iPhone Production/Release Exception Handling

Question

In advance, please excuse my lack of understanding for iPhone/Objective-C Best Practices; I come from a .NET/C# background.

Although, I've read many posts regarding exception handling for the iPhone, I'm still not exactly clear on what most people do for production code. Also, I haven't been able to find any open source apps with error handling that I would normally expect. Here are my questions:

1) If an unexpected result occurs that would cause the application to eventually fail, would you throw an exception or just wait for it to fail later? For example,

if (![fileManager createDirectoryAtPath: myNewDir
    withIntermediateDirectories: YES 
    attributes: nil 
    error: &myError]) {

    // My app shouldn't continue. Should I raise an exception? 
    // Or should I just log it and then wait for it to crash later?
}

2) Do you validate parameters? For example, in C# I would usually check for null, and if needed throw an ArgumentNullException.

3) When an app crashes, does the crash info get logged automatically or do I need to set the unhandled exception handler? Can I show a UIAlertView within this method to inform the user something bad happened, instead of having the app just disappear? (If so, I couldn't get it to work.)

4) And finally, why don't I see anyone actually using @try/@catch/@finally? It's used extensively in C#, but I haven't found open source apps using it. (Maybe I'm just looking at the wrong apps.)

Thank you so much. And sample code is much appreciated!

Answer 1

Ad 1. The example you give is a somewhat classical example of when one should use a "checked exception" in Java. The caller of the method has to be prepared, that the call can fail for reasons which are outside of what it can control. In particular, in the example given, I would allow the error descriptor object allow to propagate back to the caller:

- (BOOL) prepareDirectories: (NSString*) path error: (NSError**) location {

    if( ![fileManager createDirectoryAtPath: path
                      withIntermediateDirectories: YES 
                      attributes: nil 
                      error: location] ) {

        return NO;
    }

    // ... additional set-up here
    return YES;
}

If the error is really not recoverable, you may actually raise an exception, but that will also forfeit any chance to show a proper error message to the user of your application.

Ad 2. Yes, parameter validation is definitely something you should do. And raising an exception for unexpected nils is entirely appropriate. A bad parameter is a "programmer's error" and you cannot guarantee, that the program is still in a consistent state. For example, NSArray raises an exception, if you try to get an element using an non-existent index.

Ad 3. When the app crashes due to an uncaught exception, the fact is logged automatically. You can catch the exception if you really want to display an error message. However, the app is likely to be in an inconsistent state, and should not be allowed to continue, so simply allowing it to go down seems the best solution here.

Ad 4. I don't know.

Answer 2

1. In the specific case you mention, you should present a notification to the user about what just happened with instructions on how to fix the situation. Except in extreme circumstances your app should not quit. You should let the user fix whatever is wrong, then try again.
2. Parameter validation depends on a lot of factors. One thing to keep in mind is that in Obj-C it's perfectly valid to send messages to nil (nothing happens if you do) so in a lot of situations you don't need to check for nil. If I'm in a model class I always validate parameters in methods. If I'm not I almost never validate anything, type checking is good enough.
3. Some information should be logged but it's always helpful to log more specific information to your situation. Ideally you shouldn't ever reach the unhandled exception state.
4. Cocoa classes will rarely throw exceptions for environmental reasons, they are almost always because you did something wrong. For example, you wouldn't usually put a @try/@catch block around [NSString stringWithString:] because the only way it'll throw an exception is if you try and pass nil. Make sure you aren't passing nil and you won't have to worry about catching exceptions. When a method might fail because of something outside of your control, they usually communicate via an NSError. See the - (BOOL)save:(NSError **)error method of NSManagedObjectContext for example.