AppDownloadQueueOperation.h

#import <Foundation/Foundation.h>

@interface AppDownloadQueueOperation : NSOperation

- (instancetype)initWithURLSessionDataTask:(NSURLSessionDataTask *)dataTask;

@property (nonatomic, strong, readonly) NSURLSessionDataTask *dataTask;

@end

AppDownloadQueueOperation.m

#import "AppDownloadQueueOperation.h"

static void *AppDownloadQueueOperationTaskStateChangedContext = &AppDownloadQueueOperationTaskStateChangedContext;

@interface AppDownloadQueueOperation () {
	BOOL _executing;
	BOOL _finished;
}
@property (nonatomic) BOOL isObservingTaskState;
@end


@implementation AppDownloadQueueOperation

- (instancetype)initWithURLSessionDataTask:(NSURLSessionDataTask *)dataTask
{
	self = [super init];
	if (self) {
		_dataTask = dataTask;
	}
	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;
	}

	// If the operation is not canceled, begin executing the task.
	[self willChangeValueForKey:@"isExecuting"];
	_executing = YES;
	[self didChangeValueForKey:@"isExecuting"];

	[self.dataTask addObserver:self forKeyPath:@"state" options:NSKeyValueObservingOptionNew context:AppDownloadQueueOperationTaskStateChangedContext];
	self.isObservingTaskState = YES;

	[self.dataTask resume];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
	if (context == AppDownloadQueueOperationTaskStateChangedContext && object == self.dataTask && [keyPath isEqualToString:@"state"])
	{
		NSNumber *newStateValue = change[NSKeyValueChangeNewKey];
		NSURLSessionTaskState newState = newStateValue.integerValue;
		if (newState == NSURLSessionTaskStateCompleted) {
			[self completeOperation];
		}
	}
	else {
		[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
	}
}

- (void)completeOperation
{
	@synchronized(self) {
		if (_finished == NO || _executing == YES) {
			[self willChangeValueForKey:@"isFinished"];
			[self willChangeValueForKey:@"isExecuting"];
			_executing = NO;
			_finished = YES;
			[self didChangeValueForKey:@"isExecuting"];
			[self didChangeValueForKey:@"isFinished"];
		}
	}
}

- (void)dealloc
{
	if (self.isObservingTaskState) {
		[self.dataTask removeObserver:self forKeyPath:@"state" context:AppDownloadQueueOperationTaskStateChangedContext];
	}
}

@end