Flutter开发报错：CollectionReference无法赋值给Query参数类型

问题原因 #

Firebase Firestore默认返回的CollectionReference未指定泛型时，类型为CollectionReference<Object?>，而GeoFire的collection()方法需要的参数类型是Query<Map<String, dynamic>>，两者类型不兼容导致报错。

修复方案 #

直接在声明CollectionReference时指定泛型为Map<String, dynamic>，修改后的代码如下：

Stream nearbyATM() async* {
    GeoFirePoint point = geo.point(latitude: _position!.latitude, longitude: _position!.longitude);
    // 指定泛型为Map<String, dynamic>
    final CollectionReference<Map<String, dynamic>> atms = _firestore.collection("atms");
    double radius = 100;
    String field = "location";
    Stream<List<DocumentSnapshot>> stream = geo
                            .collection(collectionRef: atms)
                            .within(center: point, radius: radius, field: field);
}

替代方案（类型转换） #

如果不想修改原声明，也可以通过类型转换适配参数类型：

Stream nearbyATM() async* {
    GeoFirePoint point = geo.point(latitude: _position!.latitude, longitude: _position!.longitude);
    final CollectionReference atms = _firestore.collection("atms");
    double radius = 100;
    String field = "location";
    Stream<List<DocumentSnapshot>> stream = geo
                            .collection(collectionRef: atms as CollectionReference<Map<String, dynamic>>)
                            .within(center: point, radius: radius, field: field);
}

内容的提问来源于stack exchange，提问作者Abdel