Acheiving Dynamic Localisation in iOS

[ahmedk92]

(Originally published 2017-12-15)

While you can make your app support multiple languages, still there's no easy (and documented) way to dynamically change the app's localisation. You either let the user choose the iOS device's language; and your app just follows along. Or you can use that AppleLanguages solution. But if you want the change somewhere after the app has started (i.e. not in the main function), you have to close the app, and wait for the user to open it again. Which is not the best we can expect.

Alternatives

There are many libraries (on cocoapods for example) that enables you to acheive that without closing the app. But they just work on strings; either an NSLocalizedString replacement, or overriding its behaviour. Which is good, but unfortunately ignore flipping views on RTL languages like Arabic.

What worked for me

To organize, we have two problems: (1) flipping views based on the chosen language direction preference. (2) Guiding NSLocalizedString to use the correct strings.

(1) Flipping views

Since iOS 9, there's this property that enables you to decide, for a view, whether it should be always displayed left-to-right, or right-to-left (despite whatever is the language), or it should follow the language. The property is semanticContentAttribute.

Now, the question is when to set that property to take its effect? and how? Usually, UI is diversely created; maybe from storyboards, XIBs, or code. So, an early point of initialization is a good time; e.g. initWithCoder: (for views designed in Interface Builder) or iniWithFrame (for views created by code). awakeFromNib is a good time too for views designed in Interface Builder. OK, but how would we override those methods? Swizzling to the rescue.

Through swizzling, we can check the language in awakeFromNib for example, and set the relevant semanticContentAttribute. Good!, but there may be a little caveat. There may be some views that are required to be always left-to-right or righ-to-left all the time; we don't want to mess them up. We can avoid that by a simple check; we only apply the semanticContentAttribute to views with an .unspecified value.

So far so good. But what about currently visible views? what can we do about them? One can think of making every visible view observe the language change (via NSNotificationCenter for example), then try to force a redraw. But I don't like this approach, and I prefer to simply restart the flow of my app. That is re-assigning the window's root view controller with a new one, and start over the app's flow. Something like this:

for view in UIApplication.shared.window.rootViewController.view.subviews {
    view.removeFromSuperView()
}
UIApplication.shared.window.rootViewController = UIApplication.shared.window.rootViewController.storyBoard.instantiateInitialViewController 

That's it. And it works nicely.

(2) NSLocalizedString

As mentioned above, there are many libraries that do this. Most of them offer a replacement for NSLocalizedString. But sometimes, the dynamic localiztation feature is required so late in the project life-time that NSLocalizedString is scattered all over the project. So, overriding its behavior would be better. I heard about a library that does this, but couldn't find it 😑. However, that overriding is not hard.

I built my solution on this StackOverflow answer. The whole discussion is useful. The idea is that NSLocalizedString is just a macro that uses localizedStringForKey:value:table: method of the NSBundle class; specifically on [NSBundle mainBundle] instance. So, swizzling is again used here. In short, the [NSBundle mainBundle] instance's implementation is replaced by an NSBundle subclass that, on language change, makes the [NSBundle mainBundle] return a new bundle that is relevant to the selected language. Easy, but tricky.

That's it.

Notes

• Maybe it won't be enough to just have the views flipped; you may want text views (e.g. UILabel, UITextField) to flip the text alignment too. That's acheivable in our same awakeFromNib implementation explained above. All you need is checking wether the current view is a UILabel, then repeat the same logic; check wether the text alignment (NSTextAlignment) is set to .natural, then set to the correct alignment (i.e. .left, or .right).