[osx] Allow the user to configure a NSApplication subclass and use that instead.

Fixes http://hipbyte.myjetbrains.com/youtrack/issue/RM-539

lib/motion/project/template/osx/config.rb

@@ -230,20 +230,36 @@ EOS
     end
     main_txt << <<EOS
     RubyMotionInit(argc, argv);
-    NSApplication *app = [NSApplication sharedApplication];
+    NSApplication *app = [NSClassFromString(@"#{info_plist['NSPrincipalClass']}") sharedApplication];
     [app setDelegate:[NSClassFromString(@"#{delegate_class}") new]];
 EOS
     if spec_mode
       main_txt << "SpecLauncher *specLauncher = [[SpecLauncher alloc] init];\n"
       main_txt << "[[NSNotificationCenter defaultCenter] addObserver:specLauncher selector:@selector(appLaunched:) name:NSApplicationDidFinishLaunchingNotification object:nil];\n"
     end
+    if use_application_main_function?
+      main_txt << "NSApplicationMain(argc, (const char **)argv);\n"
+    else
+      main_txt << "[app run];\n"
+    end
     main_txt << <<EOS
-    NSApplicationMain(argc, (const char **)argv);
     [pool release];
     rb_exit(0);
     return 0;
 }
 EOS
     end
+
+    # If the user specifies a custom principal class the NSApplicationMain()
+    # function will only work if they have also specified a nib or storyboard.
+    def use_application_main_function?
+      info = info_plist
+      if info['NSPrincipalClass'] == 'NSApplication'
+        true
+      else
+        files = info.values_at('NSMainNibFile', 'NSMainStoryboardFile').compact
+        files.any? { |file| !file.strip.empty? }
+      end
+    end
   end
 end; end

test/test/spec/main_spec.rb

@@ -255,7 +255,7 @@ describe "NSNotFound" do
 end
 
 describe "Application singleton subclassing" do
-  xit "registers the principal class" do
+  it "registers the principal class" do
     app = (ios? ? UIApplication : NSApplication).sharedApplication
     app.class.should == ApplicationSubclass
   end