Handle export into non-file scheme

package.json

@@ -253,6 +253,7 @@
     "@types/lodash.debounce": "^4.0.6",
     "@types/markdown-it": "^12.0.3",
     "@types/node-fetch": "^2.5.12",
+    "@types/tmp": "^0.2.1",
     "@types/vscode": "~1.56.0",
     "@typescript-eslint/eslint-plugin": "^4.28.4",
     "@typescript-eslint/parser": "^4.28.4",
@@ -297,6 +298,7 @@
   },
   "dependencies": {
     "@marp-team/marp-cli": "^1.2.0",
-    "@marp-team/marp-core": "^2.1.0"
+    "@marp-team/marp-core": "^2.1.0",
+    "tmp": "^0.2.1"
   }
 }

src/__mocks__/vscode.ts

@@ -147,6 +147,7 @@ export const FileSystem = {
     type: FileType.File,
   })),
   readFile: jest.fn().mockResolvedValue(new TextEncoder().encode('readFile')),
+  isWritableFileSystem: jest.fn((scheme: string) => scheme === 'file'),
 }
 
 export enum FileType {

src/commands/export.ts

@@ -1,4 +1,7 @@
+import fs from 'fs'
 import path from 'path'
+import { promisify } from 'util'
+import { tmpName, TmpNameOptions } from 'tmp'
 import {
   commands,
   env,
@@ -19,6 +22,9 @@ import {
   WorkspaceProxyServer,
 } from '../workspace-proxy-server'
 
+const tmpNamePromise = promisify<TmpNameOptions, string>(tmpName)
+const unlink = promisify(fs.unlink)
+
 export enum Types {
   html = 'html',
   pdf = 'pdf',
@@ -92,16 +98,50 @@ export const doExport = async (uri: Uri, document: TextDocument) => {
     const input = await createWorkFile(document)
 
     try {
+      let outputPath = uri.fsPath
+      const outputToLocalFS = uri.scheme === 'file'
+
+      // NOTE: It may return `undefined` if VS Code does not know about the
+      // filesystem. In this case, Marp may be able to write to the output path.
+      if (workspace.fs.isWritableFileSystem(uri.scheme) === false) {
+        throw new Error(`Could not write to ${uri.scheme} file system.`)
+      }
+
+      if (!outputToLocalFS) {
+        outputPath = await tmpNamePromise({
+          postfix: path.extname(uri.path),
+        })
+      }
+
       // Run Marp CLI
       const conf = await createConfigFile(document, {
         allowLocalFiles: !proxyServer,
       })
 
       try {
-        await marpCli(['-c', conf.path, input.path, '-o', uri.fsPath], {
+        await marpCli(['-c', conf.path, input.path, '-o', outputPath], {
           baseUrl,
         })
-        env.openExternal(uri)
+
+        if (outputToLocalFS) {
+          env.openExternal(uri)
+        } else {
+          try {
+            await workspace.fs.copy(Uri.file(outputPath), uri, {
+              overwrite: true,
+            })
+          } finally {
+            try {
+              await unlink(outputPath)
+            } catch (e) {
+              console.warn(e)
+            }
+          }
+
+          window.showInformationMessage(
+            `Marp slide deck was successfully exported to ${uri.toString()}.`
+          )
+        }
       } finally {
         conf.cleanup()
       }
@@ -143,7 +183,7 @@ export const saveDialog = async (document: TextDocument) => {
     await window.withProgress(
       {
         location: ProgressLocation.Notification,
-        title: `Exporting Marp slide deck to ${saveURI.path}...`,
+        title: `Exporting Marp slide deck to ${saveURI.toString()}...`,
       },
       () => doExport(saveURI, document)
     )