Question

I am writing a web-connected application that needs to execute several asynchronous requests to load data needed lower down in the dependency tree.

Fig 1.

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For visualization purposes, consider an example with ASIHTTPRequests A,B,C,D,E, and F:

A’s url depends on the result of B and C,

and B’s url depends on the result of D, E, and F.

B and C can be computed concurrently, and so can D, E, and F.

NSOperationQueue = [(D,E,F),(B,C),A]

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Thus far, I have created an NSOperationQueue that contains a dependency tree of ASIHTTPRequests. However, the URLs of the ASIHTTPRequests should depend on the results of the previous operations, and, right now, they do not.

The question: What is the best way to pass the results of the computations performed by multiple NSOperations to the NSOperation that depends on them, and how can I set this up with ASIHTTPRequests?

Thanks in advance,
Julian Ceipek

Answer 1

I ended up solving this problem by wrapping the ASIHTTPRequest in a custom NSOperation object that populated the request in such a way that custom request B contained a pointer to an object in D, E, and F’s ASIHTTPRequest UserInfo Dictionary. While I liked @JosephH’s solution, I couldn’t figure out how to easily generate a dictionary or array with dependency tree intact.

A simplified version of my custom NSOperationObject is provided below; any suggestions are welcome. I used Apple’s Concurrency Programming Guide extensively as a reference, but as I have not had any prior experience extending the NSOperation class, I am sure that there is a better way to do this.

#import <Foundation/Foundation.h>
#import "SyncableData.h"
#import "ASIHTTPRequest.h"

@interface PushContentRequest : NSOperation <ASIHTTPRequestDelegate> {
    BOOL executing;
    BOOL finished;
    id <SyncableData> data;
    ASIHTTPRequest *request;
    NSURL *url;
    id <ASIHTTPRequestDelegate> delegate;
}

- (id)initWithDataObject:(id <SyncableData>)theData url:(NSURL *)theUrl delegate:(id <ASIHTTPRequestDelegate>)theDelegate;

@end


#import "PushContentRequest.h"

@implementation PushContentRequest

- (id)initWithDataObject:(id <SyncableData>)theData url:(NSURL *)theUrl delegate:(id <ASIHTTPRequestDelegate>)theDelegate {
    if ((self = [super init])) {
        executing = NO;
        finished = NO;
        data = [theData retain];
        url = [theUrl retain];
        delegate = [theDelegate retain];
    }
    return self;
}

- (BOOL)isConcurrent {
    return YES;
}

- (BOOL)isExecuting {
    return executing;
}

- (BOOL)isFinished {
    return finished;
}

- (void)start {
    if ([self isCancelled]) {
        [self willChangeValueForKey:@"isFinished"];
        finished = YES;
        [self didChangeValueForKey:@"isFinished"];
        return;
    }

    NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
    request = [ASIHTTPRequest requestWithURL:url];
    NSString *xmlToPost = [[NSString alloc] initWithString: [theData getXMLRep]];
    [request appendPostData:[xmlToPost dataUsingEncoding:NSUTF8StringEncoding]];
    [request setDelegate:self];
    NSDictionary *userInfoDict = [[NSDictionary alloc] initWithObjectsAndKeys:data, @"theData", nil];
    [request setUserInfo:userInfoDict];
    [userInfoDict release];
    [xmlToPost release];
    [self willChangeValueForKey:@"isExecuting"];
    [request start];
    executing = YES;
    [self didChangeValueForKey:@"isExecuting"];
    [pool release];

}

- (void)dealloc {
    [delegate release];
    [url release];
    [data release];
    [super dealloc];
}

- (void)requestFinished:(ASIHTTPRequest *)therequest {
    [delegate requestFinished:therequest];

    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];

    executing = NO;
    finished = YES;

    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

- (void)requestFailed:(ASIHTTPRequest *)therequest {
    [delegate requestFailed:therequest];
    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];

    executing = NO;
    finished = YES;

    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

@end

The delegate of this PushContentRequest currently handles the interpretation of the ASIHTTPRequest’s UserInfo Dictionary and the request in my implementation, though I suppose that it might make more sense to do this processing within the PushContentRequest’s body.