如何使用Dart中的镜像获取通用类型变量的具体类型？ [英] How can I get the concrete type of a generic type variable using mirrors in Dart?

问题描述

假设我有 List 的 String ，就像这样。

  var myList = new List< String>（）; 
  

如何知道 myList $ c

我尝试使用 ClassMirror 的 typeVariables ，但镜像似乎只是描述gerneric List 类。

  InstanceMirror im = reflect（myList）; // InstanceMirror on'List'的实例
 ClassMirror cm = im.type; // ClassMirror on'List'
 print（cm.typeVariables ['E']）// TypeVariableMirror on'E'
  

我在文档中也发现了这一点，但我还没有找到一个 ClassMirror 实例，其中访问 originalDeclaration 不会抛出 NoSuchMethodError 。

final ClassMirror

对于大多数类，它们都是这个类型的原始声明的镜像。是他们自己的原始声明。然而，对于通用的
类，有原始类的
声明（其具有未绑定的类型变量）和实例化
（具有绑定类型变量的泛型类）之间的区别。

解决方案

2种可能的方法。

第一种方法是使用是运算符来检查变量的类型，因为它比反射更有效： p>

  var myList = new List< String>（）; 
 print（myList is List< int>）; // false 
 print（myList is List< String>）; // true 
  

第二种方法是使用 ClassMirror.typeArguments ：

  import'dart：mirrors'; 
 var myList = new List< String>（）; 
 
映射typeArguments = reflect（myList）.type.typeArguments; 
 print（typeArguments）; // {Symbol（T）：ClassMirror on'String'} 
 
 ClassMirror firstTypeArg = typeArguments [typeArguments.keys.first]; 
 print（firstTypeArg.reflectedType）; // String 
  

Assuming I have a List of Strings like this.

var myList = new List<String>();

How can I figure out that myList is a List of Strings using mirrors?

---

I tried it using the typeVariables of ClassMirror but the mirror seems to just describe the gerneric List class.

InstanceMirror im = reflect(myList); // InstanceMirror on instance of 'List'
ClassMirror cm = im.type; // ClassMirror on 'List'
print(cm.typeVariables['E']) // TypeVariableMirror on 'E'

I also found this in the documentation but I have yet to find a ClassMirror instance where accessing originalDeclaration doesn't throw a NoSuchMethodError.

final ClassMirror originalDeclaration

A mirror on the original declaration of this type.

For most classes, they are their own original declaration. For generic classes, however, there is a distinction between the original class declaration, which has unbound type variables, and the instantiations of generic classes, which have bound type variables.

解决方案

2 possible methods.

The first method is to check the variable's type using is operator, as it's more performant than reflection:

var myList = new List<String>();
print(myList is List<int>); // false
print(myList is List<String>); // true

The second method is to use ClassMirror.typeArguments:

import 'dart:mirrors';
var myList = new List<String>();

Map typeArguments = reflect(myList).type.typeArguments;
print(typeArguments); // {Symbol("T"): ClassMirror on 'String'}

ClassMirror firstTypeArg = typeArguments[typeArguments.keys.first];
print(firstTypeArg.reflectedType); // String

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