Cocoa Tips and Tricks UsingBlocksAsContextInfo

UsingBlocksAsContextInfo - Demonstrates how to implement Objective-C "blocks" passed in as the 'contextInfo' to NSAlert, helping to handle the alert result.

// This is a quick tip on how to use the context info as a block parameter.

// You can use this type of pattern for any methods that have a delegate/selector/contextInfo pattern.

- (void)btnShowAlertClicked:(id)sender  {

    NSAlert *alert = [NSAlert alertWithMessageText:@"Alert Message"

                                     defaultButton:@"Default Button"

                                   alternateButton:@"Alternate Button"

                                       otherButton:@"Other Button"

                         informativeTextWithFormat:@"Informative Text"];

    BOOL someLocalVariable = YES;

    // We create a block that can easily access local variables to this method.

    // This is much easier than trying to package them all up into a contextInfo object

    void (^blockCallback)(NSInteger) = ^(NSInteger returnCode) {

        // Inside the block callback we can easily access locals

        if (someLocalVariable) {

            if (returnCode == NSAlertDefaultReturn) {

                [button setTitle:@"Default Return Button Clicked!"];

            } else {

                [button setTitle:@"Something else clicked...try again."];

            }

        }

    };

    // We copy the block, since it needs to stay alive for longer than the current scope

    [alert beginSheetModalForWindow:self.window

                      modalDelegate:[self class]

                     didEndSelector:@selector(alertDidEnd:returnCode:contextInfo:)

                        contextInfo:Block_copy(blockCallback)];

}

+ (void)alertDidEnd:(NSAlert *)alert returnCode:(NSInteger)returnCode contextInfo:(void (^)(NSInteger returnCode))continuationHandler {

    continuationHandler(returnCode);

    // The block must always be retained before the first call. This is the matching release

    Block_release(continuationHandler);

}