Put app on foreground programmatically on Swift

Question

I tested:

UIControl().sendAction(#selector(URLSessionTask.suspend), to: UIApplication.shared, for: nil)

which is for putting app on background and it works.

How do I put app back on foreground?

I tried:

UIControl().sendAction(#selector(URLSessionTask.resume), to: UIApplication.shared, for: nil)

But eventually it crashes...

Thank you

Answer 1

I don't think it can be done without user interaction

The option is you can generate a push notification to tell the user to bring the application to foreground

When the operating system delivers push notification and the target application is not running in the foreground, it presents the notification.

If there is a notification alert and the user taps or clicks the action button (or moves the action slider), the application launches and calls a method to pass in the local-notification object or remote-notification payload.

Answer 2

Closing/opening the app should be done explicitly by the user. Any other way of closing or opening the app is not supported by Apple and will be rejected when uploaded to app store. iOS Human Interface Guideline states:

Don’t Quit Programmatically

Never quit an iOS application programmatically because people tend to interpret this as a crash. However, if external circumstances prevent your application from functioning as intended, you need to tell your users about the situation and explain what they can do about it. Depending on how severe the application malfunction is, you have two choices.

*Display an attractive screen that describes the problem and suggests a correction. A screen provides feedback that reassures usersthat there’s nothing wrong with your application. It puts usersin control, letting them decide whether they want to take corrective action and continue using your application or press the Home button and open a different application

*If only some of your application's features are not working, display either a screen or an alert when people activate the feature. Display the alert only when people try to accessthe feature that isn’t functioning

Answer 3

Update:

Since you've indicated that you're looking for any technical solution, even those not compatible with the App Store or Apple's terms, this should be possible using the Private API LSApplicationWorkspace: openApplicationWithBundleID. Try something like this:

Create a .h file and set up an interface to the LSApplicationWorkspace class and list the required method. You will need to #import "PrivateHeaders.h" in your bridging header.

//
// PrivateHeaders.h
//

#ifndef PrivateHeaders_h
#define PrivateHeaders_h

@interface LSApplicationWorkspace : NSObject

- (bool)openApplicationWithBundleID:(id)arg1;

@end

#endif /* PrivateHeaders_h */

You should then be able to call this function and pass in the Bundle Identifier of your app as an string.

//
// SomeClass.swift
//

import MobileCoreServices

let workspace = LSApplicationWorkspace()

/**
 Launch an App given its bundle identifier
 - parameter bundleIdentifier: The bundle identifier of the app to launch
 - returns: True if app is launched, otherwise false
 */
func openApp(withBundleIdentifier bundleIdentifier: String) -> Bool {
    // Call the Private API LSApplicationWorkspace method
    return workspace.openApplication(withBundleID: bundleIdentifier)
}

Original:

What you are doing is likely a violation of the iOS Human Interface Guidelines (although the "Don’t Quit Programmatically" is no longer specifically defined), so as the comments have said, it is not suited to the App Store. Regardless, once your app is suspended in this way, I don't expect that there is a way to resume it programmatically, unless you can hook into a Background Operation to run URLSessionTask.resume, but I have not tested it and am unsure whether it can work.

Apps can be launched (and hence brought into the foreground) programmatically from another app or today extension by using a Custom URL Scheme, or via a Push Notification. It isn't possible to launch the app from the Background Operation via a URL Scheme, since it is part of the UIKit framework, which must be run in the main thread.

In summary, I think your best option is to try to use a Notification. This just means that the user will need to click on the notification to bring your app back into the foreground.

Answer 4

Just as a follow up to Jordan's excellent answer I want to give an explanation for why your code works in the first place and why that alone will get your app rejected, even without any functionality to make it active again and bring it to the foreground.

As maddy pointed out in a comment, you're basically calling a method from UIApplication's private API. This works due to the Objective-C runtime's dynamic linking. You might wonder "But I am using Swift, what does that have to do with Objective-C?" The answer lies in #selector mechanism. A Selector is basically just a symbol that the Objective-C runtime looks up in a table to get a method it invokes (for you). This is why it's technically not correct to say you "call a method" when you do something like myObjectInstance.someMethod(). The correct way to phrase that would be to "send a message" to the object, because that's what is happening in the runtime. The target-action mechanism is build around that. The sendAction(_: Selector?, to: Any?) method does the same thing. So in effect your code does the following:

1. Get the symbol that corresponds to URLSessionTask's suspend() method.
2. Tell the shared instance of UIApplication to invoke the method that it has for that symbol.

Now usually that would result in a crash with the typical "unknown selector sent to instance..." error message. But here, by sure coincidence UIApplication also has a method for that instance (or rather, the runtime also has one of its methods listed in its table for that symbol). You kind of "found" a method that is not declared in its public header. You successfully circumvented a compile-time check for this and invoke a method that is part of a private API. This is explicitly forbidden in the Apple Developer Program License Agreement

Besides all that, I would strongly advise against trying to design an app that way in the first place. As maddy pointed out it's also likely considered to violate the HIGs. Even if you're not trying to do anything malicious and properly explain the feature in your app's description, that won't make Apple let it slide (I assume). Personally, as a user, I'd also find it annoying if the app did something the system already has a specific mechanic for in a different manner, at least in terms of app's coming to background and foreground.