Drop into text editor api proposal (#143334)

* Add drop into editor api proposal

For #142990

* Use AsyncEmitter

extensions/markdown-language-features/src/extension.ts

@@ -3,7 +3,9 @@
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
 
+import * as path from 'path';
 import * as vscode from 'vscode';
+import * as URI from 'vscode-uri';
 import { CommandManager } from './commandManager';
 import * as commands from './commands/index';
 import LinkProvider from './features/documentLinkProvider';
@@ -45,6 +47,43 @@ export function activate(context: vscode.ExtensionContext) {
 		logger.updateConfiguration();
 		previewManager.updateConfiguration();
 	}));
+
+	context.subscriptions.push(vscode.workspace.onWillDropOnTextEditor(e => {
+		e.waitUntil((async () => {
+			const resourceUrls = await e.dataTransfer.get('resourceurls')?.asString();
+			if (!resourceUrls) {
+				return;
+			}
+
+			const uris: vscode.Uri[] = [];
+			for (const resource of JSON.parse(resourceUrls)) {
+				try {
+					uris.push(vscode.Uri.parse(resource));
+				} catch {
+					// noop
+				}
+			}
+
+			if (!uris.length) {
+				return;
+			}
+
+			const snippet = new vscode.SnippetString();
+			uris.forEach((uri, i) => {
+				const rel = path.relative(URI.Utils.dirname(e.editor.document.uri).fsPath, uri.fsPath);
+
+				snippet.appendText('[');
+				snippet.appendTabstop();
+				snippet.appendText(`](${rel})`);
+
+				if (i <= uris.length - 1 && uris.length > 1) {
+					snippet.appendText(' ');
+				}
+			});
+
+			return e.editor.insertSnippet(snippet, e.position);
+		})());
+	}));
 }
 
 function registerMarkdownLanguageFeatures(