Скачать или смотреть Exception handling in Objective-C

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When we programming in Objective-C, there are situations when some unexpected things may appear. For example, an object might get a type when it should be of a totally different type. And when we try to call some methods on that object, we get our program crashed, with an "unrecognised selector send to that specific object" exception.

To deal around that, we should use the @try {} @catch {} @finally {} block. All the modern object oriented programming language have such syntax, and there are many situations where we can make use of it.

Basically, inside the @try section, we insert our code that generated the exception(s). Than if one appear, we have to handle that in the @catch block. @catch has a NSException object as a parameter, and we can use it to check different things (what exactly is the type of exception, where it came from etc.). @finally is executed wether am exception was raised or not. As a more practical example, say we are working with a Database, and we have a connection opened with the DB, and after we successfully executed some queries to the DB, when trying to manipulate the result - our program crashed. We can use the @try {} @catch {} @finally {} block in this case. In @try section we'll do all the heavy work. If an @catch section we log something to the Console, or give some alert to the user in case of a Graphical User Interface. But the connection is still opened :(. And the solution is to fix that in the finally block :). If it's opened, close it, and also release the other allocated resources.