Graphics Programs Reference
In-Depth Information
Exceptions and Unrecognized Selectors
An object only responds to a message if its class implements the associated method.
Objective-C is a dynamically-typed language, so it can't always figure out at compile time
(when the application is built) whether an object will respond to a message.
Xcode
will
give you an error if it thinks you are sending a message to an object that won't respond, but
if it isn't sure, it will let the application build.
If, for some reason (and there are many), you end up sending a message to an object that
doesn't respond, your application will throw an
exception
. Exceptions are also known as
run-time errors
because they occur once your application is running as opposed to
compile-
time errors
that show up when your application is being built, or compiled. (We'll come
back to compile-time errors in
Chapter 4
.)
To practice dealing with exceptions, we're going to cause one in
RandomPossessions
. In
BNRItem.h
, declare a new method:
@interface BNRItem : NSObject
{
NSString *itemName;
NSString *serialNumber;
int valueInDollars;
NSDate *dateCreated;
}
- (void)doSomethingWeird;
+ (id)randomItem;
You are going to send the message
doSomethingWeird
to an instance of
BNRItem
.
The problem? You didn't implement
doSomethingWeird
in
BNRItem.m
- you only
declared it in
BNRItem.h
. Therefore,
BNRItem
does not implement
doSo-
methingWeird
, and an exception will be thrown. In
main.m
, send this message to a
BNRItem
.
for (int i = 0; i < 10; i++) {
BNRItem *p = [BNRItem randomItem];
[p doSomethingWeird];
[items addObject:p];
}
Build and run the application. Your application will compile, start running, and then halt.
Check your console and find the line that looks like this:
2011-11-14 12:23:47.990 RandomPossessions[10288:707] ***
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason:
