| 1 | // Copyright IBM Corp. and LoopBack contributors 2020. All Rights Reserved.
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| 2 | // Node module: @loopback/core
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| 3 | // This file is licensed under the MIT License.
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| 4 | // License text available at https://opensource.org/licenses/MIT
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| 5 |
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| 6 | import {Constructor} from '@loopback/context';
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| 7 |
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| 8 | /**
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| 9 | * A replacement for `typeof Target` to be used in mixin class definitions.
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| 10 | * This is a workaround for TypeScript limitation described in
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| 11 | * - https://github.com/microsoft/TypeScript/issues/17293
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| 12 | * - https://github.com/microsoft/TypeScript/issues/17744
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| 13 | * - https://github.com/microsoft/TypeScript/issues/36060
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| 14 | *
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| 15 | * @example
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| 16 | *
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| 17 | * ```ts
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| 18 | * export function MyMixin<T extends MixinTarget<Application>>(superClass: T) {
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| 19 | * return class extends superClass {
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| 20 | * // contribute new class members
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| 21 | * }
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| 22 | * };
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| 23 | * ```
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| 24 | *
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| 25 | * TypeScript does not allow class mixins to access protected members from
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| 26 | * the base class. You can use the following approach as a workaround:
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| 27 | *
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| 28 | * ```ts
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| 29 | * // eslint-disable-next-line @typescript-eslint/ban-ts-comment
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| 30 | * // @ts-ignore
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| 31 | * (this as unknown as {YourBaseClass}).protectedMember
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| 32 | * ```
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| 33 | *
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| 34 | * The directive `@ts-ignore` suppresses compiler error about accessing
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| 35 | * a protected member from outside. Unfortunately, it also disables other
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| 36 | * compile-time checks (e.g. to verify that a protected method was invoked
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| 37 | * with correct arguments, and so on). This is the same behavior you
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| 38 | * would get by using `Constructor<any>` instead of `MixinTarget<Application>`.
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| 39 | * The major improvement is that TypeScript can still infer the return
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| 40 | * type of the protected member, therefore `any` is NOT introduced to subsequent
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| 41 | * code.
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| 42 | *
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| 43 | * TypeScript also does not allow mixin class to overwrite a method inherited
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| 44 | * from a mapped type, see https://github.com/microsoft/TypeScript/issues/38496
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| 45 | * As a workaround, use `@ts-ignore` to disable the error.
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| 46 | *
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| 47 | * ```ts
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| 48 | * export function RepositoryMixin<T extends MixinTarget<Application>>(
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| 49 | * superClass: T,
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| 50 | * ) {
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| 51 | * return class extends superClass {
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| 52 | * // @ts-ignore
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| 53 | * public component<C extends Component = Component>(
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| 54 | * componentCtor: Constructor<C>,
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| 55 | * nameOrOptions?: string | BindingFromClassOptions,
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| 56 | * ) {
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| 57 | * const binding = super.component(componentCtor, nameOrOptions);
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| 58 | * // ...
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| 59 | * return binding;
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| 60 | * }
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| 61 | * }
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| 62 | * ```
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| 63 | */
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| 64 | export type MixinTarget<T extends object> = Constructor<{
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| 65 | // Enumerate only public members to avoid the following compiler error:
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| 66 | // Property '(name)' of exported class expression
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| 67 | // may not be private or protected.ts(4094)
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| 68 | [P in keyof T]: T[P];
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| 69 | }>;
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