How to detect if code is running in Main App or App Extension Target?

As Apple's documentation says:

When you build an extension based on an Xcode template, you get an extension bundle that ends in .appex.

So, we can use the following code:

if ([[[NSBundle mainBundle] bundlePath] hasSuffix:@".appex"]) {
    // this is an app extension
}

// Swift version
if Bundle.main.bundlePath.hasSuffix(".appex") {
    // this is an app extension
}

The preprocessor macro will work mostly, but will not work in shared library (e.g. cocoapods or shared frameworks).

Alternatively you can use following code.

@implementation ExtensionHelpers

+(BOOL) isAppExtension
{
    return [[[NSBundle mainBundle] executablePath] containsString:@".appex/"];
}

@end

This work by checking the bundle executablePath, as only App Extension has extension ".appex".

Swift 5

let bundleUrl: URL = Bundle.main.bundleURL
let bundlePathExtension: String = bundleUrl.pathExtension
let isAppex: Bool = bundlePathExtension == "appex"

// `true` when invoked inside the `Extension process`
// `false` when invoked inside the `Main process`

You can use a preprocessor macro:

In the project settings use the dropdown in the topbar to select your extension target:

Then:

1. Click Build Settings
2. Find (or search) Preprocessor Macros under Apple LLVM 6.0 - Preprocessing
3. Add TARGET_IS_EXTENSION or any other name of your choice in both the debug and release sections.

Then in your code:

#ifndef TARGET_IS_EXTENSION // if it's not defined
    // Do your calls to UIApplication
#endif