Detect text direction based on content

I've had this problem myself. With a little bit of search, made this extension for UITextView which detects the first letter's language and makes it RTL if it's needed. You need to call the function after setting text, so you may want to call it in UITextViewDelegate text change" method.

extension UITextView {
func detectRightToLeft() {
    if let text = self.text where !text.isEmpty {
        let tagschemes = NSArray(objects: NSLinguisticTagSchemeLanguage)
        let tagger = NSLinguisticTagger(tagSchemes: tagschemes as! [String], options: 0)
        tagger.string = text

        let language = tagger.tagAtIndex(0, scheme: NSLinguisticTagSchemeLanguage, tokenRange: nil, sentenceRange: nil)
        if language?.rangeOfString("he") != nil || language?.rangeOfString("ar") != nil || language?.rangeOfString("fa") != nil {
            self.text = text.stringByReplacingOccurrencesOfString("\n", withString: "\n")
            self.textAlignment = .Right
            self.makeTextWritingDirectionRightToLeft(nil)
        }else{
            self.textAlignment = .Left
            self.makeTextWritingDirectionLeftToRight(nil)
        }
    }
}
}

Surely this is messy and not perfect. But it worked for me. You can get the idea.

Swift 3:

extension UITextView {
    func detectRightToLeft() {
        if let text = self.text, !text.isEmpty {
            let tagschemes = NSArray(objects: NSLinguisticTagSchemeLanguage)
            let tagger = NSLinguisticTagger(tagSchemes: tagschemes as! [String], options: 0)
            tagger.string = text

            let language = tagger.tag(at: 0, scheme: NSLinguisticTagSchemeLanguage, tokenRange: nil, sentenceRange: nil)
            if language?.range(of: "he") != nil || language?.range(of: "ar") != nil || language?.range(of: "fa") != nil {
                self.text = text.replacingOccurrences(of: "\n", with: "\n")
                self.textAlignment = .right
                self.makeTextWritingDirectionRightToLeft(nil)
            }else{
                self.textAlignment = .left
                self.makeTextWritingDirectionLeftToRight(nil)
            }
        }
    }
}