Hydrate server-rendered apps

.gitignore

@@ -25,3 +25,6 @@
 # Ignores that should be in the global gitignore
 /coverage
 /doc
+
+# Ignore npm stuff
+/node_modules

Rakefile

@@ -14,3 +14,11 @@ require 'rspec/core/rake_task'
 RSpec::Core::RakeTask.new(:spec)
 
 task :default => [:spec, :spec_opal]
+
+namespace :vdom do
+  desc 'Compile the virtual-dom library from npm'
+  task :compile do
+    sh 'npm install'
+    sh 'npm run compile'
+  end
+end

index.js

@@ -0,0 +1,19 @@
+var diff       = require('virtual-dom/diff')
+var patch      = require('virtual-dom/patch')
+var h          = require('virtual-dom/h')
+var create     = require('virtual-dom/create-element')
+var VNode      = require('virtual-dom/vnode/vnode')
+var VText      = require('virtual-dom/vnode/vtext')
+var svg        = require('virtual-dom/virtual-hyperscript/svg')
+var virtualize = require('vdom-virtualize')
+
+module.exports = {
+  diff,
+  patch,
+  h,
+  create,
+  VNode,
+  VText,
+  svg,
+  virtualize,
+}

opal/clearwater/virtual_dom.rb

@@ -48,16 +48,26 @@ def initialize(root=Bowser.document.create_element('div'))
     end
 
     def render node
-      if rendered?
-        diff = VirtualDOM.diff @node, node
-        VirtualDOM.patch @tree, diff
-        @node = node
-      else
-        @node = node
-        @tree = create_element(node)
-        @root.inner_dom = @tree
+      if !rendered?
         @rendered = true
+
+        # If we rendered the app on the server, we hydrate the rendered app
+        # rather than replacing it. This provides a faster and more memory-
+        # efficient first vdom render.
+        if `#@root.native.children.length > 0`
+          @tree = `#@root.native.children[0]`
+          @node = `virtualDom.virtualize(#@tree)`
+        else # Not server-rendered, generate from scratch.
+          @node = node
+          @tree = VirtualDOM.create_element(node)
+          @root.inner_dom = @tree
+          return
+        end
       end
+
+      diff = VirtualDOM.diff @node, node
+      VirtualDOM.patch @tree, diff
+      @node = node
     end
 
     def create_element node