add telemetry event after destroying a live component (#3777)

* add telemetry event after destroying a live component

The people from [LiveDebugger](https://github.com/software-mansion/live-debugger)
mentioned that they struggle to react to live components that are removed
from a page. A new telemetry event should help.

* [:phoenix, :live_component, :destroyed]

Author: SteffenDE

guides/server/telemetry.md

@@ -300,3 +300,15 @@ LiveView currently exposes the following [`telemetry`](https://hexdocs.pm/teleme
             event: String.t(),
             params: unsigned_params
           }
+
+  * `[:phoenix, :live_component, :destroyed]` - Dispatched by a `Phoenix.LiveComponent`
+    after it is destroyed. No measurement.
+
+    * Metadata:
+
+        %{
+          socket: Phoenix.LiveView.Socket.t,
+          component: atom,
+          cid: integer(),
+          live_view_socket: Phoenix.LiveView.Socket.t
+        }

lib/phoenix_live_view/channel.ex

@@ -1528,12 +1528,21 @@ defmodule Phoenix.LiveView.Channel do
       {deleted_cids, new_components} = Diff.delete_component(cid, acc.components)
 
       canceled_confs =
-        deleted_cids
-        |> Enum.filter(fn deleted_cid -> deleted_cid in upload_cids end)
-        |> Enum.flat_map(fn deleted_cid ->
-          read_socket(acc, deleted_cid, fn c_socket, _ ->
-            {_new_c_socket, canceled_confs} = Upload.maybe_cancel_uploads(c_socket)
-            canceled_confs
+        Enum.flat_map(deleted_cids, fn deleted_cid ->
+          read_socket(acc, deleted_cid, fn c_socket, component ->
+            :telemetry.execute([:phoenix, :live_component, :destroyed], %{}, %{
+              socket: c_socket,
+              component: component,
+              cid: deleted_cid,
+              live_view_socket: acc.socket
+            })
+
+            if deleted_cid in upload_cids do
+              {_new_c_socket, canceled_confs} = Upload.maybe_cancel_uploads(c_socket)
+              canceled_confs
+            else
+              []
+            end
           end)
         end)
 

test/phoenix_live_view/integrations/live_components_test.exs

@@ -71,6 +71,9 @@ defmodule Phoenix.LiveView.LiveComponentsTest do
   end
 
   test "tracks removals", %{conn: conn} do
+    ref =
+      :telemetry_test.attach_event_handlers(self(), [[:phoenix, :live_component, :destroyed]])
+
     {:ok, view, html} = live(conn, "/components")
 
     assert [
@@ -101,6 +104,14 @@ defmodule Phoenix.LiveView.LiveComponentsTest do
              |> TreeDOM.normalize_to_tree(sort_attributes: true)
 
     refute view |> element("#chris") |> has_element?()
+
+    assert_received {[:phoenix, :live_component, :destroyed], ^ref, _,
+                     %{
+                       component: StatefulComponent,
+                       cid: 1,
+                       socket: %{assigns: %{name: "chris"}},
+                       live_view_socket: %{assigns: %{names: ["jose"]}}
+                     }}
   end
 
   test "tracks removals when whole root changes", %{conn: conn} do