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

你可以将当前视图或控制器作为参数发送:

+ (void)myUtilityMethod:(id)controller {
    // do stuff
    // something bad happened, display an alert.
}

其他回答

你可以使用两种方法:

-使用UIAlertView或'UIActionSheet'代替(不推荐,因为它在iOS 8中已弃用,但现在可以使用了)

-记得上次显示的视图控制器。举个例子。

@interface UIViewController (TopController)
+ (UIViewController *)topViewController;
@end

// implementation

#import "UIViewController+TopController.h"
#import <objc/runtime.h>

static __weak UIViewController *_topViewController = nil;

@implementation UIViewController (TopController)

+ (UIViewController *)topViewController {
    UIViewController *vc = _topViewController;
    while (vc.parentViewController) {
        vc = vc.parentViewController;
    }
    return vc;
}

+ (void)load {
    [super load];
    [self swizzleSelector:@selector(viewDidAppear:) withSelector:@selector(myViewDidAppear:)];
    [self swizzleSelector:@selector(viewWillDisappear:) withSelector:@selector(myViewWillDisappear:)];
}

- (void)myViewDidAppear:(BOOL)animated {
    if (_topViewController == nil) {
        _topViewController = self;
    }

    [self myViewDidAppear:animated];
}

- (void)myViewWillDisappear:(BOOL)animated {
    if (_topViewController == self) {
        _topViewController = nil;
    }

    [self myViewWillDisappear:animated];
}

+ (void)swizzleSelector:(SEL)sel1 withSelector:(SEL)sel2
{
    Class class = [self class];

    Method originalMethod = class_getInstanceMethod(class, sel1);
    Method swizzledMethod = class_getInstanceMethod(class, sel2);

    BOOL didAddMethod = class_addMethod(class,
                                        sel1,
                                        method_getImplementation(swizzledMethod),
                                        method_getTypeEncoding(swizzledMethod));

    if (didAddMethod) {
        class_replaceMethod(class,
                            sel2,
                            method_getImplementation(originalMethod),
                            method_getTypeEncoding(originalMethod));
    } else {
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}

@end 

用法:

[[UIViewController topViewController] presentViewController:alertController ...];

在WWDC上,我在一个实验室停下来,问了一个苹果工程师同样的问题:“显示UIAlertController的最佳实践是什么?”他说他们经常被问到这个问题,我们开玩笑说他们应该就此开个会。他说苹果内部创建了一个带有透明UIViewController的UIWindow,然后在上面呈现UIAlertController。基本上就是迪伦·贝特曼的答案。

但我不想使用UIAlertController的子类,因为这将需要我在整个应用程序中更改我的代码。因此,在关联对象的帮助下,我在UIAlertController上创建了一个类别,它在Objective-C中提供了一个显示方法。

以下是相关代码:

#import "UIAlertController+Window.h"
#import <objc/runtime.h>

@interface UIAlertController (Window)

- (void)show;
- (void)show:(BOOL)animated;

@end

@interface UIAlertController (Private)

@property (nonatomic, strong) UIWindow *alertWindow;

@end

@implementation UIAlertController (Private)

@dynamic alertWindow;

- (void)setAlertWindow:(UIWindow *)alertWindow {
    objc_setAssociatedObject(self, @selector(alertWindow), alertWindow, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (UIWindow *)alertWindow {
    return objc_getAssociatedObject(self, @selector(alertWindow));
}

@end

@implementation UIAlertController (Window)

- (void)show {
    [self show:YES];
}

- (void)show:(BOOL)animated {
    self.alertWindow = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    self.alertWindow.rootViewController = [[UIViewController alloc] init];

    id<UIApplicationDelegate> delegate = [UIApplication sharedApplication].delegate;
    // Applications that does not load with UIMainStoryboardFile might not have a window property:
    if ([delegate respondsToSelector:@selector(window)]) {
        // we inherit the main window's tintColor
        self.alertWindow.tintColor = delegate.window.tintColor;
    }

    // window level is above the top window (this makes the alert, if it's a sheet, show over the keyboard)
    UIWindow *topWindow = [UIApplication sharedApplication].windows.lastObject;
    self.alertWindow.windowLevel = topWindow.windowLevel + 1;

    [self.alertWindow makeKeyAndVisible];
    [self.alertWindow.rootViewController presentViewController:self animated:animated completion:nil];
}

- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    
    // precaution to ensure window gets destroyed
    self.alertWindow.hidden = YES;
    self.alertWindow = nil;
}

@end

下面是一个用法示例:

// need local variable for TextField to prevent retain cycle of Alert otherwise UIWindow
// would not disappear after the Alert was dismissed
__block UITextField *localTextField;
UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"Global Alert" message:@"Enter some text" preferredStyle:UIAlertControllerStyleAlert];
[alert addAction:[UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction *action) {
    NSLog(@"do something with text:%@", localTextField.text);
// do NOT use alert.textfields or otherwise reference the alert in the block. Will cause retain cycle
}]];
[alert addTextFieldWithConfigurationHandler:^(UITextField *textField) {
    localTextField = textField;
}];
[alert show];

当UIAlertController被释放时,创建的UIWindow将被销毁,因为它是唯一保留UIWindow的对象。但是如果你将UIAlertController分配给一个属性,或者通过在其中一个动作块中访问警报而导致其保留计数增加,UIWindow将停留在屏幕上,锁定你的UI。请参阅上面的示例使用代码,以避免在需要访问UITextField的情况下。

我用一个测试项目FFGlobalAlertController做了一个GitHub回购

斯威夫特5

在显示消息后隐藏窗口是很重要的。

func showErrorMessage(_ message: String) {
    let alertWindow = UIWindow(frame: UIScreen.main.bounds)
    alertWindow.rootViewController = UIViewController()

    let alertController = UIAlertController(title: "Error", message: message, preferredStyle: UIAlertController.Style.alert)
    alertController.addAction(UIAlertAction(title: "Close", style: UIAlertAction.Style.cancel, handler: { _ in
        alertWindow.isHidden = true
    }))
    
    alertWindow.windowLevel = UIWindow.Level.alert + 1;
    alertWindow.makeKeyAndVisible()
    alertWindow.rootViewController?.present(alertController, animated: true, completion: nil)
}

对于iOS 13,基于mythicalcoder和bobbyrehm的回答:

在iOS 13中,如果你正在创建自己的窗口来显示警报,你需要保持对该窗口的强引用,否则你的警报将不会显示,因为当它的引用退出作用域时,窗口将立即被释放。

此外,在警报解除后,您需要再次将引用设置为nil,以便删除窗口,继续允许用户在它下面的主窗口上进行交互。

你可以创建一个UIViewController子类来封装窗口内存管理逻辑:

class WindowAlertPresentationController: UIViewController {

    // MARK: - Properties

    private lazy var window: UIWindow? = UIWindow(frame: UIScreen.main.bounds)
    private let alert: UIAlertController

    // MARK: - Initialization

    init(alert: UIAlertController) {

        self.alert = alert
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {

        fatalError("This initializer is not supported")
    }

    // MARK: - Presentation

    func present(animated: Bool, completion: (() -> Void)?) {

        window?.rootViewController = self
        window?.windowLevel = UIWindow.Level.alert + 1
        window?.makeKeyAndVisible()
        present(alert, animated: animated, completion: completion)
    }

    // MARK: - Overrides

    override func dismiss(animated flag: Bool, completion: (() -> Void)? = nil) {

        super.dismiss(animated: flag) {
            self.window = nil
            completion?()
        }
    }
}

你可以这样使用它,或者如果你想在你的UIAlertController上使用一个方便的方法,你可以把它扔到一个扩展中:

extension UIAlertController {

    func presentInOwnWindow(animated: Bool, completion: (() -> Void)?) {

        let windowAlertPresentationController = WindowAlertPresentationController(alert: self)
        windowAlertPresentationController.present(animated: animated, completion: completion)
    }
}

Kevin Sliech提供了一个很好的解决方案。

我现在在我的主UIViewController子类中使用下面的代码。

我做的一个小改动是检查最好的表示控制器是不是一个普通的UIViewController。如果不是,那就一定是某个VC表示一个普通VC。因此,我们返回正在呈现的VC。

- (UIViewController *)bestPresentationController
{
    UIViewController *bestPresentationController = [UIApplication sharedApplication].keyWindow.rootViewController;

    if (![bestPresentationController isMemberOfClass:[UIViewController class]])
    {
        bestPresentationController = bestPresentationController.presentedViewController;
    }    

    return bestPresentationController;
}

在我的测试中似乎一切正常。

谢谢你,凯文!