dispatch_group based state machine

gistfile1.m

//
//  StateMachineExample
//
//  Created by Jacob Farkas on 6/4/14.
//

#import <Foundation/Foundation.h>

#define DownloadTempPath "/tmp/tacosXXXXXX"

typedef NS_ENUM(NSUInteger, MyOperationState) {
    MyOperationStateNotStarted = 1,
    MyOperationStateDownloadingPages,
    MyOperationStateProcessingData,
    MyOperationStateComplete = UINT_MAX,
};

@interface FindNewlinesOperation : NSOperation
- (instancetype)initWithURLsToDownload:(NSArray *)urlsToDownload;

@property (nonatomic, strong) NSArray /* NSURL* */ *urlsToDownload;
@property (nonatomic, copy) void (^pagesFetchedBlock)(NSUInteger newlineCount);
@end

@interface FindNewlinesOperation ()
@property (nonatomic, assign) MyOperationState state;
@property (nonatomic, strong) dispatch_group_t stateTransitionGroup;

@property (nonatomic, assign) BOOL isFinished;
@property (nonatomic, assign) BOOL isExecuting;

@property (nonatomic, strong) NSMutableArray *downloadTasks;
@property (nonatomic, strong) NSMutableArray /* NSFileHandle */ *downloadedFiles;
@property (atomic, assign) NSUInteger newlineCount;
@end

@implementation FindNewlinesOperation
- (instancetype)initWithURLsToDownload:(NSArray *)urlsToDownload {
    if ((self = [super init])) {
        _state = MyOperationStateNotStarted;
        _stateTransitionGroup = dispatch_group_create();
        _urlsToDownload = urlsToDownload;
        _downloadTasks = [NSMutableArray new];
        _downloadedFiles = [NSMutableArray new];
    }
    return self;
}

- (void)setIsFinished:(BOOL)isFinished {
    [self willChangeValueForKey:@"isFinished"];
    _isFinished = isFinished;
    [self didChangeValueForKey:@"isFinished"];
}

- (void)setIsExecuting:(BOOL)isExecuting {
    [self willChangeValueForKey:@"isExecuting"];
    _isExecuting = isExecuting;
    [self didChangeValueForKey:@"isExecuting"];
}

// Automatically advances the state machine to the next state when everything has left the dispatch group
// This only needs to be called once from the start method.
- (void)makeStateTransition {
    switch(self.state) {
        case MyOperationStateNotStarted:
            self.state = MyOperationStateDownloadingPages;
            [self downloadPages];
            break;
        case MyOperationStateDownloadingPages:
            self.state = MyOperationStateProcessingData;
            [self processData];
            break;
        case MyOperationStateProcessingData:
        case MyOperationStateComplete:
            if (self.pagesFetchedBlock) self.pagesFetchedBlock(self.newlineCount);
            self.isExecuting = NO;
            self.isFinished = YES;
            break;
    }
    
    if (!self.isFinished) {
        dispatch_group_notify(self.stateTransitionGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            [self makeStateTransition];
        });
    }
}

- (void)start {
    self.isFinished = NO;
    self.isExecuting = YES;
    
    [self makeStateTransition];
}

- (void)main {
    [self start];
}

- (void)downloadPages {
    for (NSURL *url in self.urlsToDownload) {
        dispatch_group_enter(self.stateTransitionGroup);
        
        NSURLSessionDownloadTask *downloadTask = [[NSURLSession sharedSession] downloadTaskWithURL:url completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
            if (location) {
                int fd = open([location fileSystemRepresentation], O_RDONLY);
                if (fd >= 0) {
                    NSFileHandle *fh = [[NSFileHandle alloc] initWithFileDescriptor:fd closeOnDealloc:YES];
                    @synchronized(self.downloadedFiles) {
                        [self.downloadedFiles addObject:fh];
                    }
                }
            }
            dispatch_group_leave(self.stateTransitionGroup);
        }];
        
        [self.downloadTasks addObject:downloadTask];
        [downloadTask resume];
    }
}

- (void)processData {
    for (NSFileHandle *fh in self.downloadedFiles) {
        dispatch_group_enter(self.stateTransitionGroup);
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSData *fileData = [fh readDataOfLength:getpagesize()];
            while ([fileData length]) {
                for (char *c = (char *)[fileData bytes]; c < (char *)[fileData bytes] + [fileData length]; c++) {
                    if (*c == '\n') self.newlineCount++;
                }
                fileData = [fh readDataOfLength:getpagesize()];
            }
            dispatch_group_leave(self.stateTransitionGroup);
        });
    }
}
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSMutableArray *urlsToDownload = [NSMutableArray new];
        NSArray *args = [[NSProcessInfo processInfo] arguments];
        for (NSString *urlString in [args subarrayWithRange:NSMakeRange(1, [args count] - 1)]) {
            @try {
                NSURL *curURL = [NSURL URLWithString:urlString];
                if (curURL) {
                    [urlsToDownload addObject:curURL];
                }
            } @catch (NSException *e) { }
        }
        
        FindNewlinesOperation *op = [[FindNewlinesOperation alloc] initWithURLsToDownload:urlsToDownload];
        op.pagesFetchedBlock = ^(NSUInteger newlineCount) {
            NSLog(@"%ld newlines were found on the URLs you provided", newlineCount);
        };
        op.completionBlock = ^{
            exit(0);
        };
        
        [[NSOperationQueue mainQueue] addOperation:op];
        
        dispatch_main();
    }
    return 0;
}