feat: add observable source from property changes

Closes #1

addon/index.js

@@ -1,2 +1,7 @@
 export { default as Route } from "./route";
+
 export { default as subscribe } from "./decorators/subscribe";
+
+export {
+  default as fromPropertyChange
+} from "./observables/from-property-change";

addon/observables/from-property-change.js

@@ -0,0 +1,16 @@
+import { fromEventPattern } from "rxjs";
+import { get } from "@ember/object";
+
+export default function fromPropertyChange(subject, property) {
+  return fromEventPattern(
+    handler => {
+      subject.addObserver(property, subject, handler);
+    },
+    handler => {
+      subject.removeObserver(property, subject, handler);
+    },
+    (instance, prop) => {
+      return get(instance, prop);
+    }
+  );
+}

tests/unit/observables/from-property-change-test.js

@@ -0,0 +1,48 @@
+import { module, test } from "qunit";
+import { setupScheduler } from "ember-observable/test-support";
+import td from "testdouble";
+import EmberObject from "@ember/object";
+import { fromPropertyChange } from "ember-observable";
+
+class Dummy extends EmberObject {
+  foo = "foo";
+}
+
+module("Unit | Observables | fromPropertyChange", function(hooks) {
+  setupScheduler(hooks);
+
+  test("it emits events when a property changes", function(assert) {
+    const instance = Dummy.create();
+
+    const observer = td.function();
+    fromPropertyChange(instance, "foo").subscribe(observer);
+
+    instance.set("foo", "bar");
+    instance.set("foo", "bax");
+
+    assert.verify(observer("bar"), "Called for the first change");
+    assert.verify(observer("bax"), "Called for the second change");
+  });
+
+  test("no longer emits events for changes after unsubscribe", function(assert) {
+    const instance = Dummy.create();
+
+    const observer = td.function();
+    const subscription = fromPropertyChange(instance, "foo").subscribe(
+      observer
+    );
+
+    instance.set("foo", "bar");
+
+    subscription.unsubscribe();
+
+    instance.set("foo", "bax");
+
+    assert.verify(observer("bar"), "Called for the first change");
+    assert.verify(
+      observer("bax"),
+      { times: 0 },
+      "Not called for the second change"
+    );
+  });
+});