Sweetening Your Xgrid with Cocoa
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An XGridFoundation Class Overview

To use XGridFoundation, you simply add the framework, which is located in /System/Library/Frameworks, to your Xcode project. In any source file that needs access to XGridFoundation, you import the framework header, like this:

#import <XGridFoundation/XGridFoundation.h>

I want to kick off with an overview of the main players in the XGridFoundation framework. With most Cocoa frameworks (e.g. WebKit), you tend to only use a small portion of the available classes when you are developing an application. With XGridFoundation, you really will need just about all of them, even for something as simple as Central Command. So what follows is an overview of the classes, and what they do. The classes are presented approximately in the order you would encounter them in the flow of a program.

XGConnection

This is used to represent a connection to an Xgrid server. It can be initialized with a host name, or via Bonjour.

XGAuthenticator

In order to open a connection, you will often need a means of authenticating with the Xgrid server, such as a password. XGAuthenticator is an abstract class whose subclasses are used by an XGConnection to authenticate.

XGTwoWayRandomAuthenticator

This subclass of XGAuthenticator performs password authentication.

XGGSSAuthenticator

This subclass of XGAuthenticator authenticates with Single Sign-On.

XGController

Instances of this class are proxies for Xgrid controllers. They are initialized with an XGConnection, and are used to submit jobs.

XGActionMonitor

This class is used to monitor the activity of some asynchronous requests, such as submitting a job via an XGController.

XGResource

This abstract class represents remote-grid resources, like grids and jobs. Instances of subclasses of XGResource are proxies for entities on the Xgrid server.

XGGrid

This subclass of XGResource represents grids on the Xgrid controller.

XGJob

This subclass of XGResource represents jobs running on the Xgrid controller.

XGFile

This represents a file or stream that is stored on the Xgrid controller.

XGFileDownload

This is a class used to retrieve files and streams from the Xgrid controller after a job is complete.

Putting It All Together

So how do all of these classes work together in a running program? This is one area of the current Xgrid documentation that is severely lacking, so I will try to clarify things here.

As I mentioned earlier, what makes XGridFoundation hard is that everything revolves around networking, and that spells one thing: asynchronicity. To perform just about any operation with XGridFoundation, you have to request it via a method invocation, and then wait for a "callback," which could be a call to a delegate method, but more often than not, is simply a notification arising from key-value observing (KVO). In other words, to know when any stage of the process is complete, you either have to register a delegate, or add an observer with KVO.

To give you an idea of the basic steps involved in writing an Xgrid-enabled application, I am going to list the various operations undertaken by Central Command, in the order that they occur. I will delve into the source code of each step in the coming sections.

1. The run method of the application controller class — not to be confused with the Xgrid controller — is called when the user presses the Run button. This method creates an XGTwoWayAuthenticator with the password entered by the user, and uses it (together with the host name entered) to initialize an XGConnection object. The delegate of the XGConnection is set to the application controller, and the asynchronous open method of XGConnection is invoked.
2. The XGConnection tries to connect to the Xgrid server. If successful, it calls back to its delegate, invoking connectionDidOpen:.
3. An XGController instance is created from the XGConnection. KVO is used to observe the state attribute of the XGController.
4. When the XGController state attribute changes to XGResourceStateAvailable, a job specification dictionary is created, and the job initiated by calling the XGController method performSubmitJobActionWithJobSpecification:gridIdentifier:. This method returns an instance of XGActionMonitor; the outcome attribute of this action monitor is observed using KVO.
5. If the XGActionMonitor outcome attribute changes to XGActionMonitorOutcomeSuccess, the job identifier is retrieved from the action monitor's results dictionary. The job identifier is used to retrieve the XGJob instance that represents the job on the Xgrid controller. The XGJob is extracted from an XGGrid object, which is retrieved from the XGController. (Got that? XGController begat XGGrid, which begat XGJob.) The state attribute of the XGJob is now observed with KVO. (This step is complicated by the fact that at the time that the job identifier becomes available, the XGJob may not yet have been added to the XGGrid. If this is the case, KVO is used to wait until the XGGrid is up-to-date.)
6. When the XGJob's state changes to XGResourceStateFinished, the performGetOutputStreamsAction method of XGJob is called to start retrieving information about the command output. This method returns an XGActionMonitor, and KVO is used to observe its outcome attribute.
7. When the outcome attribute changes to XGActionMonitorOutcomeSuccess, an array of XGFile objects is retrieved from the results dictionary of the XGActionMonitor. These XGFile objects encapsulate information about the output and error streams. They are used to initialize XGFileDownload objects, in order to retrieve the stream data from the Xgrid controller. The downloads' progress is monitored via XGFileDownload delegate methods.
8. The data of each stream is accumulated during repeated calls to the XGFileDownload delegate method fileDownload:didReceiveData:. Each download is complete when the delegate method fileDownloadDidFinish: is invoked.

By now, you are probably wishing you had read the tutorial about building a personal organizer without touching your keyboard. But since your here anyway, why not read on and add to the confusion? In the coming sections I will dissect the steps above, showing you what it all means in Objective-C.

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