Swift and the Command Line
Recently I’ve got a task to allow Electron-based desktop app to add events to macOS Calendar. There are not too many options to do that. I’ve made up a way to provide a macOS native layer to that JavaScript app.
I’ve written a macOS command line tool in Swift to add events and reminders using EventKit framework. This was truly great experience. Last time I made command line tools about 10 years ago in pure C.
A program importing NSFileHandle to be able to write to stderr, and C.stdlib to return a non-zero exit status via abort() function to denote failure. An exit status of 0 denotes success.
Electron framework allows web app to run shell commands. The program receives input data as command line arguments, the argument order is predefined with a frontend developers team. Usage example:
./myapp event "a title" 1497520000 1497529999An event (or reminder) is a type of entity to create. A title. A start and end dates of event are in UNIX timestamp format.
The first argument that command line program receives is always a path where its executable file is situated. For this case the first argument is dropped and the result is assigned to a payload variable.
Next, create an internal EventTask struct with arguments mapped to Swift types. Initialize CalendarController that internally initializes EKEventStore, synchronoulsy requests access to macOS Calendar, and adds events.
The app is designed to throw Swift exceptions with detailed explanation of possible errors: wrong input, saving failed, no auth to Calendar given, or even if a confirmation dialog wasn’t accepted or denied in 60 seconds:
All event-creation code is performed in do block. In case of any exception the catch block will print error description to stderr and exit with non-zero code. But the JavaScript team only observe if exit code is 0 or not.
I am very proud of the result. It seems like cross-platform apps are almost unviable without a some layer providing platform-specific solutions.
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