Allow sharing assigns between live navigation

This commit relies on a yet to be finalized feature in Phoenix Channels
to perform a custom handover between channel rejoins.

It changes the LV code to not leave the old channel before rejoining and
also instructs Phoenix to not kill the old process before starting the
new channel process. After the new channel is joined, the old one is
killed. Any `assign_new` calls in the LV mount will try to fetch assigns
from the old LV.

This is a backwards compatible optimization, so if a version of Phoenix
is used that does not support handover, it just falls back to calling the
function supplied to assign_new as usual.

Closes #3357.

assets/js/phoenix_live_view/live_socket.js

@@ -390,7 +390,7 @@ export default class LiveSocket {
     let removeEls = DOM.all(this.outgoingMainEl, `[${this.binding("remove")}]`)
     let newMainEl = DOM.cloneNode(this.outgoingMainEl, "")
     this.main.showLoader(this.loaderTimeout)
-    this.main.destroy()
+    this.main.destroy(false)
 
     this.main = this.newRootView(newMainEl, flash, liveReferer)
     this.main.setRedirect(href)
@@ -407,7 +407,7 @@ export default class LiveSocket {
           onDone()
         })
       }
-    })
+    }, true)
   }
 
   transitionRemoves(elements, skipSticky, callback){

assets/js/phoenix_live_view/view.js

@@ -205,7 +205,7 @@ export default class View {
     return val === "" ? null : val
   }
 
-  destroy(callback = function (){ }){
+  destroy(leave = true, callback = function (){ }){
     this.destroyAllChildren()
     this.destroyed = true
     delete this.root.children[this.id]
@@ -221,10 +221,14 @@ export default class View {
     DOM.markPhxChildDestroyed(this.el)
 
     this.log("destroyed", () => ["the child has been removed from the parent"])
-    this.channel.leave()
-      .receive("ok", onFinished)
-      .receive("error", onFinished)
-      .receive("timeout", onFinished)
+    if(leave){
+      this.channel.leave()
+        .receive("ok", onFinished)
+        .receive("error", onFinished)
+        .receive("timeout", onFinished)
+    } else {
+      onFinished()
+    }
   }
 
   setContainerClasses(...classes){
@@ -795,7 +799,7 @@ export default class View {
     return this.joinPush
   }
 
-  join(callback){
+  join(callback, handover = false){
     this.showLoader(this.liveSocket.loaderTimeout)
     this.bindChannel()
     if(this.isMain()){
@@ -806,7 +810,7 @@ export default class View {
       callback ? callback(this.joinCount, onDone) : onDone()
     }
 
-    this.wrapPush(() => this.channel.join(), {
+    this.wrapPush(() => this.channel.join(this.liveSocket.socket.timeout, handover), {
       ok: (resp) => this.liveSocket.requestDOMUpdate(() => this.onJoin(resp)),
       error: (error) => this.onJoinError(error),
       timeout: () => this.onJoinError({reason: "timeout"})

lib/phoenix_live_view/channel.ex

@@ -399,7 +399,7 @@ defmodule Phoenix.LiveView.Channel do
     end)
   end
 
-  def handle_call({@prefix, :child_mount, _child_pid, assign_new}, _from, state) do
+  def handle_call({@prefix, :get_assigns, _child_pid, assign_new}, _from, state) do
     assigns = Map.take(state.socket.assigns, assign_new)
     {:reply, {:ok, assigns}, state}
   end
@@ -1123,6 +1123,10 @@ defmodule Phoenix.LiveView.Channel do
       transport_pid: transport_pid
     } = phx_socket
 
+    # TODO: change this to directly pattern match on handover_pid above
+    #       when we require Phoenix 1.8
+    handover_pid = Map.get(phx_socket, :handover_pid)
+
     Process.put(:"$initial_call", {view, :mount, 3})
 
     case params do
@@ -1164,7 +1168,15 @@ defmodule Phoenix.LiveView.Channel do
     merged_session = Map.merge(socket_session, verified_user_session)
     lifecycle = load_lifecycle(config, route)
 
-    case mount_private(parent, root_view, assign_new, connect_params, connect_info, lifecycle) do
+    case mount_private(
+           parent,
+           root_view,
+           assign_new,
+           connect_params,
+           connect_info,
+           lifecycle,
+           handover_pid
+         ) do
       {:ok, mount_priv} ->
         socket = Utils.configure_socket(socket, mount_priv, action, flash, host_uri)
 
@@ -1254,20 +1266,37 @@ defmodule Phoenix.LiveView.Channel do
     socket
   end
 
-  defp mount_private(nil, root_view, assign_new, connect_params, connect_info, lifecycle) do
+  defp mount_private(
+         nil,
+         root_view,
+         assign_new,
+         connect_params,
+         connect_info,
+         lifecycle,
+         handover_pid
+       ) do
     {:ok,
      %{
        connect_params: connect_params,
        connect_info: connect_info,
        assign_new: {%{}, assign_new},
        lifecycle: lifecycle,
        root_view: root_view,
-       live_temp: %{}
+       live_temp: %{},
+       handover_pid: handover_pid
      }}
   end
 
-  defp mount_private(parent, root_view, assign_new, connect_params, connect_info, lifecycle) do
-    case sync_with_parent(parent, assign_new) do
+  defp mount_private(
+         parent,
+         root_view,
+         assign_new,
+         connect_params,
+         connect_info,
+         lifecycle,
+         _handover_pid
+       ) do
+    case get_assigns(parent, assign_new) do
       {:ok, parent_assigns} ->
         # Child live views always ignore the layout on `:use`.
         {:ok,
@@ -1278,17 +1307,18 @@ defmodule Phoenix.LiveView.Channel do
            live_layout: false,
            lifecycle: lifecycle,
            root_view: root_view,
-           live_temp: %{}
+           live_temp: %{},
+           handover_pid: nil
          }}
 
       {:error, :noproc} ->
         {:error, :noproc}
     end
   end
 
-  defp sync_with_parent(parent, assign_new) do
+  def get_assigns(pid, keys) do
     try do
-      GenServer.call(parent, {@prefix, :child_mount, self(), assign_new})
+      GenServer.call(pid, {@prefix, :get_assigns, self(), keys})
     catch
       :exit, {:noproc, _} -> {:error, :noproc}
     end

lib/phoenix_live_view/socket.ex

@@ -96,7 +96,7 @@ defmodule Phoenix.LiveView.Socket do
         }
 
   channel "lvu:*", Phoenix.LiveView.UploadChannel
-  channel "lv:*", Phoenix.LiveView.Channel
+  channel "lv:*", Phoenix.LiveView.Channel, handover_on_rejoin: true
 
   @impl Phoenix.Socket
   def connect(_params, %Phoenix.Socket{} = socket, connect_info) do
@@ -111,7 +111,7 @@ defmodule Phoenix.LiveView.Socket do
       use Phoenix.Socket
 
       channel "lvu:*", Phoenix.LiveView.UploadChannel
-      channel "lv:*", Phoenix.LiveView.Channel
+      channel "lv:*", Phoenix.LiveView.Channel, handover_on_rejoin: true
 
       def connect(params, socket, info), do: {:ok, socket}
       defdelegate id(socket), to: unquote(__MODULE__)

lib/phoenix_live_view/utils.ex

@@ -48,7 +48,7 @@ defmodule Phoenix.LiveView.Utils do
       %{assigns: %{^key => _}} ->
         socket
 
-      %{private: %{assign_new: {assigns, keys}}} ->
+      %{private: %{assign_new: {assigns, keys}} = private} ->
         # It is important to store the keys even if they are not in assigns
         # because maybe the controller doesn't have it but the view does.
         socket = put_in(socket.private.assign_new, {assigns, [key | keys]})
@@ -58,7 +58,7 @@ defmodule Phoenix.LiveView.Utils do
           key,
           case assigns do
             %{^key => value} -> value
-            %{} -> fun.(socket.assigns)
+            %{} -> maybe_handover_assign(socket, key, private[:handover_pid], fun)
           end
         )
 
@@ -72,17 +72,36 @@ defmodule Phoenix.LiveView.Utils do
       %{assigns: %{^key => _}} ->
         socket
 
-      %{private: %{assign_new: {assigns, keys}}} ->
+      %{private: %{assign_new: {assigns, keys}} = private} ->
         # It is important to store the keys even if they are not in assigns
         # because maybe the controller doesn't have it but the view does.
         socket = put_in(socket.private.assign_new, {assigns, [key | keys]})
-        Phoenix.LiveView.Utils.force_assign(socket, key, Map.get_lazy(assigns, key, fun))
+
+        Phoenix.LiveView.Utils.force_assign(
+          socket,
+          key,
+          Map.get_lazy(assigns, key, fn ->
+            maybe_handover_assign(socket, key, private[:handover_pid], fun)
+          end)
+        )
 
       %{} ->
         Phoenix.LiveView.Utils.force_assign(socket, key, fun.())
     end
   end
 
+  defp maybe_handover_assign(_socket, _key, nil, fun) when is_function(fun, 0), do: fun.()
+
+  defp maybe_handover_assign(socket, _key, nil, fun) when is_function(fun, 1),
+    do: fun.(socket.assigns)
+
+  defp maybe_handover_assign(socket, key, pid, fun) when is_pid(pid) do
+    case Phoenix.LiveView.Channel.get_assigns(pid, [key]) do
+      {:ok, %{^key => value}} -> value
+      _ -> maybe_handover_assign(socket, key, nil, fun)
+    end
+  end
+
   @doc """
   Forces an assign on a socket.
   """
@@ -194,7 +213,7 @@ defmodule Phoenix.LiveView.Utils do
     socket
     |> clear_changed()
     |> clear_temp()
-    |> drop_private([:connect_info, :connect_params, :assign_new])
+    |> drop_private([:connect_info, :connect_params, :assign_new, :handover_pid])
   end
 
   @doc """

mix.exs

@@ -41,7 +41,9 @@ defmodule Phoenix.LiveView.MixProject do
 
   defp deps do
     [
-      {:phoenix, "~> 1.6.15 or ~> 1.7.0"},
+      # {:phoenix, "~> 1.6.15 or ~> 1.7.0"},
+      # TODO: remove before merging
+      {:phoenix, github: "phoenixframework/phoenix", branch: "sd-handover-assigns", override: true},
       {:plug, "~> 1.15"},
       {:phoenix_template, "~> 1.0"},
       {:phoenix_html, "~> 3.3 or ~> 4.0 or ~> 4.1"},

test/e2e/support/navigation.ex

@@ -56,6 +56,7 @@ defmodule Phoenix.LiveViewTest.E2E.Navigation.ALive do
   def mount(_params, _session, socket) do
     socket
     |> assign(:param_current, nil)
+    |> assign_new(:foo, fn -> "bar" end)
     |> then(&{:ok, &1})
   end
 
@@ -73,6 +74,7 @@ defmodule Phoenix.LiveViewTest.E2E.Navigation.ALive do
   def render(assigns) do
     ~H"""
     <h1>This is page A</h1>
+    <p>Foo: <%= @foo %></p>
 
     <p>Current param: <%= @param_current %></p>
 
@@ -100,6 +102,7 @@ defmodule Phoenix.LiveViewTest.E2E.Navigation.BLive do
   @impl Phoenix.LiveView
   def mount(_params, _session, socket) do
     socket
+    |> assign_new(:foo, fn -> "baz" end)
     |> then(&{:ok, &1})
   end
 
@@ -128,6 +131,7 @@ defmodule Phoenix.LiveViewTest.E2E.Navigation.BLive do
   def render(assigns) do
     ~H"""
     <h1>This is page B</h1>
+    <p>Foo: <%= @foo %></p>
 
     <a
       href="#items-item-42"

test/e2e/tests/navigation.spec.js

@@ -240,3 +240,20 @@ test("navigating all the way back works without remounting (only patching)", asy
   // we patched 2 times
   await expect(webSocketEvents.filter(e => e.payload.indexOf("live_patch") !== -1)).toHaveLength(2);
 });
+
+test("sharing assigns between live navigation", async ({ page }) => {
+  await page.goto("/navigation/a");
+  await syncLV(page);
+
+  await expect(page.getByText("Foo:")).toContainText("bar");
+  await page.getByRole("link", { name: "LiveView B" }).click();
+  await syncLV(page);
+  await expect(page.getByText("Foo:")).toContainText("bar");
+
+  await page.reload();
+  await syncLV(page);
+  await expect(page.getByText("Foo:")).toContainText("baz");
+  await page.getByRole("link", { name: "LiveView A" }).click();
+  await syncLV(page);
+  await expect(page.getByText("Foo:")).toContainText("baz");
+});

test/phoenix_component_test.exs

@@ -141,6 +141,32 @@ defmodule Phoenix.ComponentUnitTest do
              }
     end
 
+    test "does handover of previous assigns when handover_pid is present" do
+      pid = spawn(fn ->
+        receive do
+          {:"$gen_call", {pid, _} = from, {:phoenix, :get_assigns, pid, [:existing]}} ->
+            GenServer.reply(from, {:ok, %{existing: "existing-handover"}})
+        end
+      end)
+
+      socket =
+        put_in(@socket.private[:assign_new], {%{}, []})
+        |> put_in([Access.key(:private), :handover_pid], pid)
+        |> assign(existing2: "existing2")
+        |> assign_new(:existing, fn -> "new-existing" end)
+        |> assign_new(:existing2, fn -> "new-existing2" end)
+        |> assign_new(:notexisting, fn -> "new-notexisting" end)
+
+      assert socket.assigns == %{
+               existing: "existing-handover",
+               existing2: "existing2",
+               notexisting: "new-notexisting",
+               live_action: nil,
+               flash: %{},
+               __changed__: %{existing: true, notexisting: true, existing2: true}
+             }
+    end
+
     test "has access to assigns" do
       socket =
         put_in(@socket.private[:assign_new], {%{existing: "existing-parent"}, []})