URL Scheme Callbacks

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[Episode Source Code]
x-callback-url spec

Sending the initial call with callback params

NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"x-callback-wordrev://x-callback-url?x-source=SchemeDemo&x-success=%@&x-error=%@&x-cancel=%@&word=%@",
                                           @"schemedemo://success",
                                           @"schemedemo://error",
                                           @"schemedemo://cancel",
                                           [word stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]]];
        if ([[UIApplication sharedApplication] canOpenURL:url]) {
            [[UIApplication sharedApplication] openURL:url];
        } else {
            NSLog(@"No app handles: %@", url);
        }

Next we receive the open url event on the receiving application, but we need to pull out all those params.

Constructing a dictionary of URL Parameters

There are probably libraries to make this easier, but it's fairly trivial to split a string of query parameters into a dictionary of key/value pairs.

    NSString *query = [url query];
        NSArray *paramValues = [query componentsSeparatedByString:@"&"];
        NSMutableDictionary *params = [NSMutableDictionary dictionary];
        [paramValues enumerateObjectsUsingBlock:^(NSString *paramValue, NSUInteger idx, BOOL *stop) {
            NSArray *parts = [paramValue componentsSeparatedByString:@"="];
            NSString *key = [parts firstObject];
            NSString *value = [parts lastObject];
            params[key] = value;
        }];

Calling back the originating application

        // x-callback-wordrev://x-callback-url?x-source=..&x-success=..&x-error=..&x-cancel&word=foo
        NSString *word = params[@"word"];
        word = [word stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
        NSString *reversed = [self reverseWord:word];

        NSString *urlString = [NSString stringWithFormat:@"%@?word=%@", params[@"x-success"], [reversed stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]];
        NSURL *callbackURL = [NSURL URLWithString:urlString];
        [[UIApplication sharedApplication] openURL:callbackURL];

Handling the callback in the originating application

We have a little code duplication on the URL parameter parsing (hence the desire for this to be in a library) but the process of extracting the result is straight-forward:

 if ([[url host] isEqualToString:@"success"]) {

        NSString *query = [url query];
        query = [query stringByReplacingOccurrencesOfString:@"?" withString:@""];
        NSArray *paramValues = [query componentsSeparatedByString:@"&"];
        NSMutableDictionary *params = [NSMutableDictionary dictionary];
        [paramValues enumerateObjectsUsingBlock:^(NSString *paramValue, NSUInteger idx, BOOL *stop) {
            NSArray *parts = [paramValue componentsSeparatedByString:@"="];
            NSString *key = [parts firstObject];
            NSString *value = [parts lastObject];
            params[key] = value;
        }];

        NSString *word = params[@"word"];
        word = [word stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
        [[NSNotificationCenter defaultCenter] postNotificationName:@"WordReversed"
                                                            object:word
                                                          userInfo:nil];

        return YES;
    }

Here we're just posting a notification with the transformed string so our view controller can pick it up.