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Support subclassing Observable with non-class constructor functions (#1)
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Original file line number | Diff line number | Diff line change |
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@@ -1 +1,29 @@ | ||
export { Observable } from "zen-observable/esm.js"; | ||
import { Observable } from "zen-observable/esm.js"; | ||
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// When a non-native class constructor function attempts to subclass | ||
// Observable, compilers like Babel and TypeScript may compile the | ||
// super(subscriber) expression to Observable.call(this, subscriber) or | ||
// Observable.apply(this, arguments). Calling a native class constructor | ||
// in this way is forbidden by the ECMAScript specification, but we can | ||
// preserve backwards compatibility by overriding the Observable.call and | ||
// Observable.apply methods with an implementation that calls | ||
// Reflect.construct instead, when available. | ||
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Observable.call = function (instance, subscriber) { | ||
// Since Observable.call is a static method, 'this' will typically be the | ||
// Observable constructor function, though it could be a subclass of the | ||
// Observable constructor, which is why we don't just hard-code it here. | ||
return construct(this, instance, subscriber); | ||
}; | ||
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Observable.apply = function (instance, args) { | ||
return construct(this, instance, args[0]); | ||
}; | ||
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function construct(Super, instance, subscriber) { | ||
return typeof Reflect === 'object' | ||
? Reflect.construct(Super, [subscriber], instance.constructor) | ||
: Function.prototype.call.call(Super, instance, subscriber) || instance; | ||
} | ||
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export { Observable } |
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@@ -1,9 +1,80 @@ | ||
const { expect } = require("chai"); | ||
const { expect } = require("chai") as typeof import("chai"); | ||
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import type { | ||
Observable, | ||
Subscriber, | ||
} from ".."; | ||
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export default function (Observable: typeof import("..").Observable) { | ||
describe("Observable", () => { | ||
it("Should be a constructor function", () => { | ||
expect(typeof Observable).to.equal("function"); | ||
}); | ||
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describe('subclassing by non-class constructor functions', () => { | ||
function check(constructor: new <T>(sub: Subscriber<T>) => Observable<T>) { | ||
constructor.prototype = Object.create(Observable.prototype, { | ||
constructor: { | ||
value: constructor, | ||
}, | ||
}); | ||
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const subscriber: Subscriber<number> = observer => { | ||
observer.next(123); | ||
observer.complete(); | ||
}; | ||
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const obs = new constructor(subscriber) as Observable<number>; | ||
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expect(typeof (obs as any).sub).to.equal("function"); | ||
expect((obs as any).sub).to.equal(subscriber); | ||
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expect(obs).to.be.instanceOf(Observable); | ||
expect(obs).to.be.instanceOf(constructor); | ||
expect(obs.constructor).to.equal(constructor); | ||
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return new Promise((resolve, reject) => { | ||
obs.subscribe({ | ||
next: resolve, | ||
error: reject, | ||
}); | ||
}).then(value => { | ||
expect(value).to.equal(123); | ||
}); | ||
} | ||
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function newify( | ||
constructor: <T>(sub: Subscriber<T>) => void, | ||
): new <T>(sub: Subscriber<T>) => Observable<T> { | ||
return constructor as any; | ||
} | ||
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it('simulating super(sub) with Observable.call(this, sub)', () => { | ||
function SubclassWithSuperCall<T>(sub: Subscriber<T>) { | ||
const self = Observable.call(this, sub) || this; | ||
self.sub = sub; | ||
return self; | ||
} | ||
return check(newify(SubclassWithSuperCall)); | ||
}); | ||
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it('simulating super(sub) with Observable.apply(this, arguments)', () => { | ||
function SubclassWithSuperApplyArgs<T>(_sub: Subscriber<T>) { | ||
const self = Observable.apply(this, arguments) || this; | ||
self.sub = _sub; | ||
return self; | ||
} | ||
return check(newify(SubclassWithSuperApplyArgs)); | ||
}); | ||
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it('simulating super(sub) with Observable.apply(this, [sub])', () => { | ||
function SubclassWithSuperApplyArray<T>(...args: [Subscriber<T>]) { | ||
const self = Observable.apply(this, args) || this; | ||
self.sub = args[0]; | ||
return self; | ||
} | ||
return check(newify(SubclassWithSuperApplyArray)); | ||
}); | ||
}); | ||
}); | ||
} |