如何在dart中创建动态getter /setter ?

Question

我试图重新创建django的QueryDict功能，并使一个对象，可以给一个地图，它是一个私有变量在对象和getter/setter是用来从地图动态拉。我已经设法重新创建它的get()方法，但我失去了动态获取值。以下是目前为止的内容:

class QueryMap {
  Map _data;
  QueryMap(Map this._data);
  dynamic get(String key, [var defaultValue]) {
    if(this._data.containsKey(key)) {
      return this._data[key];
    } else if(defaultValue) {
      return defaultValue;
    } else {
      return null;
    }
  }
}

下面是django的工作原理:https://docs.djangoproject.com/en/dev/ref/request-response/django.http.QueryDict。 getitem

Answer 1

可以覆盖noSuchMethod(仿真函数)

@proxy
class QueryMap {
  Map _data = new Map();
  QueryMap();
  noSuchMethod(Invocation invocation) {
    if (invocation.isGetter) {
      var ret = _data[invocation.memberName.toString()];
      if (ret != null) {
        return ret;
      } else {
        super.noSuchMethod(invocation);
      }
    }
    if (invocation.isSetter) {
      _data[invocation.memberName.toString().replaceAll('=', '')] =
          invocation.positionalArguments.first;
    } else {
      super.noSuchMethod(invocation);
    }
  }
}
void main() {
  QueryMap qaueryMap = new QueryMap();
  qaueryMap.greet = "Hello Dart!";
  print(qaueryMap.greet); //Hello Dart!
}

正如@PixelElephant所指出的，使用外部映射时，你必须使用真实的方法名作为映射键:

import 'dart:mirrors';
@proxy
class QueryMap {
  Map _data;
  QueryMap(this._data);
  noSuchMethod(Invocation invocation) {
    if (invocation.isGetter) {
      var ret = _data[MirrorSystem.getName(invocation.memberName)];
      if (ret != null) {
        return ret;
      } else {
        super.noSuchMethod(invocation);
      }
    }
    if (invocation.isSetter) {
      _data[MirrorSystem.getName(invocation.memberName).replaceAll('=', '')] =
          invocation.positionalArguments.first;
    } else {
      super.noSuchMethod(invocation);
    }
  }
}
void main() {
  Map myMap = new Map();
  myMap["color"] = "red";
  QueryMap qaueryMap = new QueryMap(myMap);
  qaueryMap.greet = "Hello Dart!";
  print(qaueryMap.greet); //Hello Dart!
  print(qaueryMap.color); //red
}

为了避免使用镜像，你可以在symbol的字符串序列化或转换外部映射键时使用模式匹配:

@proxy
class QueryMap {
  Map _data;
  QueryMap(Map data) {
    _data = new Map();
    data.forEach((k, v) => _data[new Symbol(k).toString()] = v);
  }
  noSuchMethod(Invocation invocation) {
    if (invocation.isGetter) {
      var ret = _data[invocation.memberName.toString()];
      if (ret != null) {
        return ret;
      } else {
        super.noSuchMethod(invocation);
      }
    }
    if (invocation.isSetter) {
      _data[invocation.memberName.toString().replaceAll('=', '')] =
          invocation.positionalArguments.first;
    } else {
      super.noSuchMethod(invocation);
    }
  }
}
void main() {
  Map myMap = new Map();
  myMap["color"] = "red";
  QueryMap qaueryMap = new QueryMap(myMap);
  qaueryMap.greet = "Hello Dart!";
  print(qaueryMap.greet); //Hello Dart!
  print(qaueryMap.color); //red
}