Add compiler flag to disable UIApplication calls (#280)

Specifying `P2_APP_EXTENSION` ensures the framework can be compiled for an app extension.

Author: MaximeLM

CONTRIBUTORS.md

@@ -3,6 +3,7 @@ Contributors
 
 Contributors to the codebase, in reverse chronological order:
 
+- Maxime Le Moine, @MaximeLM
 - Seb Skuse, @sebskuse
 - David Hardiman, @dhardiman
 - Amaury David, @amaurydavid

Sources/iOS/OAuth2Authorizer+iOS.swift

@@ -55,17 +55,13 @@ open class OAuth2Authorizer: OAuth2AuthorizerUI {
 	*/
 	public func openAuthorizeURLInBrowser(_ url: URL) throws {
 
-		// By asking for the shared instance method by using the "value for key" method on UIApplication, we are able to
-		// bypass the Swift compilation restriction that blocks the library from being compiled for an extension when 
-		// directly referencing it. We do it as an optional so in the advent of this method being called, like in an 
-		// extension, we handle it as though its not supported.
-		guard let application = UIApplication.value(forKey: "sharedApplication") as? UIApplication else {
-			throw OAuth2Error.unableToOpenAuthorizeURL
-		}
-		
-		if !application.openURL(url) {
+		#if !P2_APP_EXTENSIONS
+		if !UIApplication.shared.openURL(url) {
 			throw OAuth2Error.unableToOpenAuthorizeURL
 		}
+		#else
+		throw OAuth2Error.unableToOpenAuthorizeURL
+		#endif
 	}
 
 	/**