AsyncStorage to save and load simple text data on the device.
AsyncStorage is built into React Native (so it’s easy to get working), but it’s text only, it’s global to your app, and it isn’t built to handle large or complex data. However - it’s perfect for saving and loading small bits of data like settings and configuration values.
Instructor: [00:00] On the add review form, we can import AsyncStorage from React Native. Then, when a reviewer submits a form, we can save their name so that we can auto-fill the name in for the next time.
[00:12] We'll start with AsyncStorage.setItem, which takes a key and a value. We can name the key whatever we'd like. Just know that it's global for the entire application. We'll set reviewer name to this.state.name.
[00:28] One important point here, the values must be strings. They cannot be objects or even numbers. The value you set cannot be null or undefined, or you'll get an error. We're OK here because the name and state defaults to an empty string, but you'll want protect against null being set in AsyncStorage.
[00:49] Then, once we've set a value, we can retrieve it with AsyncStorage.getItem. If we add a componentDidMount lifecycle method, we could get the reviewer name from AsyncStorage and set it to our local state.
[01:05] Although you can't set null values to AsyncStorage, if the reviewer name isn't set here, then the name variable will come back as null. Since we don't want null values on our form, we just won't set the state at all, although you may want to handle that differently in your application.
[01:22] If we run the app now, we can submit the form with our name. The next time the app runs, when we go to submit a review, our name is being pulled from AsyncStorage and populates the text input.
[01:43] We can't set null values to AsyncStorage. If we did decide at some point that we wanted to blank out the storage for a reviewer name, instead of setting the value, we could call removeItem, which would remove that key value pair from storage.
[01:58] Once that happens, the next time we try to load it, it would come back with a null value for reviewer name since the key doesn't exist in the store anymore. Because AsyncStorage is global to your app and because the values must be strings, it's only recommended for light use, like setting and getting simple user preferences.