异步存储无法设置或启动React Native

Question

我试图使用异步存储来进行用户身份验证,以决定将呈现哪个屏幕,所以当我从异步存储中获取数据时返回给我未定义有人可以帮我吗？

我的获取代码:

var login;
AsyncStorage.getItem("login").then((value) => {
      login = value;
}).done();
alert(login);

我的设定代码:

const insert_user = (username) => {
  AsyncStorage.setItem("login", username);
  Toast.show({
    supportedOrientations: ['portrait', 'landscape'],
    text: `User registered with success`,
    position: 'bottom',
    buttonText: 'Dismiss'
  });
}

Answer 1

alert(login); 将始终未定义,因为AsyncStorage.getItem本质上是异步的意味着在从AsyncStorage.getItem接收值之前首先调用alert(login)

AsyncStorage.getItem("login").then((value) => {
      alert(value); // you will need use the alert in here because this is the point in the execution which you receive the value from getItem.
    // you could do your authentication and routing logic here but I suggest that you place them in another function and just pass the function as seen in the example below.
});

// a function that receives value
const receiveLoginDetails = (value) => {
    alert(value);
}
// pass the function that receives the login details;
AsyncStorage.getItem("login").then(receiveLoginDetails);

进一步参考

• AsyncStorage.getItem将返回一个Promise:https://www.promisejs.org/
• 异步与同步:https://www.pluralsight.com/guides/front-end-javascript/introduction-to-asynchronous-javascript