Augmenting MOLH

[ahmedk92]

(Originally published 2020-04-01)

MOLH is a popular library among developers whose audience are of Arabic background. It's common for Arabic apps to support multiple languages, and also support changing between them without restarting. I wrote here how iOS doesn't support that out-of-the-box and what were the possible workarounds back-then (this was a write-up for a solution I reached before MOLH was made public).
In this article I'll try to briefly reiterate how MOLH works to reach to its limitation, and then try to find some solutions/workarounds.

Before we start, these limitations are not specific to MOLH; it's rather general to any approach that tries to work around iOS's limitation to change the app's language without restarting.

How MOLH works

(Feel free to skip this section if you already know how MOLH works)

Two aspects of an app reflects to the user that it respects the desired localization: the language of the texts, and the direction of how the flow of the views (left-to-right or right-to-left).
As you may already know, the default behavior of iOS is to pick and suitable language at startup (of the app) and upon that it picks the relevant strings from string files in that language's lproj folder, as well as applying the corresponding language direction, and sticks to that.

Two goals here:

1. Guiding NSLocalizedString to search the new language folder (i..e ar.lproj, en.lproj, ...etc)
2. Forcing new views to flip (or not) with respect to the new language.

(1) NSLocalizedString

NSLocalizedString delegates to NSBundle.mainBundle for getting the suitable localized string value.
Upon app startup, NSBundle.mainBundle makes up its mind once and for all on what language folder it's going to search till the next run. To overcome this limitation, MOLH swizzles the main bundle and dynamically loads the relevant bundle to search for the required string.

(2) Flipping views

This is somewhat easier. UIView formally supports forcing the language direction independently of the actual app language. This is done via setting the semanticContentAttribute to the desired value.
MOLH does this for all newly created views by using the appearance proxy to set the suitable semanticContentAttribute. This is why it needs the app to virtually "restart" (i.e. start over from the root view controller, re-creating all views).

What MOLH doesn't solve (and it doesn't have to)

Formatters

There are some classes that must pick some locale information to correctly present its data. Such classes include formatters (e.g. NumberFormmatter and DateFormatter). If we don't explicitly set the locale property of such formatters to the desired language, it will pick the actual app language, causing a date to be displayed in the wrong language for example.

let dateFormatter = DateFormatter()
dateFormatter.locale = Locale(identifier: "ar")

NSTextAlignmentJustified

Formatters are easy to handle as we saw. However, justified UILabel and UITextView yield unwanted results. This is because justifying works by distributing space in every line of text so that each line starts and ends at the same start and end points respectively, and aligning the remaining last line either left or right if it's not wide enough. As you you may already guessed, left or right alignment is picked according to the actual app language.

Unlike formatters, there seems to be no explicit way to guide UILabel nor UITextView on how to force that alignment. However, there's a way to achieve the desired justifying with NSAttributedString.
NSAttributedString (a world of its own) accepts an attribute called paragraphStyle.
What matters to us from it is the baseWritingDirection property. We can set it to either to NSWritingDirectionLeftToRight or NSWritingDirectionRightToLeft. This affects decisions that rely on such information, namely justifying and natural alignment. Sample code:

let attributedString = NSAttributedString(string: string, attributes: [
    .paragraphStyle: {
        let style = NSMutableParagraphStyle()
        style.alignment = .justified
        style.baseWritingDirection = .rightToLeft
        return style
    }()
])

Demo

I made a demo of these problems (without the solutions). You can check it here. Make sure to read the usage notes before inspecting.

Thanks for reading! Corrections and suggestions are welcome.