From f7e391a912153d1ce43f9d0cf06d23121d1d49cd Mon Sep 17 00:00:00 2001
From: mgechev <mgechev@gmail.com>
Date: Mon, 12 Apr 2021 14:01:26 -0700
Subject: [PATCH] perf(core): optimize getDirectives (#41525)

This commit introduces the following optimizations:

1. We return an empty array for text nodes in `getDirectives` because
Angular does not support attaching logic to them. This optimization
improves performance of `getDirectives` significantly because text nodes
often result in expensive calls to `loadLContext` since we can't resolve
it from a parent node.
1. `getDirectives` now calls `loadLContext` with second argument `false`
so it doesn't throw an error. This brings another significant
improvement because prevents the VM from deoptimizing calls.

BREAKING CHANGE:

Previously the `ng.getDirectives` function threw an error in case a
given DOM node had no Angular context associated with it (for example
if a function was called for a DOM element outside of an Angular app).
This behavior was inconsistent with other debugging utilities under `ng`
namespace, which handled this situation without raising an exception.
Now calling the `ng.getDirectives` function for such DOM nodes would
result in an empty array returned from that function.

PR Close #41525
---
 .../core/src/render3/util/discovery_utils.ts  | 28 ++++++++++++++-----
 .../test/acceptance/discover_utils_spec.ts    |  2 ++
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/packages/core/src/render3/util/discovery_utils.ts b/packages/core/src/render3/util/discovery_utils.ts
index db5d760ff1..0546b62029 100644
--- a/packages/core/src/render3/util/discovery_utils.ts
+++ b/packages/core/src/render3/util/discovery_utils.ts
@@ -172,7 +172,7 @@ export function getInjectionTokens(element: Element): any[] {
 }
 
 /**
- * Retrieves directive instances associated with a given DOM element. Does not include
+ * Retrieves directive instances associated with a given DOM node. Does not include
  * component instances.
  *
  * @usageNotes
@@ -184,21 +184,35 @@ export function getInjectionTokens(element: Element): any[] {
  * </my-app>
  * ```
  * Calling `getDirectives` on `<button>` will return an array with an instance of the `MyButton`
- * directive that is associated with the DOM element.
+ * directive that is associated with the DOM node.
  *
  * Calling `getDirectives` on `<my-comp>` will return an empty array.
  *
- * @param element DOM element for which to get the directives.
- * @returns Array of directives associated with the element.
+ * @param node DOM node for which to get the directives.
+ * @returns Array of directives associated with the node.
  *
  * @publicApi
  * @globalApi ng
  */
-export function getDirectives(element: Element): {}[] {
-  const context = loadLContext(element)!;
+export function getDirectives(node: Node): {}[] {
+  // Skip text nodes because we can't have directives associated with them.
+  if (node instanceof Text) {
+    return [];
+  }
 
+  const context = loadLContext(node, false);
+  if (context === null) {
+    return [];
+  }
+
+  const lView = context.lView;
+  const tView = lView[TVIEW];
+  const nodeIndex = context.nodeIndex;
+  if (!tView?.data[nodeIndex]) {
+    return [];
+  }
   if (context.directives === undefined) {
-    context.directives = getDirectivesAtNodeIndex(context.nodeIndex, context.lView, false);
+    context.directives = getDirectivesAtNodeIndex(nodeIndex, lView, false);
   }
 
   // The `directives` in this case are a named array called `LComponentView`. Clone the
diff --git a/packages/core/test/acceptance/discover_utils_spec.ts b/packages/core/test/acceptance/discover_utils_spec.ts
index 5af464f790..07ae86691d 100644
--- a/packages/core/test/acceptance/discover_utils_spec.ts
+++ b/packages/core/test/acceptance/discover_utils_spec.ts
@@ -386,9 +386,11 @@ onlyInIvy('Ivy-specific utilities').describe('discovery utils deprecated', () =>
 
     it('should not throw if it cannot find LContext', () => {
       let result: any;
+
       expect(() => {
         result = getDirectives(document.createElement('div'));
       }).not.toThrow();
+
       expect(result).toEqual([]);
     });
   });