ImageFetcherOperation.swift

ImageFetcherOperation.swift

import UIKit

internal class ImageSizeFetcherOp: Operation {
	
	/// Callback to call at the end of the operation
	let callback: ImageSizeFetcher.Callback?
	
	/// Request data task
	let request: URLSessionDataTask
	
	/// Partial data
	private(set) var receivedData = Data()
	
	/// URL of the operation
	var url: URL? {
		return self.request.currentRequest?.url
	}
	
	/// Initialize a new operation for a given url.
	///
	/// - Parameters:
	///   - request: request to perform.
	///   - callback: callback to call at the end of the operation.
	init(_ request: URLSessionDataTask, callback: ImageSizeFetcher.Callback?) {
		self.request = request
		self.callback = callback
	}
	
	///MARK: - Operation Override Methods
	
	override func start() {
		guard !self.isCancelled else { return }
		self.request.resume()
	}
	
	override func cancel() {
		self.request.cancel()
		super.cancel()
	}
	
	//MARK: - Internal Helper Methods
	
	func onReceiveData(_ data: Data) {
		guard !self.isCancelled else { return }
		self.receivedData.append(data)
		
		// not enough data collected for anything
		guard data.count >= 2 else { return }
		
		// attempt to parse received data, if enough we can stop download
		do {
			if let result = try ImageSizeFetcherParser(sourceURL: self.url!, data) {
				self.callback?(nil,result)
				self.cancel()
			}
			// nothing received, continue accumulating data
		} catch let err { // parse has failed
			self.callback?(err,nil)
			self.cancel()
		}
	}
	
	func onEndWithError(_ error: Error?) {
		// download has failed, return to callback with the description of the error
		self.callback?(ImageParserErrors.network(error),nil)
		self.cancel()
	}
	
}