Function 在函数中返回字符串

我正在做一个仓库位置管理的项目。 此代码检查可用空间。 但是我对这个代码有一个问题

当我调用函数时，它返回一个字符串，但返回“locatie is bezet”不起作用。 为什么呢

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:transportmanagement/locations/AvailableLocations.dart';

final firestoreInstance = FirebaseFirestore.instance;
var locations = availableLocations;


Future<String> CheckLocation(Location_check) async{
  // This function checks the avalible space in SGF.
  String checkLocation;
  if (Location_check.isEmpty){
    checkLocation= "Vul een locatie in";
    // return "Vul een locatie in";
  }
  else{
    if (availableLocations.containsKey(Location_check)) {
      // Check if the location is in availablelocations.dart

      // Checks if there is already a pallet on this location in Firebase
      await firestoreInstance.collection("Orders").where("locatie", isEqualTo: Location_check).limit(1).get().then((querySnapshot) {
        querySnapshot.docs.forEach((result) {
          //print(result.data());
          checkLocation= "Locatie is bezet";
        });
      });
      // return null;

    }
    else{
      checkLocation= "Locatie bestaat niet";
      // return "Locatie bestaat niet";

    }
  }
  return     checkLocation;
}

---

在函数中定义一个字符串变量，然后返回该变量

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:transportmanagement/locations/AvailableLocations.dart';

final firestoreInstance = FirebaseFirestore.instance;
var locations = availableLocations;


String CheckLocation(Location_check) {
  // This function checks the avalible space in SGF.
  String checkLocation;
  if (Location_check.isEmpty){
    checkLocation= "Vul een locatie in";
    // return "Vul een locatie in";
  }
  else{
    if (availableLocations.containsKey(Location_check)) {
      // Check if the location is in availablelocations.dart

      // Checks if there is already a pallet on this location in Firebase
      firestoreInstance.collection("Orders").where("locatie", isEqualTo: Location_check).limit(1).get().then((querySnapshot) {
        querySnapshot.docs.forEach((result) {
          //print(result.data());
          checkLocation= "Locatie is bezet";
        });
      });
      // return null;

    }
    else{
      checkLocation= "Locatie bestaat niet";
            // return "Locatie bestaat niet";

    }
  }
  return     checkLocation;
}

我希望这是有帮助的

更新

querySnapshot.docs.forEach

excite after return

checkLocation

变量

所以返回null 您必须将

wait

添加到

firestoreInstance.collection（“Orders”）.where（“locatie”，isEqualTo:Location\u check）.limit（1）.get（）

看到了吗

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:transportmanagement/locations/AvailableLocations.dart';

final firestoreInstance = FirebaseFirestore.instance;
var locations = availableLocations;


String CheckLocation(Location_check) {
  // This function checks the avalible space in SGF.
  String checkLocation;
  if (Location_check.isEmpty){
    checkLocation= "Vul een locatie in";
    // return "Vul een locatie in";
  }
  else{
    if (availableLocations.containsKey(Location_check)) {
      // Check if the location is in availablelocations.dart

      // Checks if there is already a pallet on this location in Firebase
    await  firestoreInstance.collection("Orders").where("locatie", isEqualTo: Location_check).limit(1).get().then((querySnapshot) {
        querySnapshot.docs.forEach((result) {
          //print(result.data());
          checkLocation= "Locatie is bezet";
        });
      });
      // return null;

    }
    else{
      checkLocation= "Locatie bestaat niet";
            // return "Locatie bestaat niet";

    }
  }
  return     checkLocation;
}

---

什么是CheckLocation方法return？它返回一个带有荷兰语文本“此位置上已存在托盘”的字符串。所有其他方法返回工作，但此方法不返回。它认为这是因为这个返回包含在if语句中。谢谢你的帮助！你的代码看起来不错。但当我调用此代码时，它也会返回所有检查位置，但“Locatie is bezet”返回null。这意味着不满足条件，或者快照中没有文档打印documentadd wait before

firestoreInstance.collection（“Orders”）。where（“Locatie”，isEqualTo:Location\u check）。limit（1）。get（）

我在“返回checkLocation”之前添加了“print（checkLocation）”，结果为空。但当我在“checkLocation=“Locatie is bezet”下面添加“print”（“test”）时，结果是test。因此，该代码触发print语句。

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:transportmanagement/locations/AvailableLocations.dart';

final firestoreInstance = FirebaseFirestore.instance;
var locations = availableLocations;


String CheckLocation(Location_check) {
  // This function checks the avalible space in SGF.
  String checkLocation;
  if (Location_check.isEmpty){
    checkLocation= "Vul een locatie in";
    // return "Vul een locatie in";
  }
  else{
    if (availableLocations.containsKey(Location_check)) {
      // Check if the location is in availablelocations.dart

      // Checks if there is already a pallet on this location in Firebase
    await  firestoreInstance.collection("Orders").where("locatie", isEqualTo: Location_check).limit(1).get().then((querySnapshot) {
        querySnapshot.docs.forEach((result) {
          //print(result.data());
          checkLocation= "Locatie is bezet";
        });
      });
      // return null;

    }
    else{
      checkLocation= "Locatie bestaat niet";
            // return "Locatie bestaat niet";

    }
  }
  return     checkLocation;
}