如何捕获发送到实例异常的无法识别的选择器？

Question

I am getting after some time unrecognized selector sent to instance exception. When i get this i want just skip it and my app should work.

However i don't know how to catch. As this don't catch:

@property(nonatomic,retain) UIButton *button;
    @try{

       if(button.currentBackgroundImage == nil){//rises exception 
    }
    }@catch(NSException *e){
}

How i could handle this ?

Thanks.

Answer 1

The technique I use and see often is: instead of catching the exception, check if the object responds to the selector:

if(![button respondsToSelector:@selector(currentBackgroundImage)] || button.currentBackgroundImage == nil) {
  // do your thing here...
}

Answer 2

If you are getting this exception, it means there is a design flaw, a bug in your code. Patching it by ignoring the exception is not the right thing to do. Try to pin down why you are sending the wrong message to the wrong object instead. Your code will become more robust and maintainable.

Also, sometimes you get this exception when the object originally was of the right type, but is halfway in the process of being deallocated. Watch out!

If you still want to bypass the exception, read Apple's docs where it explains the multi-step process by which messages are bound to method implementations at run time. There is at least two places where you can catch it by overriding NSObject's default behavior.

Answer 3

I understand the answers telling you to prevent the unrecognized selector since that is the preferred method.

But in the case where you do not have that option (such as in my case, Cocoa internals messing up further down the call stack) you can indeed catch the unrecognized selector as you tried.

Proof of concept code:

// Do a really bad cast from NSObject to NSButton
// to get something to demonstrate on
NSButton *object = (NSButton*)[[NSObject alloc] init];

@try{
    // Log the description as the method exists 
    // on both NSObject and NSButton
    NSLog(@"%@", [object description]);

    // Send an unrecognized selector to NSObject
    [object bounds];
} @catch(NSException *e){
    NSLog(@"Catch");
} @finally {
    NSLog(@"Finally");
}

// Print the description to prove continued execution
NSLog(@"Description again: %@", [object description]);

Output:

2019-02-26 14:11:04.246050+0100 app[46152:172456] <NSObject: 0x60000000a6f0>
2019-02-26 14:11:04.246130+0100 app[46152:172456] -[NSObject bounds]: unrecognized selector sent to instance 0x60000000a6f0
2019-02-26 14:11:04.246226+0100 app[46152:172456] Catch
2019-02-26 14:11:04.246242+0100 app[46152:172456] Finally
2019-02-26 14:11:04.246258+0100 app[46152:172456] Description again: <NSObject: 0x60000000a6f0>

As you can see the exception is still logged to the console, but the code execution continues.