比较 Flutter Web 中的两个字符串 / 比较两个 Cloud Firestore 对象的文档 ID

Question

尝试将documentID与List中Firestore DocumentReference对象的id进行比较。并遇到了这种奇怪的行为。我想从列表中获取正确的对象。我做错了什么？

DocumentReference _resolveReferenceFromID(
List<DocumentReference> references, String documentID) {
references.forEach((element) {
print('element id: ${element.documentID}');
print('document id: $documentID');
if (element.documentID == documentID) {
return element;
}
});
print('something is wrong here');
return null;
}

这是一个输出：

element id: W24buSwq7cYOahJz6Gdq
document id: W24buSwq7cYOahJz6Gdq
element id: cBOIXvebYYIpox5cAcct
document id: W24buSwq7cYOahJz6Gdq
something is wrong here

颤振——版本：

Flutter 1.22.0-2.0.pre.36 • channel master • https://github.com/flutter/flutter.git
Framework • revision d30e36ba8c (6 days ago) • 2020-08-21 22:36:03 -0400
Engine • revision d495da20d0
Tools • Dart 2.10.0 (build 2.10.0-49.0.dev)

扑动医生：

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel master, 1.22.0-2.0.pre.36, on Microsoft Windows [Version 10.0.18362.1016], locale en-US)
[√] Android toolchain - develop for Android devices (Android SDK version 30.0.1)
[√] Chrome - develop for the web
[√] Android Studio (version 4.0)
[√] VS Code (version 1.48.2)
[√] Connected device (3 available)
• No issues found!

Answer 1

您正在使用forEach在元素上循环。forEach函数参数内的return element仅从该函数返回，而不从周围的_resolveReferenceFromID函数返回。

更改为：

DocumentReference _resolveReferenceFromID(
List<DocumentReference> references, String documentID) {
for (var element in references) {
print('element id: ${element.documentID}');
print('document id: $documentID');
if (element.documentID == documentID) {
return element;
}
}
print('something is wrong here');
return null;
}

然后return element;将从所需功能返回。

另请参阅：https://dart.dev/guides/language/effective-dart/usage#avoid-使用可交互的foreach-带功能的侧面