feat: add webContents.getPrintersAsync() (#31023)

This deprecates the synchronous and blocking `webContents.getPrinters()`
function and introduces `webContents.getPrintersAsync()`, which is
asynchronous and non-blocking.

Signed-off-by: Darshan Sen <darshan.sen@postman.com>

docs/api/web-contents.md

@@ -1369,12 +1369,20 @@ Decrease the capturer count by one. The page will be set to hidden or occluded s
 browser window is hidden or occluded and the capturer count reaches zero. If you want to
 decrease the hidden capturer count instead you should set `stayHidden` to true.
 
-#### `contents.getPrinters()`
+#### `contents.getPrinters()` _Deprecated_
 
 Get the system printer list.
 
 Returns [`PrinterInfo[]`](structures/printer-info.md)
 
+**Deprecated:** Should use the new [`contents.getPrintersAsync`](web-contents.md#contentsgetprintersasync) API.
+
+#### `contents.getPrintersAsync()`
+
+Get the system printer list.
+
+Returns `Promise<PrinterInfo[]>` - Resolves with a [`PrinterInfo[]`](structures/printer-info.md)
+
 #### `contents.print([options], [callback])`
 
 * `options` Object (optional)

lib/browser/api/web-contents.ts

@@ -391,6 +391,17 @@ WebContents.prototype.getPrinters = function () {
   }
 };
 
+WebContents.prototype.getPrintersAsync = async function () {
+  // TODO(nornagon): this API has nothing to do with WebContents and should be
+  // moved.
+  if (printing.getPrinterListAsync) {
+    return printing.getPrinterListAsync();
+  } else {
+    console.error('Error: Printing feature is disabled.');
+    return [];
+  }
+};
+
 WebContents.prototype.loadFile = function (filePath, options = {}) {
   if (typeof filePath !== 'string') {
     throw new Error('Must pass filePath as a string');

shell/browser/api/electron_api_printing.cc

@@ -10,7 +10,11 @@
 #include "shell/common/node_includes.h"
 
 #if BUILDFLAG(ENABLE_PRINTING)
+#include "base/task/task_traits.h"
+#include "base/task/thread_pool.h"
 #include "printing/backend/print_backend.h"
+#include "shell/common/gin_helper/promise.h"
+#include "shell/common/process_util.h"
 #endif
 
 namespace gin {
@@ -39,13 +43,16 @@ namespace electron {
 namespace api {
 
 #if BUILDFLAG(ENABLE_PRINTING)
-printing::PrinterList GetPrinterList() {
+printing::PrinterList GetPrinterList(v8::Isolate* isolate) {
+  EmitWarning(node::Environment::GetCurrent(isolate),
+              "Deprecation Warning: getPrinters() is deprecated. "
+              "Use the asynchronous and non-blocking version, "
+              "getPrintersAsync(), instead.",
+              "electron");
   printing::PrinterList printers;
   auto print_backend = printing::PrintBackend::CreateInstance(
       g_browser_process->GetApplicationLocale());
   {
-    // TODO(deepak1556): Deprecate this api in favor of an
-    // async version and post a non blocing task call.
     base::ThreadRestrictions::ScopedAllowIO allow_io;
     printing::mojom::ResultCode code =
         print_backend->EnumeratePrinters(&printers);
@@ -54,6 +61,32 @@ printing::PrinterList GetPrinterList() {
   }
   return printers;
 }
+
+v8::Local<v8::Promise> GetPrinterListAsync(v8::Isolate* isolate) {
+  gin_helper::Promise<printing::PrinterList> promise(isolate);
+  v8::Local<v8::Promise> handle = promise.GetHandle();
+
+  base::ThreadPool::PostTaskAndReplyWithResult(
+      FROM_HERE, {base::TaskPriority::USER_VISIBLE, base::MayBlock()},
+      base::BindOnce([]() {
+        printing::PrinterList printers;
+        auto print_backend = printing::PrintBackend::CreateInstance(
+            g_browser_process->GetApplicationLocale());
+        printing::mojom::ResultCode code =
+            print_backend->EnumeratePrinters(&printers);
+        if (code != printing::mojom::ResultCode::kSuccess)
+          LOG(INFO) << "Failed to enumerate printers";
+        return printers;
+      }),
+      base::BindOnce(
+          [](gin_helper::Promise<printing::PrinterList> promise,
+             const printing::PrinterList& printers) {
+            promise.Resolve(printers);
+          },
+          std::move(promise)));
+
+  return handle;
+}
 #endif
 
 }  // namespace api
@@ -64,6 +97,7 @@ namespace {
 
 #if BUILDFLAG(ENABLE_PRINTING)
 using electron::api::GetPrinterList;
+using electron::api::GetPrinterListAsync;
 #endif
 
 void Initialize(v8::Local<v8::Object> exports,
@@ -74,6 +108,8 @@ void Initialize(v8::Local<v8::Object> exports,
   gin_helper::Dictionary dict(isolate, exports);
 #if BUILDFLAG(ENABLE_PRINTING)
   dict.SetMethod("getPrinterList", base::BindRepeating(&GetPrinterList));
+  dict.SetMethod("getPrinterListAsync",
+                 base::BindRepeating(&GetPrinterListAsync));
 #endif
 }
 

spec-main/api-web-contents-spec.ts

@@ -1728,6 +1728,16 @@ describe('webContents module', () => {
     });
   });
 
+  ifdescribe(features.isPrintingEnabled())('getPrintersAsync()', () => {
+    afterEach(closeAllWindows);
+    it('can get printer list', async () => {
+      const w = new BrowserWindow({ show: false, webPreferences: { sandbox: true } });
+      await w.loadURL('about:blank');
+      const printers = await w.webContents.getPrintersAsync();
+      expect(printers).to.be.an('array');
+    });
+  });
+
   ifdescribe(features.isPrintingEnabled())('printToPDF()', () => {
     let w: BrowserWindow;
 

typings/internal-electron.d.ts

@@ -71,6 +71,7 @@ declare namespace Electron {
     _printToPDF(options: any): Promise<Buffer>;
     _print(options: any, callback?: (success: boolean, failureReason: string) => void): void;
     _getPrinters(): Electron.PrinterInfo[];
+    _getPrintersAsync(): Promise<Electron.PrinterInfo[]>;
     _init(): void;
     canGoToIndex(index: number): boolean;
     getActiveIndex(): number;