Error: unrecognized selector sent to instance

The error means an object does not respond to a selector sent to it. Various reasons can cause this error. Today I found another one: assigning to instance variable in class method can corrupt an object and cause it not to respond to a selector which it should.

For example, you have a view controller vc. By all means you expect [vc.view addSubview:anotherView] should work but you got error:

(null): unrecognized selector sent to instance 0xblah

Then you found that this view controller has a class method which updates some instance variables. Since it worked before, you think it is OK to do that. However, after some other coding, the application was suddenly broken, and you got that error. You can still NSLog(@"%@",vc.view) and it seems to be fine, but when you check [vc.view respondsToSelector:@selector(addSubview:)] you get NO.

This is totally insane. Then you noticed the compiler output:

warning: instance variable 'adView' accessed in class method

Then you remember you did something dubious: modifying instance variable in class method. In C++ such code won't compile but in objective-c it is just a warning.

After fixing this, the error disappeared.

The lessons learned:
1. Do not access instance variables in class method. Even if it works today, it may break tomorrow.
2. Pay attention to warnings. They said bad C++ code is like bullets which can shoot your toe. I would say objective-c is even worse.

UIViewController viewWillAppear not called

If you add the view of a UITabBarController as subview to another view controller. When you tap the tab bar to swap in a new view, viewWillAppear of that view's controller will not be called. This causes problem because many times you use viewWillAppear to refresh a view managed by UITabBarController.

One workaround is to subclass UIView for the view you want to update, and move the stuff from viewWillAppear to drawRect.

The Simplest Way to Add iAd Banner to Your iPhone Application

It is a bad idea to modify each view controller to add iAd banner. The simplest way to do that is to create a new root view controller and add an AdBannerView and your old root view to it. Do it in your application delegate:

ADBannerView *adView = [[ADBannerView alloc] initWithFrame:CGRectZero];

adView.currentContentSizeIdentifier = ADBannerContentSizeIdentifier320x50;

adView.frame=CGRectMake(0, 0, 320, 50);

AdVC* rootVC = [[AdVC alloc] initWithNibName:@"AdVC" bundle:nil];

[window addSubview:rootVC.view];

[rootVC.view addSubview:oldRootVC.view];

[rootVC.view addSubview:adView];

rootVC.view.frame = window.screen.applicationFrame;

oldRootVC.view.frame=CGRectMake(0, 50, 320, 480-50-20);

This will add a banner to all of your views, and it also allows you to easily remove the banner if the user choose to upgrade.

How to fix status bar covering UIView

Sometime it is annoying that the status bar covers your UIView, and there are various other problems about the view location.

A simple fix is

rootVC.view.frame = window.screen.applicationFrame;

Xcode error: No launchable executable present at path

If this error shows up after the name of the executable changes in Xcode, restarting Xcode could solve this problem.

Cocos2d v0.99.5-rc1 not working with Xcode 3.2.4

There are lots of compilation errors using the default project file of cocos2d v0.99.5-rc1 with Xcode 3.2.4.

Use gcc or llvm-gcc instead of llvm, then it is OK.

How to Enter Copyright Symbol in Inkscape

When inputing text, press Control to enter Unicode, then enter 00a9 for copyright symbol.