March 2020

Using Swift string literals to load bundle resources

My new iOS fiction project relies heavily on text.

That means I want to make it easy to create that content anywhere, and I want it to be frictionless to drop it into the project as needed.

My solution: Markdown files I can load from the bundle using string literals. Look how easy:

let markdown: MarkdownFile = "markdown.md"

Here’s how to do it.

String literals

In Swift, you use string literals all the time. Usually to initialize strings.

let string: String = "Hello, I am a string."

But Swift includes a protocol called ExpressibleByStringLiteral. Which means if your Swift type adopts it, that type can be initialized with nothing more than a string. While this is immediately convenient, it has real power for assets that need tedious boilerplate. Say, anything that needs to be loaded from a bundle.

Basic example

struct MarkdownFile: ExpressibleByStringLiteral {
    
    let bundleName: String
    let rawMarkdown: String?
    
    init(stringLiteral: String) {
        
        bundleName = stringLiteral
        
        var loadedMarkdown: String? = nil
                
        if let filepath = Bundle.main.path(forResource: bundleName, ofType: nil) {
        //By skipping the ofType argument above, we'll match to the first file whose name
        //exactly matches bundleName
            do {
                let loadedString = try String(contentsOfFile: filepath)
                loadedMarkdown = loadedString
            } catch {
                print("Could not load string: \(error)")
            }
        } else {
            print("Could not find file: \(bundleName)")
        }
        
        rawMarkdown = loadedMarkdown
    }
}

Here’s a basic example of a MarkdownFile struct. It knows two things about itself: the name of the file used to initialize it, and any string it was able to load from a file in the bundle with that name.

On init it goes looking for a bundle resource matching the name it was provided through the string literal. If it finds one, and it can load its contents as a string, those contents are stored to rawMarkdown. If not, rawMarkdown returns nil.

This is already pretty convenient. Again, to initialize, all you need is:

let markdown: MarkdownFile = "markdown.md"

But we can take it further.

Adding convenience

The MarkdownFile struct can be responsible for converting its contents into a display representation, as well. Let’s add a computed property to parse the Markdown into HTML. I’ll be using Ink for this, but you could use any project—or convert it into something else, like NSAttributedString.

var htmlRepresentation: String? {
    if let raw = rawMarkdown {
        return MarkdownParser().html(from: raw)
    } else {
        return nil
    }
}

Putting it all together

With our output property all set up, we have a small, convenient API for handling Markdown files in any way we want. Here’s how we use it:

let markdown: MarkdownFile = "markdown.md"

if let html = markdown.htmlRepresentation {
    webview.loadHTMLString(html, baseURL: nil)
}

self.title = markdown.bundleName

Behind the scenes, lots of stuff is happening to load and parse the file. But when you need Markdown across your project, you need only concern yourself with a filename. If you want to change any part of how this works later on, you have a single struct that’s responsible for all the Markdown behavior in your code.

Full example code here.