问题
Apple recommends dismissing any UIAlertViews/UIActionSheets when entering background state in iOS 4. This is to avoid any confusion on the user's part when he relaunches the application later. I wonder how I could elegantly dismiss all UIAlertViews at once, without retaining a reference to it everytime I set one up...
Any idea ?
回答1:
I was intrigued by Dad's answer (funny username :), and curious why it was down-voted.
So I tried it.
Here is the .m part of a subclass of UIAlertView.
Edit: (Cédric) I have added a way to catch calls to delegate methods and remove the observer then to avoid multiple registrations to the notification center.
Everything bundled in a class in this github repo: https://github.com/sdarlington/WSLViewAutoDismiss
#import "UIAlertViewAutoDismiss.h"
#import <objc/runtime.h>
@interface UIAlertViewAutoDismiss () <UIAlertViewDelegate> {
id<UIAlertViewDelegate> __unsafe_unretained privateDelegate;
}
@end
@implementation UIAlertViewAutoDismiss
- (id)initWithTitle:(NSString *)title
message:(NSString *)message
delegate:(id)delegate
cancelButtonTitle:(NSString *)cancelButtonTitle
otherButtonTitles:(NSString *)otherButtonTitles, ...
{
self = [super initWithTitle:title
message:message
delegate:self
cancelButtonTitle:cancelButtonTitle
otherButtonTitles:nil, nil];
if (self) {
va_list args;
va_start(args, otherButtonTitles);
for (NSString *anOtherButtonTitle = otherButtonTitles; anOtherButtonTitle != nil; anOtherButtonTitle = va_arg(args, NSString *)) {
[self addButtonWithTitle:anOtherButtonTitle];
}
privateDelegate = delegate;
}
return self;
}
- (void)dealloc
{
privateDelegate = nil;
[[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidEnterBackgroundNotification object:nil];
[super dealloc];
}
- (void)setDelegate:(id)delegate
{
privateDelegate = delegate;
}
- (id)delegate
{
return privateDelegate;
}
- (void)show
{
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(applicationDidEnterBackground:)
name:UIApplicationDidEnterBackgroundNotification
object:nil];
[super show];
}
- (void)applicationDidEnterBackground:(NSNotification *)notification
{
[super dismissWithClickedButtonIndex:[self cancelButtonIndex] animated:NO];
[[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidEnterBackgroundNotification object:nil];
}
#pragma mark - UIAlertViewDelegate
// The code below avoids to re-implement all protocol methods to forward to the real delegate.
- (id)forwardingTargetForSelector:(SEL)aSelector
{
struct objc_method_description hasMethod = protocol_getMethodDescription(@protocol(UIAlertViewDelegate), aSelector, NO, YES);
if (hasMethod.name != NULL) {
// The method is that of the UIAlertViewDelegate.
if (aSelector == @selector(alertView:didDismissWithButtonIndex:) ||
aSelector == @selector(alertView:clickedButtonAtIndex:))
{
[[NSNotificationCenter defaultCenter] removeObserver:self
name:UIApplicationDidEnterBackgroundNotification
object:nil];
}
return privateDelegate;
}
else {
return [super forwardingTargetForSelector:aSelector];
}
}
@end
It works nicely. It's great, because you can just start using it the same way that you used to use UIAlertView.
I haven't had time to test it thoroughly, but I didn't notice any side effect.
回答2:
My call would be to add a category to UIAlertview adding the following function :
- (void) hide {
[self dismissWithClickedButtonIndex:0 animated:YES];
}
And to suscribe to UIApplicationWillResignActiveNotification :
[[NSNotificationCenter defaultCenter] addObserver:alertView selector:@selector(hide) name:@"UIApplicationWillResignActiveNotification" object:nil];
回答3:
A totally different approach is a recursive search.
Recursive function for your application delegate
- (void)checkViews:(NSArray *)subviews {
Class AVClass = [UIAlertView class];
Class ASClass = [UIActionSheet class];
for (UIView * subview in subviews){
if ([subview isKindOfClass:AVClass]){
[(UIAlertView *)subview dismissWithClickedButtonIndex:[(UIAlertView *)subview cancelButtonIndex] animated:NO];
} else if ([subview isKindOfClass:ASClass]){
[(UIActionSheet *)subview dismissWithClickedButtonIndex:[(UIActionSheet *)subview cancelButtonIndex] animated:NO];
} else {
[self checkViews:subview.subviews];
}
}
}
Calling it from the applicationDidEnterBackground procedure
[self checkViews:application.windows];
回答4:
huh. Haven't tried this yet, but I wonder if it would make sense to create a subclass of UIAlertView that listens for this Notification and closes itself if so...
That'd have the "automatically" without retaining / keeping it around characteristic OP is requesting. Make sure to unregister for the notification on close (else boom!)
回答5:
As someone mentioned in a comment: the accepted answer isn't the best/cleanest one since iOS 4.0 when we have blocks! Here's how I do it:
UIAlertView* alert = [[UIAlertView alloc] initWithTitle:@"Alert!" message:@"This alert will dismiss when application resigns active!" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
[alert show];
[[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationWillResignActiveNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification* notification){
[alert dismissWithClickedButtonIndex:0 animated:NO];
}];
回答6:
UIAlertView was deprecated in iOS 8 in favor of the UIAlertController. Unfortunately, this proves to be a tricky problem because the accepted solution won't work, as Apple explicitly doesn't support subclassing UIAlertController:
The UIAlertController class is intended to be used as-is and does not support subclassing. The view hierarchy for this class is private and must not be modified.
My solution is to simply traverse the view controller tree and dismiss all UIAlertControllers that you find. You can enable this globally by creating an extension of UIApplication and then calling it in the AppDelegate applicationDidEnterBackground method.
Try this (in Swift):
extension UIApplication
{
class func dismissOpenAlerts(base: UIViewController? = UIApplication.sharedApplication().keyWindow?.rootViewController)
{
//If it's an alert, dismiss it
if let alertController = base as? UIAlertController
{
alertController.dismissViewControllerAnimated(false, completion: nil)
}
//Check all children
if base != nil
{
for controller in base!.childViewControllers
{
if let alertController = controller as? UIAlertController
{
alertController.dismissViewControllerAnimated(false, completion: nil)
}
}
}
//Traverse the view controller tree
if let nav = base as? UINavigationController
{
dismissOpenAlerts(nav.visibleViewController)
}
else if let tab = base as? UITabBarController, let selected = tab.selectedViewController
{
dismissOpenAlerts(selected)
}
else if let presented = base?.presentedViewController
{
dismissOpenAlerts(presented)
}
}
}
And then in your AppDelegate:
func applicationDidEnterBackground(application: UIApplication)
{
UIApplication.dismissOpenAlerts()
}
回答7:
I Have had solved this with the following code:
/* taken from the post above (Cédric)*/
- (void)checkViews:(NSArray *)subviews {
Class AVClass = [UIAlertView class];
Class ASClass = [UIActionSheet class];
for (UIView * subview in subviews){
NSLog(@"Class %@", [subview class]);
if ([subview isKindOfClass:AVClass]){
[(UIAlertView *)subview dismissWithClickedButtonIndex:[(UIAlertView *)subview cancelButtonIndex] animated:NO];
} else if ([subview isKindOfClass:ASClass]){
[(UIActionSheet *)subview dismissWithClickedButtonIndex:[(UIActionSheet *)subview cancelButtonIndex] animated:NO];
} else {
[self checkViews:subview.subviews];
}
}
}
/*go to background delegate*/
- (void)applicationDidEnterBackground:(UIApplication *)application
{
for (UIWindow* window in [UIApplication sharedApplication].windows) {
NSArray* subviews = window.subviews;
[self checkViews:subviews];
}
}
回答8:
The straightforward way is to hold a reference to the UIAlertView so you can dismiss it. Of course as petert mentioned you can do it with a Notification or use the delegate method on UIApplication
applicationWillResignActive:
does not always mean that you are going to the background. You will for example also receive that delegate call and notification (you get both) when the user gets a phone call or receives and SMS. So you have to decide what should happen if the user gets an SMS and presses cancel to stay in your app. You maybe want to make sure that your UIAlertView is still there.
So I would dismiss the UIAlertView and save the state in the delegate call when you really go into the background:
applicationDidEnterBackground:
Have a look at Session 105 - Adopting Multitasking on iOS4 of WWDC10 available for free at developer.apple.com. It gets interesting at 16:00 min
Check out this graphic to understand the different states of an application
回答9:
I have this on my TODO list, but my first instinct would be to listen out for the notifcation UIApplicationWillResignActiveNotification (see UIApplication) in the views where you have things like UIAlertView - here you can programmatically remove the alert view with:
(void)dismissWithClickedButtonIndex:(NSInteger)buttonIndex animated:(BOOL)animated
The discussion for this method even suggests what it's for in iOS4!
In iPhone OS 4.0, you may want to call this method whenever your application moves to the background. An alert view is not dismissed automatically when an application moves to the background. This behavior differs from previous versions of the operating system, where they were canceled automatically when the application was terminated. Dismissing the alert view gives your application a chance to save changes or abort the operation and perform any necessary cleanup in case your application is terminated later.
回答10:
if you only have one or two specific alert windows you show (as do most apps), then you can just create an assign ivar to the alert:
@property (nonatomic, assign) UIAlertView* alertview;
Then, in the app delegate:
[self.viewController.alertview dismissWithClickedButtonIndex:[self.viewController.alertview cancelButtonIndex] animated:NO];
You can put this in applicationDidEnterBackground: or wherever you see fit. It closes the alert programmatically upon application exit. I've been doing this and it works great.
回答11:
Create category on UIAlert View
Use http://nshipster.com/method-swizzling/ Swizzle "show" method
Keep track of alert view shown by keeping week references in array.
- When you want to remove all data call Dismiss on saved alert views and empty an array.
回答12:
An alternative solution, based on plkEL's, answer, where the observer is removed when the app is put in the background. If user dismisses the alert by pressing a button, the observer will still be active, but only until the app is put in the background (where the block is run - with an "nil alertView" - and the observer removed).
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:title
message:message
delegate:alertDelegate
cancelButtonTitle:cancelButtonText
otherButtonTitles:okButtonText, nil];
[alert show];
__weak UIAlertView *weakAlert = alert;
__block __weak id observer = [[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationWillResignActiveNotification object:nil queue: [NSOperationQueue mainQueue] usingBlock:^(NSNotification* notification){
[weakAlert dismissWithClickedButtonIndex:[weakAlert cancelButtonIndex] animated:NO];
[[NSNotificationCenter defaultCenter] removeObserver:observer];
observer = nil;
}];
来源：https://stackoverflow.com/questions/3105974/dismissing-uialertviews-when-entering-background-state