Merge pull request #19 from chadhietala/add-attrs-snapshot-doc

Adding docs for no-attrs-snapshot

Author: chadhietala

guides/rules/no-attrs-snapshot.md

@@ -0,0 +1,21 @@
+# No `attrs` snapshots
+
+In 2.0.0 [`didRecieveAttrs`](https://guides.emberjs.com/v2.9.0/components/the-component-lifecycle/#toc_formatting-component-attributes-with-code-didreceiveattrs-code) and [`didUpdateAttrs`](https://guides.emberjs.com/v2.9.0/components/the-component-lifecycle/#toc_resetting-presentation-state-on-attribute-change-with-code-didupdateattrs-code) hooks were introduced are called whenever the arguments to a component referencially change. These hooks do recieve params to them, however one should not use them as they force those objects to reify which can be very costly when you have a lot of components on the page. These arguments are also purposly undocumented. If for some reason you need to do a comparison of arguments we suggest that you simply keep a cache on the component.
+
+```
+export default Ember.Component({
+  init() {
+    this._super(...arguments);
+    this._valueCache = this.value;
+    this.updated = false;
+  },
+  didReceiveAttrs() {
+    if (this._valueCache !== this.value) {
+      this._valueCache = this.value;
+      this.set('updated', true);
+    } else {
+      this.set('updated', false);
+    }
+  }
+});
+```