Question

He there, i have a simple tree structure with files and folders. It’s part of a client- server application which can download virtual folders. Anyways i like using ^blocks for async programming. I have 3 urls, the first one downloads a folder description, the second one is for downloading a file and the third one is for downloading a folder contents. DSFolder is the folder description and DSFile inherits from DSFolder and contains a NSData field for the download result. The folder downloading code looks like this:

- (void)loadFolderData:(DSFolder *)folder 
              finished:(void(^)(DSFolder *))finished 
                 error:(void(^)(NSError *))error {

    if (!folder) {
        return;
    }
    for (DSFolder* fileOrFolder in folder.children) {
        if ([fileOrFolder isFolder]) {
            [self loadFolderData:fileOrFolder
                        finished:^(DSFolder * folder) {

                        // *********************************
                        // Next folder level would be a copy
                        // of the whole for loop in here
                        // *********************************

                    } error:^(NSError * err) {
                        if (error) {
                            error(err);
                        }
                    }];
        } else {
            [self loadFile:fileOrFolder.name
                    folder:folder.name
                  finished:^(NSData * data) {
                      ((DSFile *) fileOrFolder).data = data;
                  } error:^(NSError * err) {
                      if (error) {
                          error(err);
                      }
                  }];
        }
    }
}

Well i could limit the tree level depth by just copying the loop inside a few times, but that seems very ugly. I hope this question is reasonable.

Answer 1

Recursion already runs a “copy of the loop” for you:

- (void)loadFolderData:(DSFolder *)folder 
              finished:(void(^)(DSFolder *))finished 
                 error:(void(^)(NSError *))error {

    if (!folder) {
        return;
    }
    for (DSFolder* fileOrFolder in folder.children) {
        if ([fileOrFolder isFolder]) {
            [self loadFolderData:fileOrFolder finished:nil error:error];
        } else {
            [self loadFile:fileOrFolder.name
                    folder:folder.name
                  finished:^(NSData * data) {
                      ((DSFile *) fileOrFolder).data = data;
                  } error:error];
        }
    }
}

I’m assuming that -loadFile:finished:error: correctly handles a nil error-block.

A few other things:

• [self loadFile:fileOrFolder.name folder:folder.name ...] looks like it does the wrong thing if there are subdirectories: Assuming the path a/b/c has a filename of c and a folder name of b, it would appear to load b/c.
• error is called for every error. This is probably not what you want. It also does not return the load that caused the error.
• It is more normal to have a single callback, e.g. void(^)(DSFolder*,NSError*).
• finished is unused. It looks like you want it to be called when the entire hierarchy has finished loading, which is Slightly Tricky.