From 7a6b15d1dc85c7ed46fd13aecb0009143fd74ce1 Mon Sep 17 00:00:00 2001
From: Anthony Johnston <aj@antix.co.uk>
Date: Mon, 5 Aug 2019 11:36:38 +0100
Subject: [PATCH] feat(watch): watch setters on an object

---
 src/IWatchHandler.ts |  3 +++
 src/watch.spec.ts    | 64 ++++++++++++++++++++++++++++++++++++++++++++
 src/watch.ts         | 38 ++++++++++++++++++++++++++
 3 files changed, 105 insertions(+)
 create mode 100644 src/IWatchHandler.ts
 create mode 100644 src/watch.spec.ts
 create mode 100644 src/watch.ts

diff --git a/src/IWatchHandler.ts b/src/IWatchHandler.ts
new file mode 100644
index 0000000..1c2c14f
--- /dev/null
+++ b/src/IWatchHandler.ts
@@ -0,0 +1,3 @@
+export interface IWatchHandler {
+  (value?: any): void;
+}
diff --git a/src/watch.spec.ts b/src/watch.spec.ts
new file mode 100644
index 0000000..f2ccb11
--- /dev/null
+++ b/src/watch.spec.ts
@@ -0,0 +1,64 @@
+import { watch } from './watch';
+
+describe('watch', () => {
+  const prop = 'PROPERTY';
+  const value = 'VALUE';
+  let o;
+
+  beforeEach(() => {
+    o = {};
+  });
+
+  it('watches for change', () => {
+    let counter = 0;
+    watch(o, prop, () => {
+      counter++;
+    });
+
+    o[prop] = value;
+    o[prop] = value;
+
+    expect(counter).toBe(2);
+  });
+
+  it('watches for change once', () => {
+    let counter = 0;
+    watch(
+      o,
+      prop,
+      () => {
+        counter++;
+      },
+      true
+    );
+
+    o[prop] = value;
+    o[prop] = value;
+
+    expect(counter).toBe(1);
+  });
+
+  it('unwatches and keeps set value', () => {
+    const unwatch = watch(o, prop, () => {});
+
+    o[prop] = value;
+    unwatch();
+
+    expect(o[prop]).toBe(value);
+  });
+
+  it('can nest watches', () => {
+    let counter = 0;
+    watch(o, prop, () => {
+      counter++;
+    });
+    watch(o, prop, () => {
+      counter++;
+    });
+
+    o[prop] = value;
+    o[prop] = value;
+
+    expect(counter).toBe(4);
+  });
+});
diff --git a/src/watch.ts b/src/watch.ts
new file mode 100644
index 0000000..32fc123
--- /dev/null
+++ b/src/watch.ts
@@ -0,0 +1,38 @@
+import { IWatchHandler } from './IWatchHandler';
+
+export function watch(
+  o: any,
+  property: string,
+  handler: IWatchHandler,
+  once = false
+): () => void {
+
+  const descriptor = Object.getOwnPropertyDescriptor(o, property) || {};
+
+  let getValue = descriptor.get || (() => descriptor.value);
+  let setValue = descriptor.set || (value => (descriptor.value = value));
+
+  const unwatch = () => {
+    delete o[property];
+    o[property] = getValue();
+  };
+
+  Object.defineProperty(o, property, {
+    get: getValue,
+    set(value) {
+      if (once && getValue() === undefined) {
+        setValue(value);
+        unwatch();
+        handler(value);
+
+        return;
+      }
+
+      setValue(value);
+      handler(value);
+    },
+    configurable: true
+  });
+
+  return unwatch;
+}