[IMP] owl: add basic support for sub roots

In this commit, we extend the owl App class to support multiple sub
roots. This is useful for situations where we want to mount sub
components in non-managed DOM. This is exactly what the Knowledge app is
doing, with mounting views in an html editor.

Currently, this requires some difficult and fragile hacks, and still,
the result is that it is very easy to mix components from the main App
and a SubApp.  But Knowledge does not actually care about creating a sub
app. It only needs the possibility to mount sub components in dynamic
places.

closes #1640

src/runtime/app.ts

@@ -16,10 +16,13 @@ export interface Env {
   [key: string]: any;
 }
 
-export interface AppConfig<P, E> extends TemplateSetConfig {
-  name?: string;
+export interface RootConfig<P, E> {
   props?: P;
   env?: E;
+}
+
+export interface AppConfig<P, E> extends TemplateSetConfig, RootConfig<P, E> {
+  name?: string;
   test?: boolean;
   warnIfNoStaticProps?: boolean;
 }
@@ -49,6 +52,12 @@ declare global {
   }
 }
 
+interface Root<P, E> {
+  node: ComponentNode<P, E>;
+  mount(target: HTMLElement | ShadowRoot, options?: MountOptions): Promise<Component<P, E>>;
+  destroy(): void;
+}
+
 window.__OWL_DEVTOOLS__ ||= { apps, Fiber, RootFiber, toRaw, reactive };
 
 export class App<
@@ -65,6 +74,7 @@ export class App<
   props: P;
   env: E;
   scheduler = new Scheduler();
+  subRoots: Set<ComponentNode> = new Set();
   root: ComponentNode<P, E> | null = null;
   warnIfNoStaticProps: boolean;
 
@@ -91,14 +101,46 @@ export class App<
     target: HTMLElement | ShadowRoot,
     options?: MountOptions
   ): Promise<Component<P, E> & InstanceType<T>> {
-    App.validateTarget(target);
-    if (this.dev) {
-      validateProps(this.Root, this.props, { __owl__: { app: this } });
+    const root = this.createRoot(this.Root, { props: this.props });
+    this.root = root.node;
+    this.subRoots.delete(root.node);
+    return root.mount(target, options) as any;
+  }
+
+  createRoot<Props extends object, SubEnv = any>(
+    Root: ComponentConstructor<Props, E>,
+    config: RootConfig<Props, SubEnv> = {}
+  ): Root<Props, SubEnv> {
+    const props = config.props || ({} as Props);
+    // hack to make sure the sub root get the sub env if necessary. for owl 3,
+    // would be nice to rethink the initialization process to make sure that
+    // we can create a ComponentNode and give it explicitely the env, instead
+    // of looking it up in the app
+    const env = this.env;
+    if (config.env) {
+      this.env = config.env as any;
     }
-    const node = this.makeNode(this.Root, this.props);
-    const prom = this.mountNode(node, target, options);
-    this.root = node;
-    return prom;
+    const node = this.makeNode(Root, props);
+    if (config.env) {
+      this.env = env;
+    }
+    this.subRoots.add(node);
+    return {
+      node,
+      mount: (target: HTMLElement | ShadowRoot, options?: MountOptions) => {
+        App.validateTarget(target);
+        if (this.dev) {
+          validateProps(Root, props, { __owl__: { app: this } });
+        }
+        const prom = this.mountNode(node, target, options);
+        return prom;
+      },
+      destroy: () => {
+        this.subRoots.delete(node);
+        node.destroy();
+        this.scheduler.processTasks();
+      },
+    };
   }
 
   makeNode(Component: ComponentConstructor, props: any): ComponentNode {
@@ -134,6 +176,9 @@ export class App<
 
   destroy() {
     if (this.root) {
+      for (let subroot of this.subRoots) {
+        subroot.destroy();
+      }
       this.root.destroy();
       this.scheduler.processTasks();
     }