iOS - download attachment from Gmail using messageId and attachmentId

Question

I am following Google Gmail API documentation in my iOS project where I need to download an attachment from a mail by using it userId, messageId and id Try out from Google. Where userId is user's Email address, id is mailId (Try it.Enter only your EmailId) messageId is which we get by using an one of above id which has an attachment.

This is so far the code which I am have:

- (void)fetchLabels {
    GTLQueryGmail *query = [GTLQueryGmail queryForUsersThreadsList];
    [self.service executeQuery:query
                      delegate:self
            didFinishSelector:@selector(displayResultWithTicket:finishedWithObject:error:)];
}

- (void)displayResultWithTicket:(GTLServiceTicket *)ticket
             finishedWithObject:(GTLGmailListLabelsResponse *)labelsResponse
                          error:(NSError *)error {

    NSDictionary *allIDsDictionaryValue=[labelsResponse.JSON valueForKey:@"threads"];
    NSMutableArray *allIDs=[NSMutableArray new];
    for (int i=0; i<[allIDsDictionaryValue count]; i++) {
        [allIDs addObject:[allIDsDictionaryValue valueForKey:@"id"]];
    }
   NSString *mailID=[[allIDs objectAtIndex:0]objectAtIndex:0];
    NSLog(@"%@",mailID);//Ex: 14e8af86776e595b 
    //As of now I am using the first id only because it has an attachment.

   // Getting messageId from an id of a mail.
    GTLQueryGmail *query1 = [GTLQueryGmail queryForUsersMessagesGet];
    query1.identifier=mailID;
    [self.service executeQuery:query1
                      delegate:self
             didFinishSelector:@selector(displayResultWithTicketOfAttachment:finishedWithObject:error:)];
}

- (void)displayResultWithTicketOfAttachment:(GTLServiceTicket *)ticket
             finishedWithObject:(GTLGmailListLabelsResponse *)labelsResponse
                          error:(NSError *)error {

    NSDictionary *response1=labelsResponse.JSON;
    NSDictionary *response2=[[[response1 valueForKey:@"payload"] valueForKey:@"parts"]objectAtIndex:1];
    if ([[response2 valueForKey:@"mimeType"] isEqualToString:@"application/vnd.ms-excel"]) {
         attachmentId=[[response2 valueForKey:@"body"]valueForKey:@"attachmentId"];
         NSLog(@"%@",attachmentId); //Ex: ANGjdJ_Uw2o7G2Q16jfC4JSTwD2UIO6LuqDIJZvXFCft0q5ALYuiYfM2gPWG0dcty2n6ZkjnVWekIb_3e2_0TqSpctWNAABsfaSF2x2ktf9uHu63e3eIot_GFA9xgppmaRDyaJEL1V-gIvjfPRxgINK8xM0OuuwnVz2xzcCFckkwpK2XwzZG_QPGPvC2Be2bGKNJI8Ds3hGtqfHYeSWZjOjbsjBuxbUFjf1Mlp0TLol9LLAy2cjGZ_CUBQXzZhhWw9AtNHTU4jwhk4WizKRXbfuDvmRtAy1lPCnTKS6pLg

///Based on id and messageId, I am trying to download the attachment
        GTLQueryGmail *query2 = [GTLQueryGmail queryForUsersMessagesAttachmentsGet];
        query2.identifier=mailID;
        query2.messageId=attachmentId;
        [self.service executeQuery:query2
                          delegate:self
                 didFinishSelector:@selector(displayResultWithTicketOfDownloadAttachment:finishedWithObject:error:)];
    }else{
        UIAlertView *alertShow=[[UIAlertView alloc]initWithTitle:@"Error" message:@"No Attachment found" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil];
        [alertShow show];
    }
}

- (void)displayResultWithTicketOfDownloadAttachment:(GTLServiceTicket *)ticket
                         finishedWithObject:(GTLGmailListLabelsResponse *)labelsResponse
                                      error:(NSError *)error {
    NSLog(@"response is :%@",labelsResponse.JSON);
     NSLog(@"error is :%@",error);
}

But I am getting error as : Error Domain=com.google.GTLJSONRPCErrorDomain Code=400 "The operation couldn’t be completed. (Invalid attachment token)" UserInfo=0x7b68e740 {error=Invalid attachment token, GTLStructuredError=GTLErrorObject 0x7b6876e0: {message:"Invalid attachment token" code:400 data:[1]}, NSLocalizedFailureReason=(Invalid attachment token)}

Answer 1

You have the Ids mixed up a bit.

GTLQueryGmail *query2 = [GTLQueryGmail queryForUsersMessagesAttachmentsGet];
        query2.id=attachmentId;
        query2.messageId=mailID;

The data you get in your response is Base64-encoded and has been made URL safe by replacing all "+" with "-", and all "/" with "_".

Simply convert it back to regular Base64-data and decode it! Try this in your Developer Tools (press F12):

atob("SGV5IE5pbGVzaCEgSSBob3BlIHlvdSB3aWxsIGdldCBpdCB0byB3b3JrIHNvb24h".replace(/\-/g, '+').replace(/\_/g, '/'));

Do the same steps in your code and you will have your data in its original format.