Merge commit 'be3e8236086165e5e45a5a10783823874b3f3ebd' as 'lib/vscode'

lib/vscode/extensions/markdown-language-features/src/util/topmostLineMonitor.ts

@@ -0,0 +1,70 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as vscode from 'vscode';
+import { Disposable } from '../util/dispose';
+import { isMarkdownFile } from './file';
+
+export class TopmostLineMonitor extends Disposable {
+
+	private readonly pendingUpdates = new Map<string, number>();
+	private readonly throttle = 50;
+
+	constructor() {
+		super();
+		this._register(vscode.window.onDidChangeTextEditorVisibleRanges(event => {
+			if (isMarkdownFile(event.textEditor.document)) {
+				const line = getVisibleLine(event.textEditor);
+				if (typeof line === 'number') {
+					this.updateLine(event.textEditor.document.uri, line);
+				}
+			}
+		}));
+	}
+
+	private readonly _onChanged = this._register(new vscode.EventEmitter<{ readonly resource: vscode.Uri, readonly line: number }>());
+	public readonly onDidChanged = this._onChanged.event;
+
+	private updateLine(
+		resource: vscode.Uri,
+		line: number
+	) {
+		const key = resource.toString();
+		if (!this.pendingUpdates.has(key)) {
+			// schedule update
+			setTimeout(() => {
+				if (this.pendingUpdates.has(key)) {
+					this._onChanged.fire({
+						resource,
+						line: this.pendingUpdates.get(key) as number
+					});
+					this.pendingUpdates.delete(key);
+				}
+			}, this.throttle);
+		}
+
+		this.pendingUpdates.set(key, line);
+	}
+}
+
+/**
+ * Get the top-most visible range of `editor`.
+ *
+ * Returns a fractional line number based the visible character within the line.
+ * Floor to get real line number
+ */
+export function getVisibleLine(
+	editor: vscode.TextEditor
+): number | undefined {
+	if (!editor.visibleRanges.length) {
+		return undefined;
+	}
+
+	const firstVisiblePosition = editor.visibleRanges[0].start;
+	const lineNumber = firstVisiblePosition.line;
+	const line = editor.document.lineAt(lineNumber);
+	const progress = firstVisiblePosition.character / (line.text.length + 2);
+	return lineNumber + progress;
+}