试图让firebase的价值观偏离承诺

我是一个新的本地人，我很难从承诺中的firebase查询中获取值

我试图在promise中设置state，但控制台返回：TypeError:_this2.setState不是函数

_getActivites() {
      const latitude = 42.297761;
      const longitude = 4.636235;
      const radius = 5;

  var keys = [];
  var activitesToState = [];

  const firebaseRef = firebase.database().ref("activites_locations/");
  const geoFire = new GeoFire(firebaseRef);
  var geoQuery;
  var activites = [];

  geoQuery = geoFire.query({
    center: [latitude, longitude],
    radius: radius
  });

  geoQuery.on("key_entered", function(key, location, distance) {
    keys.push(key);
  });

  geoQuery.on("ready", function() {
    var promises = keys.map(function(key) {
      return firebaseRef.child(key).once("value");
    });
    Promise.all(promises).then((snapshots) => {
      snapshots.forEach(function(snapshot) {
        activites.push(snapshot.val());
      });
      this.setState({
        activitesState: activites,
      })
    }).catch((error) => {
      console.log(error);
    });

  });

};

componentDidMount() {
  firebase.auth().signInAnonymously()
    .then(() => {
      this.setState({
        isAuthenticated: true,
      });
  });

  this._getActivites();
}

---

您正在函数调用中丢失

this

的值。您应该通过将函数更新为箭头函数来绑定调用。您还可以通过将其设置为函数范围内的一个变量来停止此的丢失

_getActivites() {
      const latitude = 42.297761;
      const longitude = 4.636235;
      const radius = 5;

  var keys = [];
  var activitesToState = [];

  const firebaseRef = firebase.database().ref("activites_locations/");
  const geoFire = new GeoFire(firebaseRef);
  var geoQuery;
  var activites = [];

  geoQuery = geoFire.query({
    center: [latitude, longitude],
    radius: radius
  });

  geoQuery.on("key_entered", function(key, location, distance) {
    keys.push(key);
  });

  geoQuery.on("ready", function() {
    var promises = keys.map(function(key) {
      return firebaseRef.child(key).once("value");
    });
    Promise.all(promises).then((snapshots) => {
      snapshots.forEach(function(snapshot) {
        activites.push(snapshot.val());
      });
      this.setState({
        activitesState: activites,
      })
    }).catch((error) => {
      console.log(error);
    });

  });

};

componentDidMount() {
  firebase.auth().signInAnonymously()
    .then(() => {
      this.setState({
        isAuthenticated: true,
      });
  });

  this._getActivites();
}

重构代码时，您可能会遇到如下情况：

_getActivites = () => { // change to arrow function
  const that = this;  // capture the value of this
  const latitude = 42.297761;
  const longitude = 4.636235;
  const radius = 5;

  var keys = [];
  var activitesToState = [];

  const firebaseRef = firebase.database().ref('activites_locations/');
  const geoFire = new GeoFire(firebaseRef);
  var geoQuery;
  var activites = [];

  geoQuery = geoFire.query({
    center: [latitude, longitude],
    radius: radius
  });

  geoQuery.on('key_entered', (key, location, distance) => { // change to arrow function
    keys.push(key);
  });

  geoQuery.on('ready', () => { // change to arrow function
    var promises = keys.map((key) => { // change to arrow function
      return firebaseRef.child(key).once('value');
    });
    Promise.all(promises).then((snapshots) => {
      snapshots.forEach((snapshot) => {
        activites.push(snapshot.val());
      });
      that.setState({ activitesState: activites }); // use "that" instead of "this"
    }).catch((error) => {
      console.log(error);
    });
  });
}

---

这里有一个关于

this

和它失去上下文的好例子。

您正在函数调用中失去

this

的价值。您应该通过将函数更新为箭头函数来绑定调用。您还可以通过将其设置为函数范围内的一个变量来停止此的丢失

_getActivites() {
      const latitude = 42.297761;
      const longitude = 4.636235;
      const radius = 5;

  var keys = [];
  var activitesToState = [];

  const firebaseRef = firebase.database().ref("activites_locations/");
  const geoFire = new GeoFire(firebaseRef);
  var geoQuery;
  var activites = [];

  geoQuery = geoFire.query({
    center: [latitude, longitude],
    radius: radius
  });

  geoQuery.on("key_entered", function(key, location, distance) {
    keys.push(key);
  });

  geoQuery.on("ready", function() {
    var promises = keys.map(function(key) {
      return firebaseRef.child(key).once("value");
    });
    Promise.all(promises).then((snapshots) => {
      snapshots.forEach(function(snapshot) {
        activites.push(snapshot.val());
      });
      this.setState({
        activitesState: activites,
      })
    }).catch((error) => {
      console.log(error);
    });

  });

};

componentDidMount() {
  firebase.auth().signInAnonymously()
    .then(() => {
      this.setState({
        isAuthenticated: true,
      });
  });

  this._getActivites();
}

重构代码时，您可能会遇到如下情况：

_getActivites = () => { // change to arrow function
  const that = this;  // capture the value of this
  const latitude = 42.297761;
  const longitude = 4.636235;
  const radius = 5;

  var keys = [];
  var activitesToState = [];

  const firebaseRef = firebase.database().ref('activites_locations/');
  const geoFire = new GeoFire(firebaseRef);
  var geoQuery;
  var activites = [];

  geoQuery = geoFire.query({
    center: [latitude, longitude],
    radius: radius
  });

  geoQuery.on('key_entered', (key, location, distance) => { // change to arrow function
    keys.push(key);
  });

  geoQuery.on('ready', () => { // change to arrow function
    var promises = keys.map((key) => { // change to arrow function
      return firebaseRef.child(key).once('value');
    });
    Promise.all(promises).then((snapshots) => {
      snapshots.forEach((snapshot) => {
        activites.push(snapshot.val());
      });
      that.setState({ activitesState: activites }); // use "that" instead of "this"
    }).catch((error) => {
      console.log(error);
    });
  });
}

---

这里有一个关于

这篇

的好文章，它失去了它的上下文。

非常感谢你的代码和文章，它真的帮助了我很多！知道我是一个初学者，您认为我编写的代码是使用firebase和geofire查询数据的最佳方式（性能方面）吗？你认为我能做些改进吗？看起来不错。我喜欢你用承诺。所有，这样你只设定一次状态。谢谢！再次感谢你用React Native帮助我提高自己。非常感谢你的代码和文章，它真的帮了我很大的忙！知道我是一个初学者，您认为我编写的代码是使用firebase和geofire查询数据的最佳方式（性能方面）吗？你认为我能做些改进吗？看起来不错。我喜欢你用承诺。所有，这样你只设定一次状态。谢谢！再次感谢你帮助我用React Native提升自己。