Dart syntax: How to use a list of types <T> in dart?-CodePudding

How do you pass a list of types (NOT a list of instances!) to a method? And how do you compare if an instance matches any of the types from the list?

For example: This pseudo code wants to do something if someInstance is of a type that matches any of the types in typesOfInterest.

void doSomethingWithSpecificTypesOnly<T>(List<T> typesOfInterest) {
    Object someInstance;

    for (T thisType in typesOfInterest) {
        if (someInstance is thisType) {
            // do something...
        }
    }
}

Problem is, I can't figure out the syntax, so your help is very much appreciated. Thank you.

CodePudding user response：

I recommend creating a representation of a "type" with the operations that you need. (The Type class is very likely not that, since it has no operations other than equality.)

Something like:

class MyType<T> {
  bool isInstance(Object? object) => object is T;
  bool operator >=(MyType other) => other is MyType<T>;
  bool operator <=(MyType other) => other >= this;
  R runWith<R>(R Function<X>() function) => function<T>();
}

void doSomethingWithSpecificTypesOnly<T>(List<MyType<T>> typesOfInterest) {
  Object someInstance;

  for (T thisType in typesOfInterest) {
    if (thisType.isInstance(someInstance)) {
      // do something...
    }
  }
}

By keeping the type as a type parameter, it stays accessible as a type variable for any operation you want to do, and it can be used in most places where a type is needed. It's not possible to go back from a Type object to a type variable, and a Type variable is very restricted in what it can do.

It does mean that you need to write more to create the MyType object:

  doSomethingWithSpecificTypesOnly<num>([MyType<int>(), MyType<double()]);

You can probably create a shorthand for that, like:

typedef T<X> = MyType<X>;

and then write [T<int>(), T<double>()] instead. Slightly shorter. Or

extension StaticType<T> on T {
  MyType get staticType => MyType<T>;
}

and do something like:

[1.staticType, 1.1.staticType, "".staticType]

There are lots of options, depending on which API you want, but it won't be as short just [int, double], it's just more powerful than using Type objects.

CodePudding user response：

This could be what you're looking for, however it will match the exact type:

class SomeClass {
  
}

void doSomethingWithSpecificTypesOnly<T>(List<Type> typesOfInterest, T obj) {
    for (Type t in typesOfInterest) {
      if (obj.runtimeType == t) {
        print('ok');
        print(obj.runtimeType);
      }
    }
    
}

void main() {
  SomeClass obj = SomeClass();
  doSomethingWithSpecificTypesOnly([int, SomeClass], obj);
  doSomethingWithSpecificTypesOnly([int, double], obj);
  return; 
}