Controlling the screenshot in the iOS 7 multitasking switcher

This is the solution I worked with for my application:

As Tommy said: You can use the applicationWillResignActive. What I did was making a UIImageView with my SplashImage and add it as subview to my main window-

(void)applicationWillResignActive:(UIApplication *)application
{
    imageView = [[UIImageView alloc]initWithFrame:[self.window frame]];
    [imageView setImage:[UIImage imageNamed:@"Portrait(768x1024).png"]];
    [self.window addSubview:imageView];
}

I used this method instead of applicationDidEnterBackground because applicationDidEnterBackground won't be triggered if you doubletap the home button, and applicationWillResignActive will be. I heard people say though it can be triggered in other cases aswell, so I'm still testing around to see if it gives problem, but none so far! ;)

Here to remove the imageview:

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    if(imageView != nil) {
        [imageView removeFromSuperview];
        imageView = nil;
    }
}

Hope this helps!

Sidenote: I tested this on both the simulator and a real device: It Won't Show on the simulator, but it does on a real device!

In Preparing Your UI to Run in the Background, Apple says:

Prepare Your UI for the App Snapshot

At some point after your app enters the background and your delegate method returns, UIKit takes a snapshot of your app’s current user interface. The system displays the resulting image in the app switcher. It also displays the image temporarily when bringing your app back to the foreground.

Your app’s UI must not contain any sensitive user information, such as passwords or credit card numbers. If your interface contains such information, remove it from your views when entering the background. Also, dismiss alerts, temporary interfaces, and system view controllers that obscure your app’s content. The snapshot represents your app’s interface and should be recognizable to users. When your app returns to the foreground, you can restore data and views as appropriate.

See Technical Q&A QA1838: Preventing Sensitive Information From Appearing In The Task Switcher

In addition to obscuring/replacing sensitive information, you might also want to tell iOS 7 to not take the screen snapshot via ignoreSnapshotOnNextApplicationLaunch, whose documentation says:

If you feel that the snapshot cannot correctly reflect your app’s user interface when your app is relaunched, you can call ignoreSnapshotOnNextApplicationLaunch to prevent that snapshot image from being taken.

Having said that, it appears that the screen snapshot is still taken and I have therefore filed a bug report. But you should test further and see if using this setting helps.

If this was an enterprise app, you might also want to look into the appropriate setting of allowScreenShot outlined in the Restrictions Payload section of the Configuration Profile Reference.

---

Here is an implementation that achieves what I needed. You can present your own UIImageView, or your can use a delegate-protocol pattern to obscure the confidential information:

//  SecureDelegate.h

#import <Foundation/Foundation.h>

@protocol SecureDelegate <NSObject>

- (void)hide:(id)object;
- (void)show:(id)object;

@end

I then gave my app delegate a property for that:

@property (weak, nonatomic) id<SecureDelegate> secureDelegate;

My view controller sets it:

- (void)viewDidLoad
{
    [super viewDidLoad];

    AppDelegate *delegate = [[UIApplication sharedApplication] delegate];
    delegate.secureDelegate = self;
}

The view controller obviously implements that protocol:

- (void)hide:(id)object
{
    self.passwordLabel.alpha = 0.0;
}

- (void)show:(id)object
{
    self.passwordLabel.alpha = 1.0;
}

And, finally, my app delegate avails itself of this protocol and property:

- (void)applicationWillResignActive:(UIApplication *)application
{
    [application ignoreSnapshotOnNextApplicationLaunch];  // this doesn't appear to work, whether called here or `didFinishLaunchingWithOptions`, but seems prudent to include it

    [self.secureDelegate hide:@"applicationWillResignActive:"];  // you don't need to pass the "object", but it was useful during my testing...
}

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    [self.secureDelegate show:@"applicationDidBecomeActive:"];
}

Note, I'm using applicationWillResignActive rather than the advised applicationDidEnterBackground, because, as others have pointed out, the latter is not called when double tapping on the home button while the app is running.

I wish I could use notifications to handle all of this, rather than the delegate-protocol pattern, but in my limited testing, the notifications aren't handled in a timely-enough manner, but the above pattern works fine.

This quick and easy method will yield a black snapshot above your app's icon in the iOS7 or later app switcher.

First, take your app's key window (typically setup in AppDelegate.m in application:didFinishLaunchingWithOptions), and hide it when your app is about to move into the background:

- (void)applicationWillResignActive:(UIApplication *)application
{
    if(isIOS7Or8)
    {
        self.window.hidden = YES;
    }
}

Then, un-hide your app's key window when your app becomes active again:

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    if(isIOS7Or8)
    {
        self.window.hidden = NO;
    }
}

At this point, check out the app switcher and verify that you see a black snapshot above your app's icon. I've noticed that if you launch the app switcher immediately after moving your app into the background, there can be a delay of ~5 seconds where you'll see a snapshot of your app (the one you want to hide!), after which it transitions to an all-black snapshot. I'm not sure what's up with the delay; if anyone has any suggestions, please chime in.

If you want a color other than black in the switcher, you could do something like this by adding a subview with any background color you'd like:

- (void)applicationWillResignActive:(UIApplication *)application
{
    if(isIOS7Or8)
    {
        UIView *colorView = [[[UIView alloc] initWithFrame:self.window.frame] autorelease];
        colorView.tag = 9999;
        colorView.backgroundColor = [UIColor purpleColor];
        [self.window addSubview:colorView];
        [self.window bringSubviewToFront:colorView];
    }
}

Then, remove this color subview when your app becomes active again:

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    if(isIOS7Or8)
    {
        UIView *colorView = [self.window viewWithTag:9999];
        [colorView removeFromSuperview];
    }
}