Question

I have created a RSS parser and 3 TableViews and it parses the RSS files fine but I don’t know how to notify the TableViewController when parsing has ended so it can update the view.
The TableViewController initiates the parser and the parsing of a feed.

parser = [[RSSParser alloc] initWithURL:@"http://randomfeed.com"];

I can access the single feed items like

[parser feedItems];

In parser.m i have implemented the delegate methods of NSXMLParser:

- (void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qualifiedName attributes:(NSDictionary *)attributeDict
- (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string
- (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName
- (void)parserDidEndDocument:(NSXMLParser *)parser 

So how do i get parserDidEndDocument to notify my controllers so i can add the data to the tableview.

Cheers from a obj-c beginner.

Answer 1

Ok the delegation pattern is widely used in the Cocoa framework.

The file that parses data must have a protocol, anyone wanting callbacks form this
Class must implement the methods from the protocol:

    //XMLParser.h

    //import statements here ..

    @protocol XMLParserDelegate

    - (void) parserDidFinish:(NSArray*) theParsedData; //have as many methods as you please, didFail, doingProgress etc.

    @end

    @interface XMLParser : NSObject {

    id <XMLParserDelegate> delegate; //we don't know what type of object this will be, only that it will adhere to the XMLParserDelegate protocol.

    }

@property(assign) id delegate;
    //methods

    @end

In the implementation of the XMLParser:

@implementation XMLParser

@synthesize delegate;

- (void)parserDidEndDocument:(NSXMLParser *)parser {

[self.delegate parserDidFinish:dataYouCollectedArray]; //this is where it happens.

}

So in your controllers interface file you says that you will adhere to the XMLParserDelegate protocol.

//MyController.h

#import "XMLParser.h"

@interface MyController : UIViewController <XMLParserDelegate> { //this is where you "promise" to implement the methods of the protocol.

}

In the MyController.m file you now instantiate the XMLParser.

@implementation MyController

- (void) init {
XMLParser *parser = [[XMLParser alloc] init];
[parser setDelegate:self] //now the parser has a reference to this object.
[parser start];

}

- (void) parserDidFinish:(NSArray*) results {

//now you have your results in the controller, can set it as the data source of the tableView and call tableView.reloadData;
}

This is a great pattern that loosely couples the caller and responder without having them know anything other that what the protocol dictates, about each other.

If I have a view element that is confined to its own functionality, like, say, a clock.
I would have a ClockViewController and it would instantiate arm, dials, etc. They would all be linked to the clock and alert the clock controller about their actions using this pattern. This way I can use the clock arm or dial in other code as I please, as long as the object instantiating them adheres to the ClockArmDelegate protocol.

Hope it makes sense:) it is the, Im pretty sure, most used pattern in Cocoa.