如何使一个工厂构造函数返回作为const值 [英] How to make a factory constructor that returns as const value

问题描述

是否可能返回一个不同的const实现？

  abstract class Foo< T> {
 factory Foo（T thing）=> const FooImpl（thing）; //< =常量创建的参数必须是常量表达式
 T get thing; 
} 
 
 class FooImpl< T>实现Foo T {
 final T thing; 
 const FooImpl（this.thing）; Dart有一个委托工厂构造函数，它包含了一个代理工厂构造函数允许此
 抽象类Foo< T> {
 const factory Foo（T thing）= FooImpl; 
 T get thing; 
} 
 
 class FooImpl< T>实现Foo T {
 final T thing; 
 const FooImpl（this.thing）; 
} 
  
另请参阅
 
 
 
  https://groups.google.com/a/ dartlang.org/forum/#!topic/misc/cvjjgrwIHbU 和
  https://groups.google.com/a/dartlang.org/forum/#!topic/misc/cvjjgrwIHbU  
 
Is it possibly to return a different implementation of as const?
abstract class Foo<T> {
  factory Foo(T thing) => const FooImpl(thing); // <= Arguments of a constant creation must be constant expressions
  T get thing;
}

class FooImpl<T> implements Foo<T>{
  final T thing;
  const FooImpl(this.thing);
}

解决方案
Dart has a delegating factory constructor to allow this
abstract class Foo<T> {
  const factory Foo(T thing) = FooImpl;
  T get thing;
}

class FooImpl<T> implements Foo<T>{
  final T thing;
  const FooImpl(this.thing);
}
see also

https://groups.google.com/a/dartlang.org/forum/#!topic/misc/cvjjgrwIHbU and
https://groups.google.com/a/dartlang.org/forum/#!topic/misc/cvjjgrwIHbU

                        
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