Delegation Methods for Error Handling

Making a Subclass of NSFormatter

To subclass NSFormatter, you must, at the least, override the two primitive methods stringForObjectValue: and getObjectValue:forString:errorDescription:. In the first method you convert the cell's object to a string representation; in the second method you convert the string to the object associated with the cell.

Implement attributedStringForObjectValue:withDefaultAttributes: in addition to stringForObjectValue: when the display string has attributes associated with it. For example, if you want negative financial amounts to appear in red, you would return a string with an attribute of red text. In attributedStringForObjectValue:withDefaultAttributes: get the non-attributed NSString by invoking stringForObjectValue: and then apply the proper attributes to that NSString.

If the string for editing is different than the string for display-for example, the display version of a currency field should show a dollar sign but the editing version shouldn't-implement editingStringForObjectValue: in addition to stringForObjectValue:.

The method isPartialStringValid:newEditingString:errorDescription: allows you to edit the textual contents of a cell at each key press or to prevent entry of invalid characters. You might apply this on-the-fly editing to things like telephone numbers or social security numbers; the person entering data only needs to enter the number since the formatter automatically inserts the separator characters.

Method Types

Textual representation of cell content

- stringForObjectValue:

- attributedStringForObjectValue:withDefaultAttributes:

- editingStringForObjectValue:

Object equivalent to textual representation

- getObjectValue:forString:errorDescription:

Dynamic cell editing

- isPartialStringValid:newEditingString:errorDescription:

Instance Methods

attributedStringForObjectValue:withDefaultAttributes:

The default implementation returns nil to indicate that the formatter object does not provide an attributed string. In a subclass implementation, return an NSAttributedString if the string for display should have some attributes. For instance, you might want negative values in a financial application to appear in red text. Invoke your implementation of stringForObjectValue: to get the non-attributed string. Then create an NSAttributedString with it. The default attributes for text in the cell is passed in with attributes; use this NSDictionary to reset the attributes of the string when a change in value warrants it (for example, a negative value becomes positive). For information on creating attributed strings, see the specification for the NSAttributedString class.

See also: - editingStringForObjectValue:

editingStringForObjectValue:

The default implementation of this method invokes stringForObjectValue:. When implementing a subclass, override this method only when the string that users see and the string that they edit are different. In your implementation, return an NSString that is used for editing, following the logic recommended for implementing stringForObjectValue: (see below). As an example, you would implement this method if you want the dollar signs in displayed strings removed for editing.

See also: - attributedStringForObjectValue:withDefaultAttributes:

getObjectValue:forString:errorDescription:

The default implementation of this method raises an exception. In your subclass implementation, return by reference the object anObject after creating it from the string passed in. Return YES if the conversion from string to cell-content object was successful and NO if any error prevented the conversion. If you return NO, also return by indirection a localized user-presentable NSString (in error) that explains the reason why the conversion failed; the delegate (if any) of the NSControl managing the cell can then respond to the failure in control:didFailToFormatString:errorDescription:. However, if error is NULL, the sender is not interested in the error description, and you should not attempt to assign one.

The following implementation example (which is paired with the stringForObjectValue: example below) converts an NSString representation of a dollar amount that includes the dollar sign; it uses an NSScanner to convert this amount to a float after stripping out the initial dollar sign.

- (BOOL)getObjectValue:(id *)obj forString:(NSString *)string errorDescription:(NSString 
**)error
{
    float floatResult;
    NSScanner *scanner;
    BOOL retval = NO;
    NSString *err = nil;

    scanner = [NSScanner scannerWithString:string];
    if ([string hasPrefix:@"$"]) [scanner setScanLocation:1];    
    if ([scanner scanFloat:&floatResult] 
        && ([scanner scanLocation] == [string length] )) {
        if (obj) {
            *obj = [NSNumber numberWithFloat:floatResult];
        {
            retval = YES;
        }
        }else {
            err = NSLocalizedString(@"Couldn't convert to float");
        }
    }
    if (error) {
        *error = err;
    }
    return retval;
}

See also: - stringForObjectValue:

isPartialStringValid:newEditingString:errorDescription:

Since this method is invoked at each key press in the cell, it permits editing or evaluation of cell text as it is typed. The text as currently typed (partialString) is passed in. Evaluate this text according to the context, edit the text if necessary, and return by reference any edited NSString in newString. Return YES if partialString is acceptable and NO if partialString is unacceptable. If you return NO and newString is nil, partialString minus the last character typed is displayed. If you return NO, you can also return by indirection an NSString (in error) that explains the reason why the validation failed; the delegate (if any) of the NSControl managing the cell can then respond to the failure in control:didFailToValidatePartialString:errorDescription:.

stringForObjectValue:

The default implementation of this method raises an exception. When subclassing, return the NSString that textually represents the cell's object for display and-if editingStringForObjectValue: is unimplemented-for editing. First test the passed-in object to see if it's of the correct class. If it isn't, return nil; but if it is of the right class return a properly formatted and, if necessary, localized string. (See the specification of the NSString class for formatting and localizing details.)

The following implementation (which is paired with the getObjectValue: forString: errorDescription: example above) prefixes a two-digit float representation with a dollar sign:

- (NSString *)stringForObjectValue:(id)anObject
{
    if (![anObject isKindOfClass:[NSNumber class]]) {
        return nil;
    }
    return [NSString stringWithFormat:@"$%.2f", [anObject floatValue]];
}

See also: - attributedStringForObjectValue:withDefaultAttributes:, - editingStringForObjectValue:, - getObjectValue:forString:errorDescription:

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