Make no mistake, a tiny keyboard on a slab of glass doesn’t always lend itself to perfect typing. Whether for accuracy or hilarity, anyone typing on an iOS device notices when autocorrect steps in to help out. You might not know, however, that UIKit includes a class to help you with your user’s typing inside your app.

First introduced in iOS 3.2 (or should we call it iPhone OS 3.2, given the early date?), UITextChecker does exactly what it says: it checks text. Read on to learn how you can use this class for spell checking and text completion.

Spell Checking

What happens if you mistype a word in iOS? Type “hipstar” into a text field and iOS will offer to autocorrect to “hipster” most of the time.

Autocorrecting 'hipstar'

We can find the same suggested substitution using UITextChecker:

import UIKit

let str = "hipstar"
let textChecker = UITextChecker()
let misspelledRange =
    textChecker.rangeOfMisspelledWord(in: str,
                                      range: NSRange(0..<str.utf16.count),
                                      startingAt: 0,
                                      wrap: false,
                                      language: "en_US")

if misspelledRange.location != NSNotFound,
    let firstGuess = textChecker.guesses(forWordRange: misspelledRange,
                                         in: str,
                                         language: "en_US")?.first
    print("First guess: \(firstGuess)") // First guess: hipster
} else {
    print("Not found")

The returned array of strings might look like this one:

["hipster", "hip star", "hip-star", "hips tar", "hips-tar"]

Or it might not—UITextChecker produces context- and device-specific guesses. According to the documentation, guessesForWordRange(_:inString:language:) “returns an array of strings, in the order in which they should be presented, representing guesses for words that might have been intended in place of the misspelled word at the given range in the given string.”

So no guarantee of idempotence or correctness, which makes sense for a method with guesses... in the name. How can NSHipsters trust a method that changes its return value? We’ll find the answer if we dig further.

Learning New Words

Let’s assume that you want your users to be able to type "hipstar" exactly. Let your app know that by telling it to learn the word, using the UITextChecker.learnWord(_:) class method:


"hipstar" is now a recognized word for the whole device and won’t show up as misspelled in further checks.

let misspelledRange =
    textChecker.rangeOfMisspelledWord(in: str,
                                      range: NSRange(0..<str.utf16.count),
                                      startingAt: 0,
                                      wrap: false,
                                      language: "en_US")
misspelledRange.location == NSNotFound // true

As expected, the search above returns NSNotFound, for UITextChecker has learned the word we created. UITextChecker also provides class methods for checking and unlearning words: UITextChecker.hasLearnedWord(_:) and UITextChecker.unlearnWord(_:).

Suggesting Completions

There’s one more UITextChecker API, this time for finding possible completions for a partial word:

let partial = "hipst"
let completions = textChecker.completions(
                    forPartialWordRange: NSRange(0..<partial.utf16.count),
                    in: partial,
                    language: "en_US"
completions == ["hipster", "hipsters", "hipster's"] // true

completionsForPartialWordRange gives you an array of possible words from a group of initial characters. Although the documentation states that the returned array of strings will be sorted by probability, UITextChecker only sorts the completions alphabetically. UITextChecker’s OS X-based sibling, NSSpellChecker, does behave as it describes.

You won’t see any of the custom words you’ve taught UITextChecker show up as possible completions. Why not? Since vocabulary added via UITextChecker.learnWord(_:) is global to the device, this prevents your app’s words from showing up in another app’s autocorrections.

Building an app that leans heavily on a textual interface? Use UITextChecker to make sure the system isn’t flagging your own vocabulary. Writing a keyboard extension? With UITextChecker and UILexicon, which provides common and user-defined words from the system-wide dictionary and first and last names from the user’s address book, you can support nearly any language without creating your own dictionaries!


Questions? Corrections? Issues and pull requests are always welcome.

This article uses Swift version 4.2. Find status information for all articles on the status page.

Written by Croath Liu
Croath Liu

Croath Liu (@cr0ath) is an iOS developer and one of the lead translators for nshipster.cn. He lives in Beijing.

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