(void (^)(void))completion
{
UIViewController *rootVC = [UIApplication sharedApplication].keyWindow.rootViewController;
if (rootVC != nil) {
[self presentFromController:rootVC animated:animated completion:completion];
}
}
- (void)presentFromController:(UIViewController *)viewController
animated:(BOOL)animated
completion:(void (^)(void))completion
{
if ([viewController isKindOfClass:[UINavigationController class]]) {
UIViewController *visibleVC = ((UINavigationController *)viewController).visibleViewController;
[self presentFromController:visibleVC animated:animated completion:completion];
} else if ([viewController isKindOfClass:[UITabBarController class]]) {
UIViewController *selectedVC = ((UITabBarController *)viewController).selectedViewController;
[self presentFromController:selectedVC animated:animated completion:completion];
} else {
[viewController presentViewController:self animated:animated completion:completion];
}
}
@end
你在你的实现文件中使用这个扩展，就像这样:
#import "UIAlertController+Showable.h"
UIAlertController* alert = [UIAlertController
alertControllerWithTitle:@"Title here"
message:@"Detail message here"
preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction* defaultAction = [UIAlertAction
actionWithTitle:@"OK"
style:UIAlertActionStyleDefault
handler:^(UIAlertAction * action) {}];
[alert addAction:defaultAction];
// Add more actions if needed
[alert show];
你可以用Swift 2.2做以下事情:
let alertController: UIAlertController = ...
UIApplication.sharedApplication().keyWindow?.rootViewController?.presentViewController(alertController, animated: true, completion: nil)
Swift 3.0:
let alertController: UIAlertController = ...
UIApplication.shared.keyWindow?.rootViewController?.present(alertController, animated: true, completion: nil)
如果有人感兴趣，我创建了一个Swift 3版本的@agilityvision答案。代码:
import Foundation
import UIKit
extension UIAlertController {
var window: UIWindow? {
get {
return objc_getAssociatedObject(self, "window") as? UIWindow
}
set {
objc_setAssociatedObject(self, "window", newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
}
}
open override func viewDidDisappear(_ animated: Bool) {
super.viewDidDisappear(animated)
self.window?.isHidden = true
self.window = nil
}
func show(animated: Bool = true) {
let window = UIWindow(frame: UIScreen.main.bounds)
window.rootViewController = UIViewController(nibName: nil, bundle: nil)
let delegate = UIApplication.shared.delegate
if delegate?.window != nil {
window.tintColor = delegate!.window!!.tintColor
}
window.windowLevel = UIApplication.shared.windows.last!.windowLevel + 1
window.makeKeyAndVisible()
window.rootViewController!.present(self, animated: animated, completion: nil)
self.window = window
}
}
