The command pattern is useful to handle direction from args in command line app.\u00a0<\/p>\r\n
For example, there was an ugly code as below, which I wrote a while ago. As I simplified the code to focus on if-else statement, it may look not that bad.\u00a0 But, I was struggling with managing the mapping each method with the command from argument especially when adding a new one or modifying the existing one.\u00a0<\/p>\r\n
Many said there is code smell when you see a long if-else or case statement. I didn’t know what was the problem until I fixed it.\u00a0<\/p>\r\n
if (runOption == \"all\")\r\n {\r\n await _stockPriceManager.ReadStockPriceFromWeb(stocksToRead, _appSettings.DaysToReadFromWeb);\r\n\r\n \/\/ Iterate simulation\r\n var averageAnnualReturn = await IterateSimulationAsync(investDay, stocksToAnalyze, _tradeOption);\r\n \r\n }\r\n else if (runOption == \"recalculate\")\r\n {\r\n await _stockPriceManager.ReCalculateCoefficient(stocksToAnalyze);\r\n }\r\n else if (runOption == \"simulate\")\r\n {\r\n var averageAnnualReturn = await IterateSimulationAsync(investDay, stocksToAnalyze, _tradeOption);\r\n }\r\n else if (runOption.ToLower() == \"readprice\")\r\n {\r\n await _stockPriceManager.ReadStockPriceFromWeb(stocksToRead, _appSettings.DaysToReadFromWeb);\r\n }<\/pre>\r\n <\/p>\r\n
This code refactored as below using the command pattern. After refactoring, the selection condition was deligated to the concrete command object. To add a new action, it’s possible just to add new command object to the availablecommands array and no need to modify the code below that. Another and actually more important benefit that I didn’t show from this simple example code is that I could separate responsibility of each command under its own class. As you may imagine, most of those were inside the same class where if-else statement was. Implementing the command pattern naturally led me to implement the single responsibility principle.\u00a0<\/p>\r\n
The downside of this approach is that you need to create a new 3 command class and one ICommand interface. When it’s a simple program like a prototype, we might want to just use a simple if-else style. But, if there is any chance the code become a long-running one, which implies that continuous add\/modification will be followed by, I would use command pattern.\u00a0<\/p>\r\n
var availableCommands = new ICommand[]\r\n {\r\n new UpdatePrice(_stockPriceManager, stocksToRead, _appSettings.DaysToReadFromWeb),\r\n new UpdateStats(_stockPriceManager, stocksToAnalyze),\r\n new SimulateCommand(_stockPriceManager, stocksToAnalyze, _marketWatchRepository, investDay,\r\n _tradeOption, _logger, _appSettings)\r\n };\r\n\r\n if (runOption == \"all\")\r\n {\r\n availableCommands.Select(async c => await c.Execute());\r\n }\r\n else\r\n {\r\n var command = availableCommands.Single(c => c.Name == runOption);\r\n await command.Execute();\r\n }<\/pre>\r\n <\/p>\r\n
This was possible because each command shares the same interface, ICommand. That looks like below.\u00a0<\/p>\r\n
public interface ICommand\r\n {\r\n string Name { get; }\r\n\r\n Task Execute();\r\n }<\/pre>\r\n <\/p>\r\n
One of the implementations of ICommand will look like below.\u00a0<\/p>\r\n
public class UpdateStats : ICommand\r\n {\r\n public string Name => \"UpdateStats\";\r\n\r\n private readonly IStockPriceManager _stockPriceManager;\r\n private readonly List<string> _stocksToAnalyze;\r\n\r\n public UpdateStats(IStockPriceManager stockPriceManager, List<string> stocksToAnalyze)\r\n {\r\n _stockPriceManager = stockPriceManager;\r\n _stocksToAnalyze = stocksToAnalyze;\r\n }\r\n\r\n public async Task Execute()\r\n {\r\n await _stockPriceManager.ReCalculateCoefficient(_stocksToAnalyze);\r\n }\r\n }<\/pre>\r\nThe key here is that all command object share the member, Name, and method, Execute() so that these commands can be used in a standardized manner.\u00a0<\/p>\r\n
To see more about the command pattern, go to https:\/\/www.dofactory.com\/net\/command-design-pattern
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<\/p>","protected":false},"excerpt":{"rendered":"
The command pattern is useful to handle direction from args in command line app.\u00a0 For example, there was an ugly code as below, which I wrote a while ago. As I simplified the code to focus on if-else statement, it may look not that bad.\u00a0 But, I was struggling with managing the mapping each method… Continue reading Design Pattern – Command<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[548],"tags":[572,571],"class_list":["post-1140","post","type-post","status-publish","format-standard","hentry","category-c","tag-command-pattern","tag-design-pattern","entry"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t