Makes No Sense At All

Recently the successful iPhone applications Trein and iNap realized a tight integration, in that you can activate iNap directly from Trein. How to create the integration on the side of the ‘receiving’ application, iNap in this case, is described in the iPhone SDK example ‘Launch Me’.

When integrating applications, you typically want to pass data from one to the other, in this post I’ll describe how the incoming parameters can easily be extracted in the receiving application.

The receiving application will register itself in the OS (for details see the ‘Launch Me’ example), and the result is that it can be activated from another application or from a hyperlink in a webpage. An example of a url that activates another application is:

myapp://myapp?latitude=51.3&longitude=5.2&title=middle of nowhere

In the receiving application the method - (BOOL)application:(UIApplication *)application handleOpenURL:(NSURL *)url will be called on the application delegate object, and in this method you need to extract the parameters from the URL. The following code shows how to do that with minimal code. Like Apple’s example, it does a lot of checking to prevent undesired usage, in this case the url parameters must meet the exact specifications (i.e. no missing parameters, no extra parameters) otherwise the request will be ignored.

- (BOOL)application:(UIApplication *)application handleOpenURL:(NSURL *)url
{
  // You should be extremely careful when handling URL requests.
  // You must take steps to validate the URL before handling it.
  // If any of the validations fails, bail out returning NO.
  if (!url) {
    // The URL is nil. There's nothing more to do.
    return NO;
  }

  // Split the url into two sections, one section with the base url and one with the parameters.
  NSArray *urlSections = [[url absoluteString] componentsSeparatedByCharactersInSet:
              [NSCharacterSet characterSetWithCharactersInString:@"?"]];

  // Check that you actually got a base section and a parameter section.
  if ([urlSections count] != 2) {
    return NO;
  }

  // Check that the base part of the url is what you expect it to be.
  if (![[urlSections objectAtIndex:0] isEqualToString:@"myapp://myapp"]) {
    return NO;
  }

  // Split up the parameter section into a parameter Array.
  NSArray *parameterArray = [[urlSections objectAtIndex:1] componentsSeparatedByCharactersInSet:
              [NSCharacterSet characterSetWithCharactersInString:@"&="]];
  NSEnumerator *parameterEnum = [parameterArray objectEnumerator];
  NSString *parameter, *value;
  // The parameters values that we are expecting.
  NSString *latitude = nil, *longitude = nil, *title = nil;

  // Run through the array of parameters.
  while (parameter = [parameterEnum nextObject]) {
    // Get the value that goes with this parameter, and if it is missing break.
    if ((value = [parameterEnum nextObject]) == nil)
      break;

    // Check the parameter value against the parameters that you support/expect,
    // and assign the matching value to the right variable.
    if ([parameter isEqualToString:@"latitude"])
      latitude = value;
    else if ([parameter isEqualToString:@"longitude"])
      longitude = value;
    else if ([parameter isEqualToString:@"title"])
      title = [value stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
    else {
      // An unexpected parameter was passed, bail out.
      latitude = nil;
      longitude = nil;
      title = nil;
      break;
    }
  }

  // Check if we got all parameters (in this case all are mandatory), if not bail out.
  if (latitude == nil || longitude == nil || title == nil) {
    return NO;
  }

  // Use the parameters to do your thing.
  CLLocation *location = [[[CLLocation alloc]
                               initWithLatitude:[latitude doubleValue]
                               longitude:[longitude doubleValue]] autorelease];
  [someObject activateDestination:title location:location];

  return YES;
}