[IMP] component: add static css support for sub components

part of issue #80

doc/component.md

@@ -4,7 +4,7 @@
 
 - [Overview](#overview)
 - [Example](#example)
-- [Templates](#templates)
+- [Composition](#composition)
 - [Reference](#reference)
   - [Properties](#properties)
   - [Methods](#methods)
@@ -65,7 +65,7 @@ a state object is defined. It is not mandatory to use the state object, but it
 is certainly encouraged. The state object is [observed](observer.md), and any
 change to it will cause a rerendering.
 
-## Templates
+## Composition
 
 The example above shows a QWeb template with a `t-on-click` directive.  Widget
 templates are standard [QWeb](qweb.md) templates, but with an extra directive:
@@ -73,18 +73,39 @@ templates are standard [QWeb](qweb.md) templates, but with an extra directive:
 widgets:
 
 ```xml
-  <div>
-    <span>some text</span>
-    <t t-widget="MyWidget" t-props="{info: 13}">
-  </div>
+<div t-name="parent">
+  <span>some text</span>
+  <t t-widget="MyWidget" t-props="{info: 13}">
+</div>
 ```
 
-In this example, the template create a widget MyWidget just after the span. See
-the [QWeb](qweb.md) documentation for more information on the `t-widget` directive.
+```js
+class ParentWidget extends owl.Component {
+    template = 'parent';
+    widgets = { MyWidget: MyWidget};
+    ...
+}
+```
+
+In this example, the `ParentWidget`'s template creates a widget `MyWidget` just
+after the span. See the [QWeb](qweb.md) documentation for more information on the
+`t-widget` directive.
 
 Note that the rendering context for the template is the widget itself.  This means
 that the template can access `state`, `props`, `env`, or any methods defined in the widget.
 
+**CSS and style:** there is some specific support to allow the parent to declare
+additional css classes or style for the sub widget: css declared in `class`, `style`, `t-att-class` or `t-att-style` will be added to the
+root widget element.
+
+```xml
+<div t-name="parent">
+  <t t-widget="MyWidget" class="someClass" style="font-weight:bold;" t-props="{info: 13}">
+</div>
+```
+
+
+
 ## Reference
 
 An Owl component is a small class which represent a widget or some UI element.

src/qweb.ts

@@ -1212,8 +1212,21 @@ const widgetDirective: Directive = {
       finalizeWidgetCode += `;delete context.refs[${refKey}]`;
     }
     ctx.addIf(`isNew${widgetID}`);
+    let createHook = "";
+    let classAttr = node.getAttribute("class");
+    let styleAttr = node.getAttribute("style");
+    if (classAttr || styleAttr) {
+      const classCode = classAttr
+        ? classAttr
+            .split(" ")
+            .map(c => `vn.elm.classList.add('${c}')`)
+            .join(";") + ";"
+        : "";
+      const styleCode = styleAttr ? `vn.elm.style = '${styleAttr}'` : "";
+      createHook = `vnode.data.hook = {create(_, vn){${classCode}${styleCode}}};`;
+    }
     ctx.addLine(
-      `def${defID} = def${defID}.then(vnode=>{let pvnode=h(vnode.sel, {key: ${templateID}});c${
+      `def${defID} = def${defID}.then(vnode=>{${createHook}let pvnode=h(vnode.sel, {key: ${templateID}});c${
         ctx.parentNode
       }[_${dummyID}_index]=pvnode;pvnode.data.hook = {insert(vn){let nvn=w${widgetID}._mount(vnode, vn.elm);pvnode.elm=nvn.elm;${refExpr}},remove(){${finalizeWidgetCode}},destroy(){${finalizeWidgetCode}}}; w${widgetID}.__owl__.pvnode = pvnode;});`
     );

tests/component.test.ts

@@ -1224,6 +1224,37 @@ describe("props evaluation (with t-props directive)", () => {
   });
 });
 
+describe("class and style attributes with t-widget", () => {
+  test("class is properly added on widget root el", async () => {
+    class ParentWidget extends Widget {
+      inlineTemplate = `
+        <div>
+            <t t-widget="child" class="a b"/>
+        </div>`;
+      widgets = { child: Child };
+    }
+    class Child extends Widget {
+        inlineTemplate = `<div class="c"/>`
+    }
+    const widget = new ParentWidget(env);
+    await widget.mount(fixture);
+    expect(fixture.innerHTML).toBe(`<div><div class="c a b"></div></div>`);
+  });
+
+  test("style is properly added on widget root el", async () => {
+    class ParentWidget extends Widget {
+      inlineTemplate = `
+        <div>
+            <t t-widget="child" style="font-weight: bold;"/>
+        </div>`;
+      widgets = { child: Widget };
+    }
+    const widget = new ParentWidget(env);
+    await widget.mount(fixture);
+    expect(fixture.innerHTML).toBe(`<div><div style="font-weight: bold;"></div></div>`);
+  });
+});
+
 describe("other directives with t-widget", () => {
   test("t-on works as expected", async () => {
     let n = 0;