iOS how to catch exception unrecognized selector sent to instance when target object is NULL already?

I read alot about what is the problem for exception "unrecog..." But i need something else

I have view with two buttons: Start and Delete, also i have two UILabels: oneLabel and secondLabel

So the I press button Start I start NSOperation thread started

And I give him labels (oneLabel,secondLabel) as params

to change text of labels on main loop i use

[oneLabel performSelectorOnMainThread:@selector(setText:) withObject:someString waitUntilDone:YES];

All works fine, but when i press button Delete - it's deleting secondLabel from view with method

[secondLabel removeFromSuperview] 

and then

s开发者_开发问答econdLabel=nil;

So, after what i get exception. I understand why it happened - because the target object for message with selector setText if not available now becaus it's nil.

And i get exception and app crashes.

How i can Catch this exception in this case?

For what i need it for? When use tableView controller with ImageView wich loads images in separate thread.

try this:

id yourObject;
    if (yourObject != nil && [yourObject respondsToSelector:@selector(yourSelector)]) {
    // Do your stuff here
}

This will only call your method to be executed if your objects is still available and it responds to the specifed selector.

Hope this helps, Vlad

setText: it is not an object, it should be a method in your implementation. someString is the object that is send to the setText method.

When the command [oneLabel performSelectorOnMainThread:@selector(setText:) withObject:someString waitUntilDone:YES]; is executed, then the method setText is called.

You are seeing unrecognized selector sent to instance because the method
-(void)setText:(id)sender does not exist or you have misspelled it.

You dont need to catch the exception. You can make a method that waits for the selector to excecute and catches it when it is really available.

You need to probably remake the secondLabel if I understood correctly because you are deleting it. Do this first and the exception won't be there to catch.

This is not a problem to send a message to nil, it is legal. Unrecognised selector exception means you have wrong spelling in your method name. A common mistake is to forget the colon if the method requires a parameter. For example @selector(setText) when you meant @selector(setText:).