New Util: ReactiveArray (#9)

* Copy code from tracked-built-ins

* Update README

* Add footnote

* Noice

* yay

* More reactivity tests

README.md

@@ -4,11 +4,17 @@ Utils for the [Signal's Proposal](https://github.com/proposal-signals/proposal-s
 
 ## APIs 
 
+> [!NOTE]
+> All examples either use JavaScript or a mixed-language psuedocode[^syntax-based-off] to convey the reactive intention of using Signals. 
+> These utilities can be used in any framework that wires up Signals to their rendering implementation.
+
+[^syntax-based-off]: The syntax is based of a mix of [Glimmer-flavored Javascript](https://tutorial.glimdown.com) and [Svelte](https://svelte.dev/). The main thing being focused around JavaScript without having a custom file format. The `<template>...</template>` blocks may as well be HTML, and `{{ }}` escapes out to JS. I don't have a strong preference on `{{ }}` vs `{ }`, the important thing is only to be consistent within an ecosystem.
+
 ### `@signal`
 
 A utility decorator for easily creating signals 
 
-```ts
+```jsx
 import { signal } from 'signal-utils';
 
 class State {
@@ -23,17 +29,34 @@ class State {
 
 let state = new State();
 
-state.doubled // 6
-state.increment()
-state.doubled // 8
+
+// output: 6
+// button clicked
+// output: 8
+<template>
+  <output>{{state.doubled}}</output>
+  <button onclick={{state.increment}}>+</button>
+</template>
 ```
 
 ### `Array`
 
-wip
-
 A reactive Array
 
+```jsx
+import { ReactiveArray } from 'signal-utils/array';
+
+let arr = new ReactiveArray([1, 2, 3]);
+
+// output: 3
+// button clicked
+// output: 2
+<template>
+  <output>{{arr.at(-1)}}</output>
+  <button onclick={{() => arr.pop()}}>pop</button>
+</template>
+```
+
 ### `Object`
 
 A reactive Object
@@ -47,9 +70,13 @@ let obj = new ReactiveObject({
     result: null,
 });
 
-obj.isLoading // true
-obj.isLoading = false
-obj.isLoading // false
+// output: true
+// button clicked
+// output: false
+<template>
+  <output>{{obj.isLoading}}</output>
+  <button onclick={{() => obj.isLoading = false}}>finish</button>
+</template>
 ```
 
 In this example, we could use a reactive object for quickly and dynamically creating an object of signals -- useful for when we don't know all the keys boforehand, or if we want a shorthand to creating many named signals.

src/-private/util.ts

@@ -0,0 +1,15 @@
+import { Signal } from "signal-polyfill";
+
+/**
+ * equality check here is always false so that we can dirty the storage
+ * via setting to _anything_
+ *
+ *
+ * This is for a pattern where we don't *directly* use signals to back the values used in collections
+ * so that instanceof checks and getters and other native features "just work" without having
+ * to do nested proxying.
+ *
+ * (though, see deep.ts for nested / deep behavior)
+ */
+export const createStorage = () =>
+  new Signal.State(null, { equals: () => false });

src/array.ts

@@ -1,5 +1,218 @@
-export class Array {
-  constructor() {
-    throw new Error("Not implemented");
+/* eslint-disable @typescript-eslint/no-explicit-any */
+// Unfortunately, TypeScript's ability to do inference *or* type-checking in a
+// `Proxy`'s body is very limited, so we have to use a number of casts `as any`
+// to make the internal accesses work. The type safety of these is guaranteed at
+// the *call site* instead of within the body: you cannot do `Array.blah` in TS,
+// and it will blow up in JS in exactly the same way, so it is safe to assume
+// that properties within the getter have the correct type in TS.
+
+import { Signal } from "signal-polyfill";
+import { createStorage } from "./-private/util.ts";
+
+const ARRAY_GETTER_METHODS = new Set<string | symbol | number>([
+  Symbol.iterator,
+  "concat",
+  "entries",
+  "every",
+  "filter",
+  "find",
+  "findIndex",
+  "flat",
+  "flatMap",
+  "forEach",
+  "includes",
+  "indexOf",
+  "join",
+  "keys",
+  "lastIndexOf",
+  "map",
+  "reduce",
+  "reduceRight",
+  "slice",
+  "some",
+  "values",
+]);
+
+// For these methods, `Array` itself immediately gets the `.length` to return
+// after invoking them.
+const ARRAY_WRITE_THEN_READ_METHODS = new Set<string | symbol>([
+  "fill",
+  "push",
+  "unshift",
+]);
+
+function convertToInt(prop: number | string | symbol): number | null {
+  if (typeof prop === "symbol") return null;
+
+  const num = Number(prop);
+
+  if (isNaN(num)) return null;
+
+  return num % 1 === 0 ? num : null;
+}
+
+class ReactiveArrayImpl<T = unknown> {
+  /**
+   * Creates an array from an iterable object.
+   * @param iterable An iterable object to convert to an array.
+   */
+  static from<T>(iterable: Iterable<T> | ArrayLike<T>): ReactiveArrayImpl<T>;
+
+  /**
+   * Creates an array from an iterable object.
+   * @param iterable An iterable object to convert to an array.
+   * @param mapfn A mapping function to call on every element of the array.
+   * @param thisArg Value of 'this' used to invoke the mapfn.
+   */
+  static from<T, U>(
+    iterable: Iterable<T> | ArrayLike<T>,
+    mapfn: (v: T, k: number) => U,
+    thisArg?: unknown,
+  ): ReactiveArrayImpl<U>;
+
+  static from<T, U>(
+    iterable: Iterable<T> | ArrayLike<T>,
+    mapfn?: (v: T, k: number) => U,
+    thisArg?: unknown,
+  ): ReactiveArrayImpl<T> | ReactiveArrayImpl<U> {
+    return mapfn
+      ? new ReactiveArrayImpl(Array.from(iterable, mapfn, thisArg))
+      : new ReactiveArrayImpl(Array.from(iterable));
+  }
+
+  static of<T>(...arr: T[]): ReactiveArrayImpl<T> {
+    return new ReactiveArrayImpl(arr);
+  }
+
+  constructor(arr: T[] = []) {
+    let clone = arr.slice();
+    // eslint-disable-next-line @typescript-eslint/no-this-alias
+    let self = this;
+
+    let boundFns = new Map<string | symbol, (...args: any[]) => any>();
+
+    /**
+      Flag to track whether we have *just* intercepted a call to `.push()` or
+      `.unshift()`, since in those cases (and only those cases!) the `Array`
+      itself checks `.length` to return from the function call.
+     */
+    let nativelyAccessingLengthFromPushOrUnshift = false;
+
+    return new Proxy(clone, {
+      get(target, prop /*, _receiver */) {
+        let index = convertToInt(prop);
+
+        if (index !== null) {
+          self.#readStorageFor(index);
+          self.#collection.get();
+
+          return target[index];
+        }
+
+        if (prop === "length") {
+          // If we are reading `.length`, it may be a normal user-triggered
+          // read, or it may be a read triggered by Array itself. In the latter
+          // case, it is because we have just done `.push()` or `.unshift()`; in
+          // that case it is safe not to mark this as a *read* operation, since
+          // calling `.push()` or `.unshift()` cannot otherwise be part of a
+          // "read" operation safely, and if done during an *existing* read
+          // (e.g. if the user has already checked `.length` *prior* to this),
+          // that will still trigger the mutation-after-consumption assertion.
+          if (nativelyAccessingLengthFromPushOrUnshift) {
+            nativelyAccessingLengthFromPushOrUnshift = false;
+          } else {
+            self.#collection.get();
+          }
+
+          return target[prop];
+        }
+
+        // Here, track that we are doing a `.push()` or `.unshift()` by setting
+        // the flag to `true` so that when the `.length` is read by `Array` (see
+        // immediately above), it knows not to dirty the collection.
+        if (ARRAY_WRITE_THEN_READ_METHODS.has(prop)) {
+          nativelyAccessingLengthFromPushOrUnshift = true;
+        }
+
+        if (ARRAY_GETTER_METHODS.has(prop)) {
+          let fn = boundFns.get(prop);
+
+          if (fn === undefined) {
+            fn = (...args) => {
+              self.#collection.get();
+              return (target as any)[prop](...args);
+            };
+
+            boundFns.set(prop, fn);
+          }
+
+          return fn;
+        }
+
+        return (target as any)[prop];
+      },
+
+      set(target, prop, value /*, _receiver */) {
+        (target as any)[prop] = value;
+
+        let index = convertToInt(prop);
+
+        if (index !== null) {
+          self.#dirtyStorageFor(index);
+          self.#collection.set(null);
+        } else if (prop === "length") {
+          self.#collection.set(null);
+        }
+
+        return true;
+      },
+
+      getPrototypeOf() {
+        return ReactiveArrayImpl.prototype;
+      },
+    }) as ReactiveArrayImpl<T>;
+  }
+
+  #collection = createStorage();
+
+  #storages = new Map<PropertyKey, Signal.State<null>>();
+
+  #readStorageFor(index: number) {
+    let storage = this.#storages.get(index);
+
+    if (storage === undefined) {
+      storage = createStorage();
+      this.#storages.set(index, storage);
+    }
+
+    storage.get();
+  }
+
+  #dirtyStorageFor(index: number): void {
+    const storage = this.#storages.get(index);
+
+    if (storage) {
+      storage.set(null);
+    }
   }
 }
+
+// This rule is correct in the general case, but it doesn't understand
+// declaration merging, which is how we're using the interface here. This says
+// `ReactiveArray` acts just like `Array<T>`, but also has the properties
+// declared via the `class` declaration above -- but without the cost of a
+// subclass, which is much slower that the proxied array behavior. That is: a
+// `ReactiveArray` *is* an `Array`, just with a proxy in front of accessors and
+// setters, rather than a subclass of an `Array` which would be de-optimized by
+// the browsers.
+//
+// eslint-disable-next-line @typescript-eslint/no-empty-interface
+interface ReactiveArrayImpl<T = unknown> extends Array<T> {}
+
+type ReactiveArray = ReactiveArrayImpl;
+
+export const ReactiveArray: ReactiveArray =
+  ReactiveArrayImpl as unknown as ReactiveArray;
+
+// Ensure instanceof works correctly
+Object.setPrototypeOf(ReactiveArrayImpl.prototype, Array.prototype);

src/object.ts

@@ -1,10 +1,5 @@
 import { Signal } from "signal-polyfill";
-
-/**
- * equality check here is always false so that we can dirty the storage
- * via setting to _anything_
- */
-const createStorage = () => new Signal.State(null, { equals: () => false });
+import { createStorage } from "./-private/util.ts";
 
 /**
  * Implementation based of tracked-built-ins' TrackedObject