Question

How do I display a popup informing a NSTextField can’t be empty in cocoa ?

If the user click apply and the NSTextField is empty a popup should appear saying the field can’t be empty.

thanks

Answer 1

The answer by @beryllium only tells part of the story.

In fact, to properly validate text field input in Cocoa you should be using an NSFormatter attached to your text field cell and then in your NSTextFieldDelegate you should implement the method: control:didFailToFormatString:errorDescription:. In this delegate method you can prompt the user to correct their input.

All you need to do in your NSFormatter subclass is something like this:

@implementation RKTextFormatter

- (NSString*)stringForObjectValue:(id)object 
{
    return (NSString*)object;
}

- (BOOL)getObjectValue:(id*)object forString:(NSString*)string errorDescription:(NSString**)error {

    BOOL stringPassesTest = NO;

    //put your test here 

    if(!stringPassesTest)
    {
        //set the error and return NO
        if(error)
            *error = [NSError errorWithDomain:@"YourErrorDomain" code:1 userInfo:[NSDictionary dictionaryWithObject:@"It's a bingo" forKey:NSLocalizedDescriptionKey]];
        return NO;
    }

    //otherwise, just assign the string
    *object = string;
    return YES;
}
@end

You would assign the formatter to your text field like so:

RKTextFormatter* formatter = [[RKTextFormatter alloc] init];
[[textField cell] setFormatter:formatter];

And then in your NSTextFieldDelegate you handle any invalid input:

- (BOOL)control:(NSControl *)control didFailToFormatString:(NSString *)string errorDescription:(NSString *)error
{
//display an alert, obviously it would be more useful than this
NSAlert* alert = [NSAlert alertWithMessageText:@"You have failed me for the last time" defaultButton:@"Revise Input" alternateButton:nil otherButton:nil informativeTextWithFormat:@"%@",error];

[alert beginSheetModalForWindow:control.window modalDelegate:nil didEndSelector:NULL contextInfo:NULL];

//if you return NO here, the user's input will not be accepted
//and the field will remain in focus
return NO;
}