Translate text with ML Kit on iOS

This page describes an old version of the Translation API, which was part of ML Kit for Firebase. Development of this API has been moved to the standalone ML Kit SDK, which you can use with or without Firebase. Learn more.

See Translate text with ML Kit on iOS for the latest documentation.

You can use ML Kit to translate text between languages. ML Kit currently supports translation between 59 languages.

Before you begin

1. If you have not already added Firebase to your app, do so by following the steps in the getting started guide.
2. Include the ML Kit libraries in your Podfile:

   pod 'Firebase/MLNLTranslate', '6.25.0'
   

   After you install or update your project's Pods, be sure to open your Xcode project using its .xcworkspace.
3. In your app, import Firebase:

   Swift

   import Firebase

   Objective-C

   @import Firebase;

Translate a string of text

To translate a string between two languages:

1. Create a Translator object, configuring it with the source and target languages:

   Swift

   // Create an English-German translator:
   let options = TranslatorOptions(sourceLanguage: .en, targetLanguage: .de)
   let englishGermanTranslator = NaturalLanguage.naturalLanguage().translator(options: options)
   

   Objective-C

   // Create an English-German translator:
   FIRTranslatorOptions *options =
       [[FIRTranslatorOptions alloc] initWithSourceLanguage:FIRTranslateLanguageEN
                                             targetLanguage:FIRTranslateLanguageDE];
   FIRTranslator *englishGermanTranslator =
       [[FIRNaturalLanguage naturalLanguage] translatorWithOptions:options];
   

   If you don't know the language of the input text, you can use the language identification API first. (But be sure you don't keep too many language models on the device at once.)

2. Make sure the required translation model has been downloaded to the device. Don't call translate(_:completion:) until you know the model is available.

   Swift

   let conditions = ModelDownloadConditions(
       allowsCellularAccess: false,
       allowsBackgroundDownloading: true
   )
   englishGermanTranslator.downloadModelIfNeeded(with: conditions) { error in
       guard error == nil else { return }
   
       // Model downloaded successfully. Okay to start translating.
   }
   

   Objective-C

   FIRModelDownloadConditions *conditions =
       [[FIRModelDownloadConditions alloc] initWithAllowsCellularAccess:NO
                                            allowsBackgroundDownloading:YES];
   [englishGermanTranslator downloadModelIfNeededWithConditions:conditions
                                                     completion:^(NSError *_Nullable error) {
     if (error != nil) {
       return;
     }
     // Model downloaded successfully. Okay to start translating.
   }];
   

   Language models are around 30MB, so don't download them unnecessarily, and only download them using WiFi, unless the user has specified otherwise. You should also delete unneeded models. See Explicitly manage translation models.

3. After you confirm the model has been downloaded, pass a string of text in the source language to translate(_:completion:):

   Swift

   englishGermanTranslator.translate(text) { translatedText, error in
       guard error == nil, let translatedText = translatedText else { return }
   
       // Translation succeeded.
   }
   

   Objective-C

   [englishGermanTranslator translateText:text
                               completion:^(NSString *_Nullable translatedText,
                                            NSError *_Nullable error) {
     if (error != nil || translatedText == nil) {
       return;
     }
   
     // Translation succeeded.
   }];
   

   ML Kit translates the text to the target language you configured and passes the translated text to the completion handler.

Explicitly manage translation models

When you use the translation API as described above, ML Kit automatically downloads language-specific translation models to the device as required. You can also explicitly manage the translation models you want available on the device by using ML Kit's translation model management API. This can be useful if you want to download models ahead of time, or delete unneeded models from the device.

To get the translation models stored on the device:

Swift

let localModels = ModelManager.modelManager().downloadedTranslateModels

Objective-C

NSSet<FIRTranslateRemoteModel *> *localModels =
    [FIRModelManager modelManager].downloadedTranslateModels;

To delete a model:

Swift

// Delete the German model if it's on the device.
let deModel = TranslateRemoteModel.translateRemoteModel(language: .de)
ModelManager.modelManager().deleteDownloadedModel(deModel) { error in
    guard error == nil else { return }
    // Model deleted.
}

Objective-C

// Delete the German model if it's on the device.
FIRTranslateRemoteModel *deModel =
    [FIRTranslateRemoteModel translateRemoteModelWithLanguage:FIRTranslateLanguageDE];
[[FIRModelManager modelManager] deleteDownloadedModel:deModel
                                           completion:^(NSError * _Nullable error) {
                                               if (error != nil) {
                                                   return;
                                               }
                                               // Model deleted.
                                           }];

To download a model:

Swift

// Download the French model.
let frModel = TranslateRemoteModel.translateRemoteModel(language: .fr)

// Keep a reference to the download progress so you can check that the model
// is available before you use it.
progress = ModelManager.modelManager().download(
    frModel,
    conditions: ModelDownloadConditions(
        allowsCellularAccess: false,
        allowsBackgroundDownloading: true
    )
)

If you want to get the download status with NotificationCenter, register observers for firebaseMLModelDownloadDidSucceed and firebaseMLModelDownloadDidFail. Be sure to use a weak reference to self in the observer block, since downloads can take some time, and the originating object can be freed by the time the download finishes. For example:

NotificationCenter.default.addObserver(
    forName: .firebaseMLModelDownloadDidSucceed,
    object: nil,
    queue: nil
) { [weak self] notification in
    guard let strongSelf = self,
        let userInfo = notification.userInfo,
        let model = userInfo[ModelDownloadUserInfoKey.remoteModel.rawValue]
            as? TranslateRemoteModel,
        model == frModel
        else { return }
    // The model was downloaded and is available on the device
}

NotificationCenter.default.addObserver(
    forName: .firebaseMLModelDownloadDidFail,
    object: nil,
    queue: nil
) { [weak self] notification in
    guard let strongSelf = self,
        let userInfo = notification.userInfo,
        let model = userInfo[ModelDownloadUserInfoKey.remoteModel.rawValue]
            as? TranslateRemoteModel
        else { return }
    let error = userInfo[ModelDownloadUserInfoKey.error.rawValue]
    // ...
}

Objective-C

// Download the French model.
FIRModelDownloadConditions *conditions =
    [[FIRModelDownloadConditions alloc] initWithAllowsCellularAccess:NO
                                         allowsBackgroundDownloading:YES];
FIRTranslateRemoteModel *frModel =
    [FIRTranslateRemoteModel translateRemoteModelWithLanguage:FIRTranslateLanguageFR];

// Keep a reference to the download progress so you can check that the model
// is available before you use it.
self.downloadProgress = [[FIRModelManager modelManager] downloadModel:frModel
                                                           conditions:conditions];

If you want to get the download status with NSNotificationCenter, register observers for FIRModelDownloadDidSucceedNotification and FIRModelDownloadDidFailNotification. Be sure to use a weak reference to self in the observer block, since downloads can take some time, and the originating object can be freed by the time the download finishes.

__block MyViewController *weakSelf = self;

[NSNotificationCenter.defaultCenter
 addObserverForName:FIRModelDownloadDidSucceedNotification
 object:nil
 queue:nil
 usingBlock:^(NSNotification * _Nonnull note) {
     if (weakSelf == nil | note.userInfo == nil) {
         return;
     }

     FIRTranslateRemoteModel *model = note.userInfo[FIRModelDownloadUserInfoKeyRemoteModel];
     if ([model isKindOfClass:[FIRTranslateRemoteModel class]]
         && model == frModel) {
         // The model was downloaded and is available on the device
     }
 }];

[NSNotificationCenter.defaultCenter
 addObserverForName:FIRModelDownloadDidFailNotification
 object:nil
 queue:nil
 usingBlock:^(NSNotification * _Nonnull note) {
     if (weakSelf == nil | note.userInfo == nil) {
         return;
     }

     NSError *error = note.userInfo[FIRModelDownloadUserInfoKeyError];
 }];