Checking Text Spelling

To check the spelling of some text, an application performs the following actions:

For example, you might use the following statement to create a spell checker:

range = [[NSSpellChecker sharedSpellChecker] checkSpellingOfString:aString startingAt:0];

The checkSpellingOfString:startingAt: method checks the spelling of the words in the specified string beginning at the specified offset (this example uses 0 to start at the beginning of the string) until it finds a word that is misspelled. Then it returns an NSRange to indicate the location of the misspelled word.

In a graphical application, whenever a misspelled word is found, you’ll probably want to highlight the word in the document, using the NSRange that checkSpellingOfString:startingAt: returns to determine the text to highlight. Then you should show the misspelled word in the Spelling panel’s misspelled-word field by calling updateSpellingPanelWithMisspelledWord:. If checkSpellingOfString:startingAt: does not find a misspelled word, you should call updateSpellingPanelWithMisspelledWord: with the empty string. This causes the system to beep, letting the user know that the spell check is complete and no misspelled words were found. None of these steps is required, but if you do one, you should do them all.

The object that provides the string being checked should adopt the following protocols:




A message in this protocol (changeSpelling:) is sent down the responder chain when the user presses the Correct button.


When the object being checked responds to this protocol, the spell server keeps a list of words that are acceptable in the document and enables the Ignore button in the Spelling panel.

The application may choose to split a document’s text into segments and check them separately. This is necessary when the text has segments in different languages. Spell checking is invoked for one language at a time, so a document that contains portions in three languages requires at least three checks.

Dictionaries and Word Lists

The process of checking spelling makes use of three references:

A word is considered to be misspelled if none of these three accepts it.

Matching a List of Ignored Words with the Document It Belongs To

The NSString being checked isn’t the same as the document. In the course of processing a document, an application might run several checks based on different parts or different versions of the text, but they all belong to the same document. The NSSpellChecker keeps a separate “ignored words” list for each document that it checks. To help match “ignored words” lists to documents, you should call uniqueSpellDocumentTag once for each document. This method returns a unique arbitrary integer that will serve to distinguish one document from the others being checked and to match each “ignored words” list to a document. When searching for misspelled words, pass the tag as the fourth argument of checkSpellingOfString:startingAt:language:wrap:inSpellDocumentWithTag:wordCount:. (The convenience method checkSpellingOfString:startingAt: takes no tag. This method is suitable when the first responder does not conform to the NSIgnoreMisspelledWords protocol.)

When the application saves a document, it may choose to retrieve the “ignored words” list and save it along with the document. To get back the right list, it must send the NSSpellChecker an ignoredWordsInSpellDocumentWithTag: message. When the application has closed a document, it should notify the NSSpellChecker that the document’s “ignored words” list can now be discarded, by sending it a closeSpellDocumentWithTag: message. When the application reopens the document, it should restore the “ignored words” list with the message setIgnoredWords:inSpellDocumentWithTag:.