I have encountered an issue while trying to implement a Model/Collection pattern with various typings. Both the Model and Collection classes have a method called serialize(). When this method is called on the Collection, it serializes all the Model(s) within.
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Having investigated various approaches to address this issue, I have identified three distinct strategies based on your preference for maintaining type information within the specific ModelInterface in CollectionInterface, as well as specifying the desired type parameter for CollectionInterface.
To begin with, let's rectify the typings for ModelInterface and SerializedCollection by leveraging generic constraints:
// Definition of a type that can be serialized into a M, where M represents a SerializedModel
interface ModelInterface<M extends SerializedModel> {
serialize(): M;
}
// Implementation of Contact serializing into SerializedContact
class Contact implements ModelInterface<SerializedContact> { /* ... */ }
// A serialization of collection of Ms
interface SerializedCollection<M extends SerializedModel> {
count: number;
items: M[];
}
ModelInterfaceThis method maintains the type information for specific ModelInterfaces within the CollectionInterface. However, there were challenges with accurate type inference; explicit declaration of type parameters is required when constructing a Collection.
The code snippet below demonstrates the use of parameter properties for Collection, offering a more streamlined syntax than traditional class property assignment.
// Type capable of serializing into collections of Ms from the serializable Is
interface CollectionInterface<M extends SerializedModel, I extends ModelInterface<M>> {
count: number;
items: I[];
serialize(): SerializedCollection<M>;
}
// Class implementing CollectionInterface for a model M and model interface I
class Collection<M extends SerializedModel, I extends ModelInterface<M>> implements CollectionInterface<M, I> {
constructor(public items: I[], public count: number) {}
public serialize(): SerializedCollection<M> { /* ... */ }
}
// TypeScript automatically infers contactCollection as Collection<SerializedModel, Contact> if type parameters are not explicitly specified
const contactCollection = new Collection<SerializedContact, Contact>(
[new Contact("Bill"), new Contact("Bob")],
2
);
// Represents type Contact[]
const contacts = contactCollection.items;
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