widget/PrintBackgroundTask.h

   1 /* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* This Source Code Form is subject to the terms of the Mozilla Public
   3  * License, v. 2.0. If a copy of the MPL was not distributed with this
   4  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   5
   6 #ifndef mozilla_PrintBackgroundTask_h_
   7 #define mozilla_PrintBackgroundTask_h_
   8
   9 #include "mozilla/dom/Promise.h"
  10 #include "mozilla/ErrorResult.h"
  11 #include "mozilla/Telemetry.h"
  12
  13 #include <tuple>
  14 #include <utility>
  15
  16 // A helper to resolve a DOM Promise with the result of a const method, executed
  17 // in another thread.
  18 //
  19 // Once in the main thread, the caller can turn the result of the method into a
  20 // JSValue by specializing ResolveOrReject.
  21 namespace mozilla {
  22
  23 template <typename T, typename Result>
  24 void ResolveOrReject(dom::Promise& aPromise, T&, Result& aResult) {
  25   aPromise.MaybeResolve(std::forward<Result>(aResult));
  26 }
  27
  28 template <typename T, typename Result, typename... Args>
  29 using PrintBackgroundTask = Result (T::*)(Args...) const;
  30
  31 template <typename T, typename Result, typename... Args>
  32 void SpawnPrintBackgroundTask(
  33     T& aReceiver, dom::Promise& aPromise, const nsCString& aTelemetryKey,
  34     PrintBackgroundTask<T, Result, Args...> aBackgroundTask, Args... aArgs) {
  35   auto promiseHolder = MakeRefPtr<nsMainThreadPtrHolder<dom::Promise>>(
  36       "nsPrinterBase::SpawnBackgroundTaskPromise", &aPromise);
  37   // We actually want to allow to access the printer data from the callback, so
  38   // disable strict checking. They should of course only access immutable
  39   // members.
  40   auto holder = MakeRefPtr<nsMainThreadPtrHolder<T>>(
  41       "nsPrinterBase::SpawnBackgroundTaskPrinter", &aReceiver,
  42       /* strict = */ false);
  43   // See
  44   // https://stackoverflow.com/questions/47496358/c-lambdas-how-to-capture-variadic-parameter-pack-from-the-upper-scope
  45   // about the tuple shenanigans. It could be improved with C++20
  46   NS_DispatchBackgroundTask(
  47       NS_NewRunnableFunction(
  48           "SpawnPrintBackgroundTask",
  49           [holder = std::move(holder), promiseHolder = std::move(promiseHolder),
  50            aTelemetryKey, startRoundTrip = TimeStamp::Now(),
  51            backgroundTask = aBackgroundTask,
  52            aArgs = std::make_tuple(std::forward<Args>(aArgs)...)] {
  53             auto start = TimeStamp::Now();
  54             Result result = std::apply(
  55                 [&](auto&&... args) {
  56                   return (holder->get()->*backgroundTask)(args...);
  57                 },
  58                 std::move(aArgs));
  59             Telemetry::AccumulateTimeDelta(
  60                 Telemetry::PRINT_BACKGROUND_TASK_TIME_MS, aTelemetryKey, start);
  61             NS_DispatchToMainThread(NS_NewRunnableFunction(
  62                 "SpawnPrintBackgroundTaskResolution",
  63                 [holder = std::move(holder),
  64                  promiseHolder = std::move(promiseHolder),
  65                  telemetryKey = std::move(aTelemetryKey), startRoundTrip,
  66                  result = std::move(result)] {
  67                   Telemetry::AccumulateTimeDelta(
  68                       Telemetry::PRINT_BACKGROUND_TASK_ROUND_TRIP_TIME_MS,
  69                       telemetryKey, startRoundTrip);
  70                   ResolveOrReject(*promiseHolder->get(), *holder->get(),
  71                                   result);
  72                 }));
  73           }),
  74       NS_DISPATCH_EVENT_MAY_BLOCK);
  75 }
  76
  77 // Gets a fresh promise into aResultPromise, that resolves whenever the print
  78 // background task finishes.
  79 template <typename T, typename Result, typename... Args>
  80 nsresult PrintBackgroundTaskPromise(
  81     T& aReceiver, JSContext* aCx, dom::Promise** aResultPromise,
  82     const nsCString& aTelemetryKey,
  83     PrintBackgroundTask<T, Result, Args...> aTask, Args... aArgs) {
  84   ErrorResult rv;
  85   RefPtr<dom::Promise> promise =
  86       dom::Promise::Create(xpc::CurrentNativeGlobal(aCx), rv);
  87   if (MOZ_UNLIKELY(rv.Failed())) {
  88     return rv.StealNSResult();
  89   }
  90
  91   SpawnPrintBackgroundTask(aReceiver, *promise, aTelemetryKey, aTask,
  92                            std::forward<Args>(aArgs)...);
  93
  94   promise.forget(aResultPromise);
  95   return NS_OK;
  96 }
  97
  98 // Resolves an async attribute via a background task, creating and storing a
  99 // promise as needed in aPromiseSlot.
 100 template <typename T, typename Result, typename... Args>
 101 nsresult AsyncPromiseAttributeGetter(
 102     T& aReceiver, RefPtr<dom::Promise>& aPromiseSlot, JSContext* aCx,
 103     dom::Promise** aResultPromise, const nsCString& aTelemetryKey,
 104     PrintBackgroundTask<T, Result, Args...> aTask, Args... aArgs) {
 105   if (RefPtr<dom::Promise> existing = aPromiseSlot) {
 106     existing.forget(aResultPromise);
 107     return NS_OK;
 108   }
 109
 110   nsresult rv =
 111       PrintBackgroundTaskPromise(aReceiver, aCx, aResultPromise, aTelemetryKey,
 112                                  aTask, std::forward<Args>(aArgs)...);
 113   NS_ENSURE_SUCCESS(rv, rv);
 114
 115   aPromiseSlot = *aResultPromise;
 116   return NS_OK;
 117 }
 118
 119 }  // namespace mozilla
 120
 121 #endif