First part of this series introduced how scripts can be written in Apple’s Swift programming language while also having referenced a library of our own. sayHello.swift script was implemented by calling a public field, abc.text, located in this library.

This second part is showing how a script evolves from writing a few short and dirty lines to a rather complex ensemble of writing functions and calling 3rd party library tools — all under swift-sh’s umbrella .

The scope of this second script is to write some code that retrieves the price of a cryptocurrency pair, say BTCUSDT, published by the cryptocurrency exchange Binance . The script life-cycle is described in this pull request :

  1. First commit introduces the command line arguments that the script is expecting, namely that the cryptocurrency pair like BTCUSDT must be specified as the only required input parameter.

  2. Second commit introduces the 3rd party libraries that the script will be using : Alamofire and SwiftyJSON . Their versions are specified as per the pattern introduced by swift-sh tool:

     import Alamofire // @Alamofire ~> 5.2
 import SwiftyJSON // @SwiftyJSON ~> 5.0.0

    Then it follows a very quick and dirty approach of getting the work done, assuming that everything will work just fine and no errors will ever happen. Running the script first time , without any input parameters, will make swift-sh tool fetch the 3rd party libraries then execute the script code; running the script the second time, with the correct input parameter BTCUSDT will make the script output the expected price for the cryptocurrency pair; finally, running the script with an invalid input parameter such as BTCUSDTX will make the script output some nasty error — which is expected, considering that the code does not handle any kind of error. Here’s the output of these three runs :

    2-writing-scripts-in-swift-language-1.png

    What comes next are several steps that would improve the source code sufficiently such that it can be used in a production setting :

  3. Third commit introduces the synchronisation via DispatchGroup as well as the cURL debug-like output so we’d know what kind of request is actually being sent to the exchange

  4. Fourth commit introduces the error handling of the exchange response in case of various scenarios : invalid pair, wrong response etc.

  5. Fifth commit replaces the entire logic written thus far with a function with signature :

     func getPrice(ofPair pair:String) throws -> (String, Decimal)

    It returns the a tuple of (pair,price) upon successful processing or it throws an exception of type Failure in case of an error .

  6. Finally, the sixth commit decides to replace DispatchGroup with DispatchSemaphore for better and easier understanding of the source code.

All in all, the script is now doing a rather decent job of retrieving the price of a cryptocurrency pair while also dealing with potential errors that might come to its way :

2-writing-scripts-in-swift-language-2.png

That’s it for now. Enjoy !