I'm making a plugin for Aurelia and need a class decorator that
I've looked through examples, and so far I've put together ("pseudo-ish" code)
return function addAndCall(target: any): any {
var original = target;
var newConstructor = function (...args) {
original.apply(this, args);
this.newAttribute = "object instance value";
ExternalModule.externalFunction(this);
};
newConstructor.prototype = Object.create(original.prototype);
newConstructor.prototype.constructor = original;
return <any>newConstructor;
}
but
Any help and explanation would be greatly appreciated!
Decorators provide a way to annotate or modify a class or class member in TypeScript. However, Decorators are still an experimental feature of the language. To learn more about Decorators in TypeScript, visit the Decorators proposal page.
The TypeScript website describes it as: “A Decorator is a special kind of declaration that can be attached to a class declaration, method, accessor, property, or parameter.” I would describe it as “a special declaration to add extra features on an existing class declaration, method, accessor, property, or parameter.”
The decorator is applied to the Property Descriptor for the method, and can be used to observe, modify, or replace a method definition. A method decorator cannot be used in a declaration file, on an overload, or in any other ambient context (such as in a declare class).
In TypeScript, you can create decorators using the special syntax @expression , where expression is a function that will be called automatically during runtime with details about the target of the decorator. The target of a decorator depends on where you add them.
Why not just assign those properties to the prototype, and subsequently assign to the instance on first invocation
// decorator
function addAndCall(cb: Function, newField: string) {
// cb is now available in the decorator
return function(ctor: Function): void {
Object.defineProperty(ctor.prototype, newField, {
value: function(...args: any[]) {
return Object.defineProperty(this, newField, {
value: function(...args: any[]) {
console.log(newField, ...args);
}
})[newField](...args);
}
});
cb(ctor);
}
}
let callMe = (decoratedCtor) => console.log(decoratedCtor);
@addAndCall(callMe, 'propertyName')
class AddToMe {}
let addToMe = new AddToMe();
(<any>addToMe).propertyName(1, 2);
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