Add API documentation to '@theia/preview'

* Document preview-handler.ts
* Add custom preview guide to README.md

Signed-off-by: Stefan Dirix <sdirix@eclipsesource.com>
Contributed on behalf of STMicroelectronics

packages/preview/README.md

@@ -15,6 +15,20 @@
 The `@theia/preview` extension adds the ability to display rendered previews of supported resources.\
 The extension comes with built-in support for rendering `markdown` files.
 
+## Contribute Custom Previews
+
+To provide custom previews implement and bind the `PreviewHandler` interface, e.g.
+
+```typescript
+@injectable
+class MyPreviewHandler implements PreviewHandler {
+  ...
+}
+// in container
+bind(MyPreviewHandler).toSelf().inSingletonScope();
+bind(PreviewHandler).toService(MyPreviewHandler);
+```
+
 ## Additional Information
 
 - [API documentation for `@theia/preview`](https://eclipse-theia.github.io/theia/docs/next/modules/preview.html)

packages/preview/src/browser/preview-handler.ts

@@ -20,8 +20,17 @@ import { ContributionProvider, MaybePromise, Prioritizeable } from '@theia/core'
 
 export const PreviewHandler = Symbol('PreviewHandler');
 
+/**
+ * The parameters given to the preview handler to render the preview content.
+ */
 export interface RenderContentParams {
+    /**
+     * Textual content of the resource.
+     */
     content: string;
+    /**
+     * URI identifying the source resource.
+     */
     originUri: URI;
 }
 
@@ -31,15 +40,72 @@ export namespace RenderContentParams {
     }
 }
 
+/**
+ * A PreviewHandler manages the integration of one or more previews.
+ *
+ * It indicates whether a preview shall be rendered for a given resource URI and, if yes, renders the content.
+ * Additionally it optionally provides methods with which the scroll state of the preview and corresponding
+ * editor can be managed.
+ *
+ * See {@link MarkdownPreviewHandler} for an example implementation.
+ */
 export interface PreviewHandler {
+    /**
+     * One or more classes which specify the preview widget icon.
+     */
     readonly iconClass?: string;
+    /**
+     * Indicates whether and with which priority (larger is better) this preview handler is responsible for the resource identified by the given URI.
+     * If multiple handlers return the same priority it's undefined which one will be used.
+     *
+     * @param uri the URI identifying a resource.
+     *
+     * @returns a number larger than 0 if the handler is applicable, 0 or a negative number otherwise.
+     */
     canHandle(uri: URI): number;
+    /**
+     * Render the preview content by returning appropriate HTML.
+     *
+     * @param params information for the handler to render its content.
+     *
+     * @returns the HTMLElement which will be attached to the preview widget.
+     */
     renderContent(params: RenderContentParams): MaybePromise<HTMLElement | undefined>;
+    /**
+     * Search and return the HTMLElement which corresponds to the given fragment.
+     * This is used to initially reveal elements identified via the URI fragment.
+     *
+     * @param content the preview widget element containing the content previously rendered by {@link PreviewHandler.renderContent}.
+     * @param fragment the URI fragment for which the corresponding element shall be returned
+     *
+     * @returns the HTMLElement which is part of content and corresponds to the given fragment, undefined otherwise.
+     */
     findElementForFragment?(content: HTMLElement, fragment: string): HTMLElement | undefined;
+    /**
+     * Search and return the HTMLElement which corresponds to the given line number.
+     * This is used to scroll the preview when the source editor scrolls.
+     *
+     * @param content the preview widget element containing the previously rendered by {@link PreviewHandler.renderContent}.
+     * @param sourceLine the line number for which the corresponding element shall be returned.
+     *
+     * @returns the HTMLElement which is part of content and corresponds to the given line number, undefined otherwise.
+     */
     findElementForSourceLine?(content: HTMLElement, sourceLine: number): HTMLElement | undefined;
+    /**
+     * Returns the line number which corresponds to the preview element at the given offset.
+     * This is used to scroll the source editor when the preview scrolls.
+     *
+     * @param content the preview widget element containing the previously rendered by {@link PreviewHandler.renderContent}.
+     * @param offset the total amount by which the preview widget is scrolled.
+     *
+     * @returns the source line number which corresponds to the preview element at the given offset, undefined otherwise.
+     */
     getSourceLineForOffset?(content: HTMLElement, offset: number): number | undefined;
 }
 
+/**
+ * Provider managing the available PreviewHandlers.
+ */
 @injectable()
 export class PreviewHandlerProvider {
 
@@ -48,15 +114,28 @@ export class PreviewHandlerProvider {
         protected readonly previewHandlerContributions: ContributionProvider<PreviewHandler>
     ) { }
 
+    /**
+     * Find PreviewHandlers for the given resource identifier.
+     *
+     * @param uri the URI identifying a resource.
+     *
+     * @returns the list of all supported `PreviewHandlers` sorted by their priority.
+     */
     findContribution(uri: URI): PreviewHandler[] {
         const prioritized = Prioritizeable.prioritizeAllSync(this.previewHandlerContributions.getContributions(), contrib =>
             contrib.canHandle(uri)
         );
         return prioritized.map(c => c.value);
     }
 
+    /**
+     * Indicates whether any PreviewHandler can process the resource identified by the given URI.
+     *
+     * @param uri the URI identifying a resource.
+     *
+     * @returns `true` when a PreviewHandler can process the resource, `false` otherwise.
+     */
     canHandle(uri: URI): boolean {
         return this.findContribution(uri).length > 0;
     }
-
 }