[IMP] qweb: add renderToString method

closes #230

doc/qweb.md

@@ -122,6 +122,13 @@ It's API is quite simple:
   const vnode = qweb.render("App", component);
   ```
 
+- **`renderToString(name, context)`**: renders a template, but returns an html
+  string.
+
+  ```js
+  const str = qweb.renderToString("someTemplate", somecontext);
+  ```
+
 - **`register(name, Component)`**: static function to register an OWL Component
   to QWeb's global registry. Globally registered Components can be used in
   templates (see the `t-component` directive). This is useful for commonly used

src/qweb_core.ts

@@ -1,4 +1,4 @@
-import { VNode, h } from "./vdom";
+import { VNode, h, patch } from "./vdom";
 import { QWebVar, compileExpr } from "./qweb_expressions";
 import { EventBus } from "./event_bus";
 
@@ -288,6 +288,23 @@ export class QWeb extends EventBus {
     return template.fn.call(this, context, extra);
   }
 
+  /**
+   * Render a template to a html string.
+   *
+   * Note that this is more limited than the `render` method: it is not suitable
+   * to render a full component tree, since this is an asynchronous operation.
+   * This method can only render templates without components.
+   */
+  renderToString(name: string, context: EvalContext = {}): string {
+    const vnode = this.render(name, context);
+    if (vnode.sel === undefined) {
+      return vnode.text!;
+    }
+    const node = document.createElement(vnode.sel);
+    const result = patch(node, vnode);
+    return (<HTMLElement>result.elm).outerHTML;
+  }
+
   _compile(name: string, elem: Element, parentContext?: Context): CompiledTemplate {
     const isDebug = elem.attributes.hasOwnProperty("t-debug");
     const ctx = new Context(name);

tests/helpers.ts

@@ -68,9 +68,22 @@ export function renderToDOM(
   return result.elm as HTMLElement;
 }
 
+/**
+ * Render a template to an html string. The big difference with the
+ * renderToString method in QWeb is that we use the renderToDom method, which
+ * snapshots the resulting template function.  Doing so gives us a large body
+ * of reference to evaluate changes in the generated QWeb code.
+ *
+ * Note that the result of renderToString is guaranteed to be the same as the
+ * one from QWeb.
+ */
 export function renderToString(qweb: QWeb, t: string, context: EvalContext = {}): string {
   const node = renderToDOM(qweb, t, context);
-  return node instanceof Text ? node.textContent! : node.outerHTML;
+  const result = node instanceof Text ? node.textContent! : node.outerHTML;
+  if (result !== qweb.renderToString(t, context)) {
+      throw new Error("HTML string returned by renderToString helper does not match QWeb render");
+  }
+  return result;
 }
 
 // hereafter, we define two helpers to patch/unpatch the nextFrame utils. This