[IMP] qweb: add t-async directive on t-widget

Closes #98

doc/component.md

@@ -10,6 +10,7 @@
   - [Methods](#methods)
   - [Lifecycle](#lifecycle)
   - [Composition](#composition)
+  - [Asynchronous rendering](#asynchronous-rendering)
   - [Event Handling](#event-handling)
   - [`t-key` directive](#t-key-directive)
   - [`t-mounted` directive](#t-mounted-directive)
@@ -309,7 +310,7 @@ the DOM. This is a good place to remove some listeners, for example.
 
 This is the opposite method of `mounted`.
 
-## Composition
+### 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:
@@ -828,9 +829,25 @@ There are two different common problems with Owl asynchronous rendering model:
 
 Here are a few tips on how to work with asynchronous widgets:
 
-1. minimize the use of asynchronous widgets!
+1. Minimize the use of asynchronous widgets!
 2. Maybe move the asynchronous logic in a store, which then triggers (mostly)
    synchronous renderings
 3. Lazy loading external libraries is a good use case for async rendering. This
    is mostly fine, because we can assume that it will only takes a fraction of a
    second, and only once (see `owl.utils.loadJS`)
+4. For all the other cases, the `t-async` directive (to use alongside
+   `t-widget`) is there to help you. When this directive is met, a new rendering
+   sub tree is created, such that the rendering of that component (and its
+   children) is not tied to the rendering of the rest of the interface. It can
+   be used on an asynchronous component, to prevent it from delaying the
+   rendering of the whole interface, or on a synchronous one, such that its
+   rendering isn't delayed by other (asynchronous) components. Note that this
+   directive has no effect on the first rendering, but only on subsequent ones
+   (triggered by state or props changes).
+
+   ```xml
+   <div t-name="ParentWidget">
+     <t t-widget="SyncChild"/>
+     <t t-widget="AsyncChild" t-async="1"/>
+   </div>
+   ```

doc/qweb.md

@@ -66,8 +66,8 @@ The component system in Owl requires additional directives, to express various
 needs. Here is a list of all Owl specific directives:
 
 | Name                                 | Description                                                                             |
-| ------------------------- | --------------------------------------------------------------------------------------- |
-| `t-widget`, `t-keepalive` | [Defining a sub component](component.md#composition)                                    |
+| ------------------------------------ | --------------------------------------------------------------------------------------- |
+| `t-widget`, `t-keepalive`, `t-async` | [Defining a sub component](component.md#composition)                                    |
 | `t-ref`                              | [Setting a reference to a dom node or a sub component](component.md#keeping-references) |
 | `t-key`                              | [Defining a key (to help virtual dom reconciliation)](component.md#t-key-directive)     |
 | `t-on-*`                             | [Event handling](component.md#event-handling)                                           |

src/component.ts

@@ -274,20 +274,7 @@ export class Component<T extends Env, Props extends {}, State extends {}> {
     if (shouldPatch && __owl__.isMounted && renderId === __owl__.renderId) {
       // we only update the vnode and the actual DOM if no other rendering
       // occurred between now and when the render method was initially called.
-      const patchLen = patchQueue!.length;
-      for (let i = 0; i < patchLen; i++) {
-        const patch = patchQueue![i];
-        patch.push(patch[0].willPatch());
-      }
-      for (let i = 0; i < patchLen; i++) {
-        const patch = patchQueue![i];
-        patch[0]._patch(patch[1]);
-      }
-
-      for (let i = patchLen - 1; i >= 0; i--) {
-        const patch = patchQueue![i];
-        patch[0].patched(patch[2]);
-      }
+      this._applyPatchQueue(<any[]>patchQueue);
     }
   }
 
@@ -568,6 +555,29 @@ export class Component<T extends Env, Props extends {}, State extends {}> {
     return <Props>props;
   }
 
+  /**
+   * Apply the given patch queue. A patch is a pair [c, vn], where c is a
+   * Component instance and vn a VNode.
+   *   1) Call 'willPatch' on the component of each patch
+   *   2) Call '_patch' on the component of each patch
+   *   3) Call 'patched' on the component of each patch, in inverse order
+   */
+  _applyPatchQueue(patchQueue: any[]) {
+    const patchLen = patchQueue.length;
+    for (let i = 0; i < patchLen; i++) {
+      const patch = patchQueue[i];
+      patch.push(patch[0].willPatch());
+    }
+    for (let i = 0; i < patchLen; i++) {
+      const patch = patchQueue[i];
+      patch[0]._patch(patch[1]);
+    }
+    for (let i = patchLen - 1; i >= 0; i--) {
+      const patch = patchQueue[i];
+      patch[0].patched(patch[2]);
+    }
+  }
+
   /**
    * Validate the component props (or next props) against the (static) props
    * description.  This is potentially an expensive operation: it may needs to

src/qweb_extensions.ts

@@ -225,13 +225,6 @@ const T_WIDGET_MODS_CODE = Object.assign({}, MODS_CODE, {
  * // t-widget directive: we start by evaluating the expression given by t-key:
  * let key5 = "somestring";
  *
- * // We keep the index of the position of the widget in the closure.  We push
- * // null to reserve the slot, and will replace it later by the widget vnode,
- * // when it will be ready (do not forget that preparing/rendering a widget is
- * // asynchronous)
- * let _2_index = c1.length;
- * c1.push(null);
- *
  * // def3 is the promise that will contain later either the new widget
  * // creation, or the props update...
  * let def3;
@@ -246,6 +239,13 @@ const T_WIDGET_MODS_CODE = Object.assign({}, MODS_CODE, {
  *   ? context.__owl__.children[context.__owl__.cmap[key5]]
  *   : false;
  *
+ * // We keep the index of the position of the widget in the closure.  We push
+ * // null to reserve the slot, and will replace it later by the widget vnode,
+ * // when it will be ready (do not forget that preparing/rendering a widget is
+ * // asynchronous)
+ * let _2_index = c1.length;
+ * c1.push(null);
+ *
  * // we evaluate here the props given to the component. It is done here to be
  * // able to easily reference it later, and also, it might be an expensive
  * // computation, so it is certainly better to do it only once
@@ -360,7 +360,7 @@ const T_WIDGET_MODS_CODE = Object.assign({}, MODS_CODE, {
 
 QWeb.addDirective({
   name: "widget",
-  extraNames: ["props", "keepalive"],
+  extraNames: ["props", "keepalive", "async"],
   priority: 100,
   atNodeEncounter({ ctx, value, node, qweb }): boolean {
     ctx.addLine("//WIDGET");
@@ -368,6 +368,7 @@ QWeb.addDirective({
     ctx.rootContext.shouldDefineQWeb = true;
     ctx.rootContext.shouldDefineUtils = true;
     let keepAlive = node.getAttribute("t-keepalive") ? true : false;
+    let async = node.getAttribute("t-async") ? true : false;
 
     // t-on- events and t-transition
     const events: [string, string[], string, string][] = [];
@@ -413,8 +414,6 @@ QWeb.addDirective({
       // want to evaluate it only once)
       ctx.addLine(`let key${keyID} = ${key};`);
     }
-    ctx.addLine(`let _${dummyID}_index = c${ctx.parentNode}.length;`);
-    ctx.addLine(`c${ctx.parentNode}.push(null);`);
     ctx.addLine(`let def${defID};`);
     let templateID = key
       ? `key${keyID}`
@@ -505,6 +504,13 @@ QWeb.addDirective({
     ctx.addLine(
       `let w${widgetID} = ${templateID} in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[${templateID}]] : false;`
     );
+    ctx.addLine(`let _${dummyID}_index = c${ctx.parentNode}.length;`);
+    if (async) {
+      ctx.addLine(`const patchQueue${widgetID} = [];`);
+      ctx.addLine(`c${ctx.parentNode}.push(w${widgetID} && w${widgetID}.__owl__.pvnode || null);`);
+    } else {
+      ctx.addLine(`c${ctx.parentNode}.push(null);`);
+    }
     ctx.addLine(`let props${widgetID} = {${propStr}};`);
     ctx.addIf(
       `w${widgetID} && w${widgetID}.__owl__.renderPromise && !w${widgetID}.__owl__.vnode`
@@ -551,7 +557,6 @@ QWeb.addDirective({
 
     ctx.addLine(`def${defID} = w${widgetID}._prepare();`);
     // hack: specify empty remove hook to prevent the node from being removed from the DOM
-    // FIXME: click to re-add widget during remove transition -> leak
     ctx.addLine(
       `def${defID} = def${defID}.then(vnode=>{${createHook}let pvnode=h(vnode.sel, {key: ${templateID}, hook: {insert(vn) {let nvn=w${widgetID}._mount(vnode, pvnode.elm);pvnode.elm=nvn.elm;${refExpr}${transitionsInsertCode}},remove() {},destroy(vn) {${finalizeWidgetCode}}}});c${
         ctx.parentNode
@@ -560,8 +565,9 @@ QWeb.addDirective({
 
     ctx.addElse();
     // need to update widget
+    const patchQueueCode = async ? `patchQueue${widgetID}` : "extra.patchQueue";
     ctx.addLine(
-      `def${defID} = def${defID} || w${widgetID}._updateProps(props${widgetID}, extra.forceUpdate, extra.patchQueue);`
+      `def${defID} = def${defID} || w${widgetID}._updateProps(props${widgetID}, extra.forceUpdate, ${patchQueueCode});`
     );
     let keepAliveCode = "";
     if (keepAlive) {
@@ -576,7 +582,13 @@ QWeb.addDirective({
     );
     ctx.closeIf();
 
+    if (async) {
+      ctx.addLine(
+        `def${defID}.then(w${widgetID}._applyPatchQueue.bind(w${widgetID}, patchQueue${widgetID}));`
+      );
+    } else {
       ctx.addLine(`extra.promises.push(def${defID});`);
+    }
 
     if (
       node.hasAttribute("t-if") ||

tests/__snapshots__/animations.test.ts.snap

@@ -10,10 +10,10 @@ exports[`animations t-transition combined with t-widget 1`] = `
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -54,10 +54,10 @@ exports[`animations t-transition combined with t-widget and t-if 1`] = `
     var vn1 = h('div', p1, c1);
     if (context['state'].display) {
         //WIDGET
-        let _2_index = c1.length;
-        c1.push(null);
         let def3;
         let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+        let _2_index = c1.length;
+        c1.push(null);
         let props4 = {};
         if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
             if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {

tests/__snapshots__/component.test.ts.snap

@@ -1,5 +1,248 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`async rendering delayed t-widget with t-async directive 1`] = `
+"function anonymous(context,extra
+) {
+    let utils = this.utils;
+    let QWeb = this.constructor;
+    let owner = context;
+    var h = this.utils.h;
+    let c1 = [], p1 = {key:1};
+    var vn1 = h('div', p1, c1);
+    let c2 = [], p2 = {key:2,on:{}};
+    var vn2 = h('button', p2, c2);
+    c1.push(vn2);
+    if (!context['updateApp']) {
+        throw new Error('Missing handler \\\\'' + 'updateApp' + \`\\\\' when evaluating template 'Parent'\`)
+    }
+    extra.handlers['click' + 2] = extra.handlers['click' + 2] || context['updateApp'].bind(owner);
+    p2.on['click'] = extra.handlers['click' + 2];
+    c2.push({text: \`Update App State\`});
+    var _3 = 'children';
+    let c4 = [], p4 = {key:4,attrs:{class: _3}};
+    var vn4 = h('div', p4, c4);
+    c1.push(vn4);
+    //WIDGET
+    let def6;
+    let w7 = 7 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[7]] : false;
+    let _5_index = c4.length;
+    c4.push(null);
+    let props7 = {val:context['state'].val};
+    if (w7 && w7.__owl__.renderPromise && !w7.__owl__.vnode) {
+        if (utils.shallowEqual(props7, w7.__owl__.renderProps)) {
+            def6 = w7.__owl__.renderPromise;
+        } else {
+            w7.destroy();
+            w7 = false;
+        }
+    }
+    if (!w7) {
+        let widgetKey7 = \`Child\`;
+        let W7 = context.widgets && context.widgets[widgetKey7] || QWeb.widgets[widgetKey7];
+        if (!W7) {throw new Error('Cannot find the definition of widget \\"' + widgetKey7 + '\\"')}
+        w7 = new W7(owner, props7);
+        context.__owl__.cmap[7] = w7.__owl__.id;
+        def6 = w7._prepare();
+        def6 = def6.then(vnode=>{let pvnode=h(vnode.sel, {key: 7, hook: {insert(vn) {let nvn=w7._mount(vnode, pvnode.elm);pvnode.elm=nvn.elm;},remove() {},destroy(vn) {w7.destroy();}}});c4[_5_index]=pvnode;w7.__owl__.pvnode = pvnode;});
+    } else {
+        def6 = def6 || w7._updateProps(props7, extra.forceUpdate, extra.patchQueue);
+        def6 = def6.then(()=>{if (w7.__owl__.isDestroyed) {return};let pvnode=w7.__owl__.pvnode;c4[_5_index]=pvnode;});
+    }
+    extra.promises.push(def6);
+    //WIDGET
+    let def9;
+    let w10 = 10 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[10]] : false;
+    let _8_index = c4.length;
+    const patchQueue10 = [];
+    c4.push(w10 && w10.__owl__.pvnode || null);
+    let props10 = {val:context['state'].val};
+    if (w10 && w10.__owl__.renderPromise && !w10.__owl__.vnode) {
+        if (utils.shallowEqual(props10, w10.__owl__.renderProps)) {
+            def9 = w10.__owl__.renderPromise;
+        } else {
+            w10.destroy();
+            w10 = false;
+        }
+    }
+    if (!w10) {
+        let widgetKey10 = \`AsyncChild\`;
+        let W10 = context.widgets && context.widgets[widgetKey10] || QWeb.widgets[widgetKey10];
+        if (!W10) {throw new Error('Cannot find the definition of widget \\"' + widgetKey10 + '\\"')}
+        w10 = new W10(owner, props10);
+        context.__owl__.cmap[10] = w10.__owl__.id;
+        def9 = w10._prepare();
+        def9 = def9.then(vnode=>{let pvnode=h(vnode.sel, {key: 10, hook: {insert(vn) {let nvn=w10._mount(vnode, pvnode.elm);pvnode.elm=nvn.elm;},remove() {},destroy(vn) {w10.destroy();}}});c4[_8_index]=pvnode;w10.__owl__.pvnode = pvnode;});
+    } else {
+        def9 = def9 || w10._updateProps(props10, extra.forceUpdate, patchQueue10);
+        def9 = def9.then(()=>{if (w10.__owl__.isDestroyed) {return};let pvnode=w10.__owl__.pvnode;c4[_8_index]=pvnode;});
+    }
+    def9.then(w10._applyPatchQueue.bind(w10, patchQueue10));
+    return vn1;
+}"
+`;
+
+exports[`async rendering fast t-widget with t-async directive 1`] = `
+"function anonymous(context,extra
+) {
+    let utils = this.utils;
+    let QWeb = this.constructor;
+    let owner = context;
+    var h = this.utils.h;
+    let c1 = [], p1 = {key:1};
+    var vn1 = h('div', p1, c1);
+    let c2 = [], p2 = {key:2,on:{}};
+    var vn2 = h('button', p2, c2);
+    c1.push(vn2);
+    if (!context['updateApp']) {
+        throw new Error('Missing handler \\\\'' + 'updateApp' + \`\\\\' when evaluating template 'Parent'\`)
+    }
+    extra.handlers['click' + 2] = extra.handlers['click' + 2] || context['updateApp'].bind(owner);
+    p2.on['click'] = extra.handlers['click' + 2];
+    c2.push({text: \`Update App State\`});
+    var _3 = 'children';
+    let c4 = [], p4 = {key:4,attrs:{class: _3}};
+    var vn4 = h('div', p4, c4);
+    c1.push(vn4);
+    //WIDGET
+    let def6;
+    let w7 = 7 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[7]] : false;
+    let _5_index = c4.length;
+    const patchQueue7 = [];
+    c4.push(w7 && w7.__owl__.pvnode || null);
+    let props7 = {val:context['state'].val};
+    if (w7 && w7.__owl__.renderPromise && !w7.__owl__.vnode) {
+        if (utils.shallowEqual(props7, w7.__owl__.renderProps)) {
+            def6 = w7.__owl__.renderPromise;
+        } else {
+            w7.destroy();
+            w7 = false;
+        }
+    }
+    if (!w7) {
+        let widgetKey7 = \`Child\`;
+        let W7 = context.widgets && context.widgets[widgetKey7] || QWeb.widgets[widgetKey7];
+        if (!W7) {throw new Error('Cannot find the definition of widget \\"' + widgetKey7 + '\\"')}
+        w7 = new W7(owner, props7);
+        context.__owl__.cmap[7] = w7.__owl__.id;
+        def6 = w7._prepare();
+        def6 = def6.then(vnode=>{let pvnode=h(vnode.sel, {key: 7, hook: {insert(vn) {let nvn=w7._mount(vnode, pvnode.elm);pvnode.elm=nvn.elm;},remove() {},destroy(vn) {w7.destroy();}}});c4[_5_index]=pvnode;w7.__owl__.pvnode = pvnode;});
+    } else {
+        def6 = def6 || w7._updateProps(props7, extra.forceUpdate, patchQueue7);
+        def6 = def6.then(()=>{if (w7.__owl__.isDestroyed) {return};let pvnode=w7.__owl__.pvnode;c4[_5_index]=pvnode;});
+    }
+    def6.then(w7._applyPatchQueue.bind(w7, patchQueue7));
+    //WIDGET
+    let def9;
+    let w10 = 10 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[10]] : false;
+    let _8_index = c4.length;
+    c4.push(null);
+    let props10 = {val:context['state'].val};
+    if (w10 && w10.__owl__.renderPromise && !w10.__owl__.vnode) {
+        if (utils.shallowEqual(props10, w10.__owl__.renderProps)) {
+            def9 = w10.__owl__.renderPromise;
+        } else {
+            w10.destroy();
+            w10 = false;
+        }
+    }
+    if (!w10) {
+        let widgetKey10 = \`AsyncChild\`;
+        let W10 = context.widgets && context.widgets[widgetKey10] || QWeb.widgets[widgetKey10];
+        if (!W10) {throw new Error('Cannot find the definition of widget \\"' + widgetKey10 + '\\"')}
+        w10 = new W10(owner, props10);
+        context.__owl__.cmap[10] = w10.__owl__.id;
+        def9 = w10._prepare();
+        def9 = def9.then(vnode=>{let pvnode=h(vnode.sel, {key: 10, hook: {insert(vn) {let nvn=w10._mount(vnode, pvnode.elm);pvnode.elm=nvn.elm;},remove() {},destroy(vn) {w10.destroy();}}});c4[_8_index]=pvnode;w10.__owl__.pvnode = pvnode;});
+    } else {
+        def9 = def9 || w10._updateProps(props10, extra.forceUpdate, extra.patchQueue);
+        def9 = def9.then(()=>{if (w10.__owl__.isDestroyed) {return};let pvnode=w10.__owl__.pvnode;c4[_8_index]=pvnode;});
+    }
+    extra.promises.push(def9);
+    return vn1;
+}"
+`;
+
+exports[`async rendering t-widget with t-async directive: mixed re-renderings 1`] = `
+"function anonymous(context,extra
+) {
+    let utils = this.utils;
+    let QWeb = this.constructor;
+    let owner = context;
+    var h = this.utils.h;
+    let c1 = [], p1 = {key:1};
+    var vn1 = h('div', p1, c1);
+    let c2 = [], p2 = {key:2,on:{}};
+    var vn2 = h('button', p2, c2);
+    c1.push(vn2);
+    if (!context['updateApp']) {
+        throw new Error('Missing handler \\\\'' + 'updateApp' + \`\\\\' when evaluating template 'Parent'\`)
+    }
+    extra.handlers['click' + 2] = extra.handlers['click' + 2] || context['updateApp'].bind(owner);
+    p2.on['click'] = extra.handlers['click' + 2];
+    c2.push({text: \`Update App State\`});
+    var _3 = 'children';
+    let c4 = [], p4 = {key:4,attrs:{class: _3}};
+    var vn4 = h('div', p4, c4);
+    c1.push(vn4);
+    //WIDGET
+    let def6;
+    let w7 = 7 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[7]] : false;
+    let _5_index = c4.length;
+    c4.push(null);
+    let props7 = {val:context['state'].val};
+    if (w7 && w7.__owl__.renderPromise && !w7.__owl__.vnode) {
+        if (utils.shallowEqual(props7, w7.__owl__.renderProps)) {
+            def6 = w7.__owl__.renderPromise;
+        } else {
+            w7.destroy();
+            w7 = false;
+        }
+    }
+    if (!w7) {
+        let widgetKey7 = \`Child\`;
+        let W7 = context.widgets && context.widgets[widgetKey7] || QWeb.widgets[widgetKey7];
+        if (!W7) {throw new Error('Cannot find the definition of widget \\"' + widgetKey7 + '\\"')}
+        w7 = new W7(owner, props7);
+        context.__owl__.cmap[7] = w7.__owl__.id;
+        def6 = w7._prepare();
+        def6 = def6.then(vnode=>{let pvnode=h(vnode.sel, {key: 7, hook: {insert(vn) {let nvn=w7._mount(vnode, pvnode.elm);pvnode.elm=nvn.elm;},remove() {},destroy(vn) {w7.destroy();}}});c4[_5_index]=pvnode;w7.__owl__.pvnode = pvnode;});
+    } else {
+        def6 = def6 || w7._updateProps(props7, extra.forceUpdate, extra.patchQueue);
+        def6 = def6.then(()=>{if (w7.__owl__.isDestroyed) {return};let pvnode=w7.__owl__.pvnode;c4[_5_index]=pvnode;});
+    }
+    extra.promises.push(def6);
+    //WIDGET
+    let def9;
+    let w10 = 10 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[10]] : false;
+    let _8_index = c4.length;
+    const patchQueue10 = [];
+    c4.push(w10 && w10.__owl__.pvnode || null);
+    let props10 = {val:context['state'].val};
+    if (w10 && w10.__owl__.renderPromise && !w10.__owl__.vnode) {
+        if (utils.shallowEqual(props10, w10.__owl__.renderProps)) {
+            def9 = w10.__owl__.renderPromise;
+        } else {
+            w10.destroy();
+            w10 = false;
+        }
+    }
+    if (!w10) {
+        let widgetKey10 = \`AsyncChild\`;
+        let W10 = context.widgets && context.widgets[widgetKey10] || QWeb.widgets[widgetKey10];
+        if (!W10) {throw new Error('Cannot find the definition of widget \\"' + widgetKey10 + '\\"')}
+        w10 = new W10(owner, props10);
+        context.__owl__.cmap[10] = w10.__owl__.id;
+        def9 = w10._prepare();
+        def9 = def9.then(vnode=>{let pvnode=h(vnode.sel, {key: 10, hook: {insert(vn) {let nvn=w10._mount(vnode, pvnode.elm);pvnode.elm=nvn.elm;},remove() {},destroy(vn) {w10.destroy();}}});c4[_8_index]=pvnode;w10.__owl__.pvnode = pvnode;});
+    } else {
+        def9 = def9 || w10._updateProps(props10, extra.forceUpdate, patchQueue10);
+        def9 = def9.then(()=>{if (w10.__owl__.isDestroyed) {return};let pvnode=w10.__owl__.pvnode;c4[_8_index]=pvnode;});
+    }
+    def9.then(w10._applyPatchQueue.bind(w10, patchQueue10));
+    return vn1;
+}"
+`;
+
 exports[`class and style attributes with t-widget dynamic t-att-style is properly added and updated on widget root el 1`] = `
 "function anonymous(context,extra
 ) {
@@ -10,11 +253,11 @@ exports[`class and style attributes with t-widget dynamic t-att-style is properl
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     const _5 = context['state'].style;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -51,11 +294,11 @@ exports[`class and style attributes with t-widget t-att-class is properly added/
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     const _5 = {a:context['state'].a,b:context['state'].b};
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -112,10 +355,10 @@ exports[`composition sub widgets with some state rendered in a loop 1`] = `
         context.number_value = _4[i];
         //WIDGET
         let key8 = context['number'];
-        let _5_index = c1.length;
-        c1.push(null);
         let def6;
         let w7 = key8 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[key8]] : false;
+        let _5_index = c1.length;
+        c1.push(null);
         let props7 = {};
         if (w7 && w7.__owl__.renderPromise && !w7.__owl__.vnode) {
             if (utils.shallowEqual(props7, w7.__owl__.renderProps)) {
@@ -153,10 +396,10 @@ exports[`composition t-widget with dynamic value 1`] = `
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -193,10 +436,10 @@ exports[`composition t-widget with dynamic value 2 1`] = `
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -233,10 +476,10 @@ exports[`other directives with t-widget t-on with handler bound to argument 1`]
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -273,10 +516,10 @@ exports[`other directives with t-widget t-on with handler bound to empty object
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -313,10 +556,10 @@ exports[`other directives with t-widget t-on with handler bound to empty object
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -353,10 +596,10 @@ exports[`other directives with t-widget t-on with handler bound to object 1`] =
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -393,10 +636,10 @@ exports[`other directives with t-widget t-on with prevent and self modifiers (or
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -433,10 +676,10 @@ exports[`other directives with t-widget t-on with self and prevent modifiers (or
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -473,10 +716,10 @@ exports[`other directives with t-widget t-on with self modifier 1`] = `
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -513,10 +756,10 @@ exports[`other directives with t-widget t-on with stop and/or prevent modifiers
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -554,10 +797,10 @@ exports[`random stuff/miscellaneous snapshotting compiled code 1`] = `
     var vn1 = h('div', p1, c1);
     //WIDGET
     let key5 = 'somestring';
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = key5 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[key5]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {flag:context['state'].flag};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {
@@ -594,10 +837,10 @@ exports[`t-slot directive can define and call slots 1`] = `
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
     //WIDGET
-    let _2_index = c1.length;
-    c1.push(null);
     let def3;
     let w4 = 4 in context.__owl__.cmap ? context.__owl__.children[context.__owl__.cmap[4]] : false;
+    let _2_index = c1.length;
+    c1.push(null);
     let props4 = {};
     if (w4 && w4.__owl__.renderPromise && !w4.__owl__.vnode) {
         if (utils.shallowEqual(props4, w4.__owl__.renderProps)) {

tests/component.test.ts

@@ -2352,6 +2352,194 @@ describe("async rendering", () => {
     await nextTick();
     expect(destroyCount).toBe(0);
   });
+
+  test("delayed t-widget with t-async directive", async () => {
+    env.qweb.addTemplates(`
+      <templates>
+        <div t-name="Parent">
+          <button t-on-click="updateApp">Update App State</button>
+          <div class="children">
+            <t t-widget="Child" val="state.val"/>
+            <t t-widget="AsyncChild" t-async="1" val="state.val"/>
+          </div>
+        </div>
+        <span t-name="Child"><t t-esc="props.val"/></span>
+      </templates>
+    `);
+
+    let def;
+    class Parent extends Widget {
+      widgets = { Child, AsyncChild };
+      state = { val: 0 };
+
+      updateApp() {
+        this.state.val++;
+      }
+    }
+    class Child extends Widget {}
+    class AsyncChild extends Child {
+      willUpdateProps() {
+        return def;
+      }
+    }
+
+    const parent = new Parent(env);
+    await parent.mount(fixture);
+
+    expect(env.qweb.templates.Parent.fn.toString()).toMatchSnapshot();
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>0</span><span>0</span>"
+    );
+
+    // click on button to increment Parent counter
+    def = makeDeferred();
+    fixture.querySelector("button")!.click();
+    await nextTick();
+
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>1</span><span>0</span>"
+    );
+
+    def.resolve();
+    await nextTick();
+
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>1</span><span>1</span>"
+    );
+  });
+
+  test("fast t-widget with t-async directive", async () => {
+    env.qweb.addTemplates(`
+      <templates>
+        <div t-name="Parent">
+          <button t-on-click="updateApp">Update App State</button>
+          <div class="children">
+            <t t-widget="Child" t-async="1" val="state.val"/>
+            <t t-widget="AsyncChild" val="state.val"/>
+          </div>
+        </div>
+        <span t-name="Child"><t t-esc="props.val"/></span>
+      </templates>
+    `);
+
+    let def;
+    class Parent extends Widget {
+      widgets = { Child, AsyncChild };
+      state = { val: 0 };
+
+      updateApp() {
+        this.state.val++;
+      }
+    }
+    class Child extends Widget {}
+    class AsyncChild extends Child {
+      willUpdateProps() {
+        return def;
+      }
+    }
+
+    const parent = new Parent(env);
+    await parent.mount(fixture);
+
+    expect(env.qweb.templates.Parent.fn.toString()).toMatchSnapshot();
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>0</span><span>0</span>"
+    );
+
+    // click on button to increment Parent counter
+    def = makeDeferred();
+    fixture.querySelector("button")!.click();
+    await nextTick();
+
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>1</span><span>0</span>"
+    );
+
+    def.resolve();
+    await nextTick();
+
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>1</span><span>1</span>"
+    );
+  });
+
+  test("t-widget with t-async directive: mixed re-renderings", async () => {
+    env.qweb.addTemplates(`
+      <templates>
+        <div t-name="Parent">
+          <button t-on-click="updateApp">Update App State</button>
+          <div class="children">
+            <t t-widget="Child" val="state.val"/>
+            <t t-widget="AsyncChild" t-async="1" val="state.val"/>
+          </div>
+        </div>
+        <span t-name="Child" t-on-click="increment">
+          <t t-esc="state.val"/>/<t t-esc="props.val"/>
+        </span>
+      </templates>
+    `);
+
+    let def;
+    class Parent extends Widget {
+      widgets = { Child, AsyncChild };
+      state = { val: 0 };
+
+      updateApp() {
+        this.state.val++;
+      }
+    }
+    class Child extends Widget {
+      state = { val: 0 };
+
+      increment() {
+        this.state.val++;
+      }
+    }
+    class AsyncChild extends Child {
+      willUpdateProps() {
+        return def;
+      }
+    }
+
+    const parent = new Parent(env);
+    await parent.mount(fixture);
+
+    expect(env.qweb.templates.Parent.fn.toString()).toMatchSnapshot();
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>0/0</span><span>0/0</span>"
+    );
+
+    // click on button to increment Parent counter
+    def = makeDeferred();
+    fixture.querySelector("button")!.click();
+    await nextTick();
+
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>0/1</span><span>0/0</span>"
+    );
+
+    // click on each Child to increment their local counter
+    const children = parent.el!.querySelectorAll("span");
+    children[0]!.click();
+    await nextTick();
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>1/1</span><span>0/0</span>"
+    );
+
+    children[1]!.click();
+    await nextTick();
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>1/1</span><span>1/0</span>"
+    );
+
+    // finalize first re-rendering (coming from the props update)
+    def.resolve();
+    await nextTick();
+
+    expect(fixture.querySelector(".children")!.innerHTML).toBe(
+      "<span>1/1</span><span>1/1</span>"
+    );
+  });
 });
 
 describe("updating environment", () => {

tools/playground/samples.js

@@ -1196,6 +1196,94 @@ const SLOTS_CSS = `.main {
     border-top: 1px solid white;
 }`;
 
+const ASYNC_COMPONENTS = `// This example will not work if your browser does not support ESNext class fields
+
+// In this example, we have 2 sub widgets, one of them being async. However, we don't want renderings
+// of the sync sub widget to be delayed because of the async one. We use the 't-async' directive for
+// this purpose.
+
+class App extends owl.Component {
+    widgets = {AsyncChild, NotificationManager};
+    state = { value: 0 };
+
+    increment() {
+        this.state.value++;
+        this.refs.notificationManager.notify("Value will be set to " + this.state.value);
+    }
+}
+
+class AsyncChild extends owl.Component {
+    willUpdateProps() {
+        // simulate a widget that needs to perform async stuff (e.g. an RPC)
+        // with the updated props before re-rendering itself
+        return new Promise(function (resolve) {
+            setTimeout(resolve, 1000);
+        });
+    }
+}
+
+class NotificationManager extends owl.Component {
+    state = { notifs: [] };
+
+    notify(notif) {
+        this.state.notifs.push(notif);
+        setTimeout(() => {
+            var index = this.state.notifs.indexOf(notif);
+            this.state.notifs.splice(index, 1);
+        }, 3000);
+    }
+}
+
+const qweb = new owl.QWeb(TEMPLATES);
+const app = new App({ qweb });
+app.mount(document.body);`;
+
+const ASYNC_COMPONENTS_XML = `<templates>
+  <div t-name="App" class="app">
+    <button t-on-click="increment">Increment</button>
+    <t t-widget="AsyncChild" t-async="1" value="state.value"/>
+    <t t-widget="NotificationManager" t-ref="notificationManager"/>
+  </div>
+
+  <div class="value" t-name="AsyncChild">
+    Current value: <t t-esc="props.value"/>
+  </div>
+
+  <div class="notification_manager" t-name="NotificationManager">
+    <t t-foreach="state.notifs" t-as="notif">
+      <div class="notification"><t t-esc="notif"/></div>
+    </t>
+  </div>
+</templates>`;
+
+const ASYNC_COMPONENTS_CSS = `.app {
+    width: 70%;
+}
+
+button {
+    color: darkred;
+    font-size: 30px;
+    width: 220px;
+}
+
+.value {
+    font-size: 26px;
+    padding: 20px;
+}
+
+.notification_manager {
+    position: absolute;
+    top: 0;
+    right: 0;
+}
+.notification {
+    width: 150px;
+    margin: 4px 8px;
+    padding: 16px;
+    border: 1px solid: black;
+    background-color: lightgray;
+}`;
+
 const EMPTY = `class App extends owl.Component {
 }
 
@@ -1252,6 +1340,12 @@ export const SAMPLES = [
     xml: SLOTS_XML,
     css: SLOTS_CSS
   },
+  {
+    description: "Asynchronous components",
+    code: ASYNC_COMPONENTS,
+    xml: ASYNC_COMPONENTS_XML,
+    css: ASYNC_COMPONENTS_CSS
+  },
   {
     description: "Empty",
     code: EMPTY