Add API proposal for reading files in data transfer (#148596)

* Add experimental support for reading files in data transfer

Adds a new `DataTransfer.asFile` method which lets you get file objects from a `DataTransfer`. This is currently only hooked up for drop into editors.

A few follow ups:

- Right now the file data is also read eagerly when it is transfered to the extension host. Before shipping this we would make this happen lazily instead
- The drop into editor api does not provide a nice way to do anything with the dropped files.

    We should at least support returning a `WorkspaceEdit`. However `WorkspaceEdit` only supports text files, so we would also need to add an API that lets it deal with binary files

* Make `asFile` return a value instead of a promise

`asFile().data()` already returns a promise so `asFile` doesn't also need to be async

* Trying resolving data files transfer lazily

* Cleaning up code for lazy drop

* Remove testing code

* Remove unneeded buffer serialize

* 💄

extensions/markdown-language-features/src/languageFeatures/dropIntoEditor.ts

@@ -25,14 +25,19 @@ const imageFileExtensions = new Set<string>([
 
 export function registerDropIntoEditor(selector: vscode.DocumentSelector) {
 	return vscode.languages.registerDocumentOnDropProvider(selector, new class implements vscode.DocumentOnDropProvider {
-		async provideDocumentOnDropEdits(document: vscode.TextDocument, position: vscode.Position, dataTransfer: vscode.DataTransfer, _token: vscode.CancellationToken): Promise<vscode.SnippetTextEdit | undefined> {
+		async provideDocumentOnDropEdits(document: vscode.TextDocument, position: vscode.Position, dataTransfer: vscode.DataTransfer, token: vscode.CancellationToken): Promise<vscode.SnippetTextEdit | undefined> {
 			const enabled = vscode.workspace.getConfiguration('markdown', document).get('editor.drop.enabled', true);
 			if (!enabled) {
 				return;
 			}
 
+			const replacementRange = new vscode.Range(position, position);
+			return this.tryInsertUriList(document, replacementRange, dataTransfer, token);
+		}
+
+		private async tryInsertUriList(document: vscode.TextDocument, replacementRange: vscode.Range, dataTransfer: vscode.DataTransfer, token: vscode.CancellationToken): Promise<vscode.SnippetTextEdit | undefined> {
 			const urlList = await dataTransfer.get('text/uri-list')?.asString();
-			if (!urlList) {
+			if (!urlList || token.isCancellationRequested) {
 				return undefined;
 			}
 
@@ -65,7 +70,7 @@ export function registerDropIntoEditor(selector: vscode.DocumentSelector) {
 				}
 			});
 
-			return new vscode.SnippetTextEdit(new vscode.Range(position, position), snippet);
+			return new vscode.SnippetTextEdit(replacementRange, snippet);
 		}
 	});
 }

extensions/markdown-language-features/tsconfig.json

@@ -7,5 +7,6 @@
 		"src/**/*",
 		"../../src/vscode-dts/vscode.d.ts",
 		"../../src/vscode-dts/vscode.proposed.textEditorDrop.d.ts",
+		"../../src/vscode-dts/vscode.proposed.dataTransferFiles.d.ts"
 	]
 }