Error propagatin inside try/catch in cocoa - objective-c

Question

I want to raise an exception inside a catch block of a method and handle it in catch of another method from where earlier method was called.
I tried this logic-
method B()
{
#try{
.......
// calling method where is probable chance of exception
method A();
}
catch(NSException e)
{
//catching the exception thrown in the method B()
NSString* theError=[e reason];
NSLog(#"the error is == %#",theError);
}
}
method A()
{
#try{
.............
//throw an exception incase of some condition
throw e;
}
catch(NSException e)
{
//rethrowing the exception, want to catch in the method from where this method is called.
throw e;
}
}
But the catch block of method B() is never accessible.
The control never returns to the catch block of method B().
Please suggest.
Thanks,
Sudhansu
Here is a bit of code. I am calling a method (populateData) of TableController from MyController.
The exception supposed to occur in another method of TableController(initializeTest) and i am throwing it
inside try block of FinderCompleted method. In the catch block of same method , rethrowing the exception, as
I don't want to handle it here.
The control is restricted only to the innermost catch block of method - (void)FinderCompleted:(id)args
the NSLog prints like this-
m here 1
Inside the error
You got nothing in ur bucket :D
Throwing exception for the 1st time
Gotcha--1st time exception , throwing it 2nd time
the error is == Something unexpected happened --EXCEPTION
After that i don't know where the control is going. I want the control to go catch block of the outer method which calls FinderCompleted method,
and print the other logs like-
Gotcha--2st time exception, throwing it 3rd time
Gotcha--3rd time exception
Throwing exception for the 4th time
Gotcha--4th time exception
the error is Something unexpected happened --EXCEPTION
in MyController.m
- (IBAction)fetchResults:(id)sender
{
NSArray *tableColumnArray = ...............;//some values initialized
NSArray *identifierArray = ................;//some values initialized
NSArray *bindVariableArray = ................;//some values initialized
TableController *pTC = [[TableController alloc] init];
#try
{
[pTC populateData :tableColumnArray :identifierArray :bindVariableArray];// calling populate DataForMD method defined in TableController class
[pTC release];
}
#catch (NSException * e)
{
NSLog(#"Gotcha--4th time exception");
//want to handle the exception here
NSString* theError=[e reason];
NSLog(#"the error is %#",theError);
}
}
in TableController.m
-(void)populateData:(NSArray *)tableColumnArray:(NSArray *)identifierArray:(NSArray *)bindVariableArray
{
[self setTableColumnArray:tableColumnArray];
[self setColumnIdentifierArray:identifierArray];
[self setBindVarArray:bindVariableArray];
#try
{
NSLog(#"m here 1");
[self initializeTest];// calling initializeTest method
}
#catch (NSException * e)
{
//Do not want to handle it here
NSLog(#"Gotcha--3rd time exception");
NSLog(#"Throwing exception for the 4th time");
#throw e;
}
}
-(void)initializeTest
{
#try
{
ISTQuery* theQuery = (ISTQuery*)[ISTQueryGenerator getQueryByName:[self queryClassName]];
..........
...........//some loc here
[theQuery run];
.................//some loc here
if(theQuery)
{
//Calling FinderCompleted method
//supposed to get error here
[[self modelFinder] startWithRecipient:self andNotificationSelector:#selector(FinderCompleted:)];
}
}
#catch(NSException *e)
{
NSLog(#"Gotcha--2st time exception, throwing it 3rd time");
//Do not want to handle it here
#throw e; // rethrows e implicitly
}
}
- (void)FinderCompleted:(id)args
{
#try
{ //getting some error while back-end transaction
NSString* theError = [ISTModelFinder errorMessageFromFinderArgs:args];
if (theError)
{
NSLog(#"Inside the error");
NSLog(#"You got nothing in ur bucket :D");
NSException *e = [NSException
exceptionWithName:#"InternalErrorException"
reason:#"Something unexpected happened --EXCEPTION"
userInfo:nil];
NSLog(#"Throwing exception for the 1st time");
#throw e;
}
else
{
//do sth else
}
}
#catch(NSException *e)
{
NSLog(#"Gotcha--1st time exception , throwing it 2nd time");
NSString* theError=[e reason];
//Do not want to handle it here
NSLog(#"the error is == %#",theError);
#throw e; // rethrows e implicitly
}
}

Answer 1

You cannot use exceptions for flow control in Cocoa or iOS programming. Exceptions are purely for identifying unrecoverable errors and, typically, the program crashes purposefully very shortly thereafter. (With a handful of exceptions to this rule, most of which have bugs filed against them to deprecate and eliminate the related API.)
Use the NSError pattern to manage user recoverable errors.
It isn't clear why your code isn't working. But that doesn't look like real code. What have you tried?
I had added the code snippet, please suggest how to achieve that
functionality.
The first thing you do is do not use exceptions.
Use the NSError pattern. You will need to refactor your code to do so. It is worth it.
See the documentation on error handling in cocoa.