blocks|key|341567|text|只需让视图控制器注册UIApplicationWillEnterForegroundNotification通知并做出相应的反应即可。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|341568|entityMap^0|A|18|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

Just have your view controller register for the <code>UIApplicationWillEnterForegroundNotification</code> notification and react accordingly.

blocks|key|123672|text|出于对事件的确切顺序的好奇，我检测了一个应用程序，如下所示：(@Zohaib，您可以使用下面的NSNotificationCenter代码来回答您的问题)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|123673|//+AppDelegate.m

-+(void)applicationWillEnterForeground:(UIApplication+*)application
{
++++NSLog(@"app+will+enter+foreground");
}

-+(void)applicationDidBecomeActive:(UIApplication+*)application
{
++++NSLog(@"app+did+become+active");
}

//+ViewController.m

-+(void)viewDidLoad
{
++++[super+viewDidLoad];
++++NSLog(@"view+did+load");

++++[[NSNotificationCenter+defaultCenter]+addObserver:self+selector:@selector(appDidBecomeActive:)+name:UIApplicationDidBecomeActiveNotification+object:nil];
++++[[NSNotificationCenter+defaultCenter]+addObserver:self+selector:@selector(appWillEnterForeground:)+name:UIApplicationWillEnterForegroundNotification+object:nil];
}

-+(void)appDidBecomeActive:(NSNotification+*)notification+{
++++NSLog(@"did+become+active+notification");
}

-+(void)appWillEnterForeground:(NSNotification+*)notification+{
++++NSLog(@"will+enter+foreground+notification");
}

-+(void)viewWillAppear:(BOOL)animated+{
++++[super+viewWillAppear:animated];
++++NSLog(@"view+will+appear");
}

-+(void)viewDidAppear:(BOOL)animated+{
++++[super+viewDidAppear:animated];
++++NSLog(@"view+did+appear");
}|code-block|syntax|javascript|123674|在启动时，输出如下所示：|123675|2013-04-07+09:31:06.505+myapp[15459:11303]+view+did+load
2013-04-07+09:31:06.507+myapp[15459:11303]+view+will+appear
2013-04-07+09:31:06.511+myapp[15459:11303]+app+did+become+active
2013-04-07+09:31:06.512+myapp[15459:11303]+did+become+active+notification
2013-04-07+09:31:06.517+myapp[15459:11303]+view+did+appear|123676|进入背景，然后重新进入前台：|123677|2013-04-07+09:32:05.923+myapp[15459:11303]+app+will+enter+foreground
2013-04-07+09:32:05.924+myapp[15459:11303]+will+enter+foreground+notification
2013-04-07+09:32:05.925+myapp[15459:11303]+app+did+become+active
2013-04-07+09:32:05.926+myapp[15459:11303]+did+become+active+notification|123678|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

Curious about the exact sequence of events, I instrumented an app as follows: (@Zohaib, you can use the NSNotificationCenter code below to answer your question).

<pre><code>// AppDelegate.m

- (void)applicationWillEnterForeground:(UIApplication *)application
{
 NSLog(@"app will enter foreground");
}

- (void)applicationDidBecomeActive:(UIApplication *)application
{
 NSLog(@"app did become active");
}

// ViewController.m

- (void)viewDidLoad
{
 [super viewDidLoad];
 NSLog(@"view did load");

 [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appDidBecomeActive:) name:UIApplicationDidBecomeActiveNotification object:nil];
 [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appWillEnterForeground:) name:UIApplicationWillEnterForegroundNotification object:nil];
}

- (void)appDidBecomeActive:(NSNotification *)notification {
 NSLog(@"did become active notification");
}

- (void)appWillEnterForeground:(NSNotification *)notification {
 NSLog(@"will enter foreground notification");
}

- (void)viewWillAppear:(BOOL)animated {
 [super viewWillAppear:animated];
 NSLog(@"view will appear");
}

- (void)viewDidAppear:(BOOL)animated {
 [super viewDidAppear:animated];
 NSLog(@"view did appear");
}
</code></pre>

At launch, the output looks like this:

<pre><code>2013-04-07 09:31:06.505 myapp[15459:11303] view did load
2013-04-07 09:31:06.507 myapp[15459:11303] view will appear
2013-04-07 09:31:06.511 myapp[15459:11303] app did become active
2013-04-07 09:31:06.512 myapp[15459:11303] did become active notification
2013-04-07 09:31:06.517 myapp[15459:11303] view did appear
</code></pre>

Enter the background then reenter the foreground:

<pre><code>2013-04-07 09:32:05.923 myapp[15459:11303] app will enter foreground
2013-04-07 09:32:05.924 myapp[15459:11303] will enter foreground notification
2013-04-07 09:32:05.925 myapp[15459:11303] app did become active
2013-04-07 09:32:05.926 myapp[15459:11303] did become active notification
</code></pre>

blocks|key|341636|text|我认为注册UIApplicationWillEnterForegroundNotification是有风险的，因为您可能最终会有多个控制器对该通知做出反应。当收到通知时，没有任何保证这些控制器仍然可见。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|341637|下面是我要做的:我直接从应用的委托didBecomeActive方法中强制调用active控制器上的viewDidAppear：|341638|将以下代码添加到-+(void)applicationDidBecomeActive:(UIApplication+*)application|offset|length|style|CODE|341639|UIViewController+*activeController+=+window.rootViewController;
if+([activeController+isKindOfClass:[UINavigationController+class]])+{
++++activeController+=+[(UINavigationController*)window.rootViewController+topViewController];
}
[activeController+viewDidAppear:NO];|code-block|syntax|javascript|341640|entityMap^0|0|0|8|1R|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|S|8|@$F|T|G|U|H|I]]|9|@]|A|$]]|$1|J|3|K|5|L|7|V|8|@]|9|@]|A|$M|N]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

I think registering for the UIApplicationWillEnterForegroundNotification is risky as you may end up with more than one controller reacting to that notification. Nothing garanties that these controllers are still visible when the notification is received.

Here is what I do: I force call viewDidAppear on the active controller directly from the App's delegate didBecomeActive method:

Add the code below to <code>- (void)applicationDidBecomeActive:(UIApplication *)application</code>

<pre><code>UIViewController *activeController = window.rootViewController;
if ([activeController isKindOfClass:[UINavigationController class]]) {
 activeController = [(UINavigationController*)window.rootViewController topViewController];
}
[activeController viewDidAppear:NO];
</code></pre>

blocks|key|341673|text|尝试在AppDelegate+applicationWillEnterForeground中添加此内容。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|341674|func+applicationWillEnterForeground(_+application:+UIApplication)+{++++++++
++++//+makes+viewWillAppear+run
++++self.window?.rootViewController?.beginAppearanceTransition(true,+animated:+false)
++++self.window?.rootViewController?.endAppearanceTransition()
}|code-block|syntax|javascript|341675|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

try adding this in AppDelegate applicationWillEnterForeground.

<pre><code>func applicationWillEnterForeground(_ application: UIApplication) { 
 // makes viewWillAppear run
 self.window?.rootViewController?.beginAppearanceTransition(true, animated: false)
 self.window?.rootViewController?.endAppearanceTransition()
}
</code></pre>

blocks|key|123801|text|Swift+3.0+%2B%2B版本|type|unstyled|depth|inlineStyleRanges|entityRanges|data|123802|在您的viewDidLoad中，在通知中心注册以收听从后台打开的操作|offset|length|style|CODE|123803|NotificationCenter.default.addObserver(self,+selector:#selector(doSomething),+name:+NSNotification.Name.UIApplicationWillEnterForeground,+object:+nil)|code-block|syntax|javascript|123804|然后添加此函数并执行所需的操作|123805|func+doSomething(){
++++//...
}|123806|最后，添加此函数，以便在视图控制器被销毁时清理通知观察者。|123807|deinit+{
++++NotificationCenter.default.removeObserver(self)
}|123808|entityMap^0|0|3|B|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|10|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|11|8|@]|9|@]|A|$]]|$1|O|3|P|5|J|7|12|8|@]|9|@]|A|$K|L]]|$1|Q|3|R|5|6|7|13|8|@]|9|@]|A|$]]|$1|S|3|T|5|J|7|14|8|@]|9|@]|A|$K|L]]|$1|U|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|V|$]]

<h1>Swift 3.0 ++ version</h1>
In your <code>viewDidLoad</code>, register at notification center to listen to this opened from background action
<pre><code>NotificationCenter.default.addObserver(self, selector:#selector(doSomething), name: NSNotification.Name.UIApplicationWillEnterForeground, object: nil)
 
</code></pre>
Then add this function and perform needed action
<pre><code>func doSomething(){
 //...
}
</code></pre>
Finally add this function to clean up the notification observer when your view controller is destroyed.
<pre><code>deinit {
 NotificationCenter.default.removeObserver(self)
}
</code></pre>

blocks|key|341749|text|Swift+4.2。版本|type|unstyled|depth|inlineStyleRanges|entityRanges|data|341750|在viewDidLoad中注册NotificationCenter，以便在应用程序从后台返回时收到通知|offset|length|style|CODE|341751|NotificationCenter.default.addObserver(self,+selector:+#selector(doSomething),+name:+UIApplication.willEnterForegroundNotification,+object:+nil)|code-block|syntax|javascript|341752|实现应该调用的方法。|341753|@objc+private+func+doSomething()+{
++++//+Do+whatever+you+want,+for+example+update+your+view.
}|341754|一旦ViewController被销毁，您可以移除观察者。这仅在iOS9和macOS+10.11版本中是必需的|341755|deinit+{
++++NotificationCenter.default.removeObserver(self)
}|341756|entityMap^0|0|1|B|0|0|0|0|2|E|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|10|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|11|8|@]|9|@]|A|$]]|$1|O|3|P|5|J|7|12|8|@]|9|@]|A|$K|L]]|$1|Q|3|R|5|6|7|13|8|@$D|14|E|15|F|G]]|9|@]|A|$]]|$1|S|3|T|5|J|7|16|8|@]|9|@]|A|$K|L]]|$1|U|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|V|$]]

<h2>Swift 4.2. version</h2>

Register with the NotificationCenter in <code>viewDidLoad</code> to be notified when the app returns from background

<pre><code>NotificationCenter.default.addObserver(self, selector: #selector(doSomething), name: UIApplication.willEnterForegroundNotification, object: nil)
</code></pre>

Implement the method that should be called.

<pre><code>@objc private func doSomething() {
 // Do whatever you want, for example update your view.
}
</code></pre>

You can remove the observer once the <code>ViewController</code> is destroyed. This is only required below iOS9 and macOS 10.11

<pre><code>deinit {
 NotificationCenter.default.removeObserver(self)
}
</code></pre>

I have a View Controller in which my value is 0 (label) and when I open that View Controller from another <code>ViewController</code> I have set <code>viewDidAppear</code> to set value 20 on label. It works fine but when I close my app and than again I open my app but the value doesn't change because <code>viewDidLoad</code>, <code>viewDidAppear</code> and <code>viewWillAppear</code> nothing get called. How can I call when I open my app. Do I have to do anything from <code>applicationDidBecomeActive</code>?

ViewDidAppear is not called when opening app from background

Swift

我有一个视图控制器，其中我的值是0(标签)，当我从另一个ViewController打开这个视图控制器时，我已经将viewDidAppear设置为标签上的值20。它工作得很好，但当我关闭我的应用程序，然后再次打开我的应用程序，但值不会改变，因为viewDidLoad，viewDidAppear和viewWillAppe...

问后台打开应用时不调用ViewDidAppear
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