可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试):

问题:

I'm trying to get information (true/false) from AsyncStorage in a function and create a string which is importent to fetch data in the next step. My problem is, the function is not finished until the string is required.

I tried many solutions from the internet like async function and await getItem or .done() or .then(), but none worked out for me.

The current behaviour is that the console displays first "channel required: " than "channel: channel_id0".

回答1:

Aspects in your question are unclear:

You don't say when this.state.firstValue is set, and how that relates to what you are trying to accomplish.
You have a for-loop where you could be setting the same value multiple times.
You mutate the state rather than set it. This is not good, see this SO question for more on that.

There are somethings we can do to make your code easier to understand. Below I will show a possible refactor. Explaining what I am doing at each step. I am using async/await because it can lead to much tidier and easier to read code, rather than using promises where you can get lost in callbacks.

Get all the keys from AsyncStorage
Make sure that there is a value for all the keys.
Filter the keys so that we only include the ones that do not contain the string 'not'.
Use a Promise.all, this part is important as it basically gets all the values for each of the keys that we just found and puts them into an array called items
Each object in the items array has a key and a value property.
We then filter the items so that only the ones with a item.value === 'true' remain.
We then filter the items so that only the ones with a item.value !== 'true' remain. (this may be optional it is really dependent on what you want to do)
What do we return? You need to add that part.

Here is the refactor:

_getFetchData = async () => {
  let allKeys = await AsyncStorage.getAllKeys();                             // 1
  if (allKeys.length) {                                                      // 2

    let filteredKeys = allKeys.filter(key => !key.includes('not'));          // 3
    let items = await Promise.all(filteredKeys.map(async key => {            // 4
      let value = await AsyncStorage.getItem(key);
      return { key, value };                                                 // 5
    }))

    let filteredTrueItems = items.filter(item => items.value === 'true');    // 6
    let filteredFalseItems = items.filter(item => items.value !== 'true');   // 7
    // now you have two arrays one with the items that have the true values 
    // and one with the items that have the false values
    // at this points you can decide what to return as it is not 
    // that clear from your question

    // return the value that your want                                       // 8
  } else {
    // return your default value if there are no keys                        // 8
  }
}

You would call this function as follows:

_fetchData = async () => {
  let channel = await this._getFetchData();
  console.log("channel required: " + channel);
}

Although the above will work, it will not currently return a value as you haven't made it clear which value you wish to return. I would suggest you build upon the code that I have written here and update it so that it returns the values that you want.

回答2:

Try this instead. Async functions and Promises can be tricky to get right and can be difficult to debug but you're on the right track.

async _getFetchData() {
    let channels = "";

    let results = await AsyncStorage.getAllKeys();

    results.forEach((result) => {
        if (result.includes("not") === false) {
            let item = await AsyncStorage.getItem(result);

            if (item === 'true') {
                console.log(`channel: ${result}`)

                channels = `channel_id ${result}`;
            }
        }
    });

    return channels;
}

_fetchData() {
    this._getFetchData().then((channels) => {
        console.log(`channel required: ${channel}`);
    });
}

回答3:

what if you wrap the _getFetchData() in a Promise? This would enable you to use

var channel = this._getFetchData().then(console.log("channel required: " + channel));

Otherwise the console.log won't wait for the execution of the _getFetchData(). This is what the console.log is telling you. it just logs the string. the variable is added after the async operation is done.

UPDATE

I would try this:

//_getFetchData()
AsyncStorage.getAllKeys().then((result) => { //get all stored Keys
  valuelength = result.length;
  if (valuelength !== 0) {
    for (let i = 0; i < valuelength; i++) {
      if (result[i].includes("not") == false) { //get Keys without not
        AsyncStorage.getItem(result[i]).then((resultvalue) => {
          if (resultvalue === 'true') {
            if (this.state.firstValue) {
              this.state.channels = this.state.channels + "channel_id" + result[i];
              console.log("channel: " + this.state.channels);
            }
            else {
              this.state.channels = this.state.channels + "channel" + result[i];
            }
          }
        });
      }
return new Promise((resolve, reject) => {
    this.state.channels !=== undefined ? resolve(this.state.channels) : reject(Error('error '));
}

_fetchData() {
var channel = this._getFetchData().then(console.log("channel required: " + channel));
}

maybe you must change the this.state.channels !=== undefined to an expression that's matches the default value of this.state.channels.