1. Async Made Simple with C++ PPL
Microsoft Consumer Channels and Central Marketing Group
4/26/2017
Async Made Simple with C++ PPL
Rahul V. Patil
Lead Program Manager
Microsoft Corporation
© 2012 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

2. Topics Mental model of async
C++ constructs for async, including best practices

3. mental model of async

4. UI Thread: You Block = You Break
Update UI control
Fetch an image from network
Fetch an image from network
Synchronous?
Process Image

5. Fetch an image from network
Async
Advantages
App stays responsive to many requests
Runs concurrently (takes advantage of more CPUs)
Operations can be canceled UI
Update UI control
then
Process Image
then
Fetch an image from network
Threadpool

6. async concepts in VC++

7. .then returns another task more than one continuation is ok
4/26/2017
PPL Task
task<int> t([]() {
return fib(40);
});
task<void> t2 = t.then ([](int n) …
task<void> t3 = t.then ([](int n)
int result = t.get() //blocking & optional
Must Match
.then returns another task
more than one continuation is ok
© 2012 Microsoft Corporation. All rights reserved. Microsoft, Windows, and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

8. .then ladder Continuation Graphs task<int> t([]() { return 1;
}).then([](int n)
return n+1; })

9. Join and Choice task<string> tasks[] = {t1, t2, t3};
auto taskResult = when_all (begin(tasks), end(tasks))
.then([](std::vector<string> results) {
(t1 && t2 && t3).then
auto taskResult = when_any (begin(tasks), end(tasks))
.then([](string result) {
(t1 || t2 || t3).then

10. Error Handling task<int> t([]() { try int x = a(); } catch(…)
{…}
return x;
}).then([](int n)
use(n); })
throws exception?
should continue?

11. Error Handling – Task Based Continuation
task<int> t([]() {
int x = a(); ……
return x;
}).then([](task<int> tn){
try {
int n = tn.get();
use(n); }
catch(…)
{…}
});
throws exception?
throws exception? X X
catch

12. Cancellation cancellation_token_source cts;
auto token = cts.get_token();
task<void> t([]() { { …
if (is_task_cancellation_requested()) …
}, token);
cts.cancel()

13. Windows Runtime Async Interfaces
IAsyncAction
Async operation that has no return value
Set .Completed property to schedule continuation
IAsyncOperation<TResult>
Async operation that returns TResult
Call .GetResults to get the value
IAsyncActionWithProgress<TProgress> IAsyncOperationWithProgress<TResult,tProgress>
Set .Progress property to schedule a function/lambda to be called on every progress update

14. Example: Using Built-In Async File I/O
template<typename Callback>
void ReadString(String^ fileName, Callback func) {
StorageFolder^ item = KnownFolders::PicturesLibrary;
IAsyncOperation<StorageFile^>^ getFileOp = item->GetFileAsync(fileName);
getFileOp->Completed = ref new AsyncOperationCompletedHandler<StorageFile^>
([=](IAsyncOperation<StorageFile^>^ operation, AsyncStatus status) ……
}, Platform::CallbackContext::Same); }
task<String^> ReadStringTask(String^ fileName) {
StorageFolder^ item = KnownFolders::PicturesLibrary;
task<StorageFile^> getFileTask = create_task(item->GetFileAsync(fileName));
return getFileTask.then([](StorageFile^ storageFile) { ….
}); }

15. Fetch an image from network
Execution Context
.then([]() {
// scheduled back to the same COM apartment (UI)
}, task_continuation_context::use_current()); // default UI
Update UI control
then
Process Image
.then([]() {
// scheduled on a COM MTA apartment (threadpool)
}, task_continuation_context::use_arbitrary());
then
Fetch an image from network
create_task(GetFileAsync(filename));
// scheduled on a threadpool
Threadpool

16. Wrap Your Synchronous: create_async
IAsyncOperation<BitmapImage^>^ ProcessImageAsync(BitmapImage^ image) {
return create_async([=]() {
return SynchronousProcessImage(image);
}); }
// register as a Windows Runtime component
// use from C++
// use from javascript
processImageAsync.then(
function() …
// or use from C#

17. Genevieve Fernandes (Geni)
Demo
Genevieve Fernandes (Geni)

18. advanced

19. Nesting Async GetFileAsync .then GetFileAsync .then return OpenAsync
LoadAsync
.then
return LoadAsync
Error?
.then
return <t>
.then(..task<t>) //catch

20. Nesting Async VS Unwrapping

21. Recap Async keeps your application responsive
PPL makes async simple and powerful
How to do Async
Tasks and continuation graphs
Errors and cancellation
C++ PPL tasks wrapper over IAsync* objects
Understanding execution context
Create your own Windows Runtime IAsync* objects
Advanced: async and task unwrapping

22. Thank You! Contact me: rahul.patil @ the company I work for
Follow our blog posts:
Download our sample pack:
Ask a question on our msdn forums:

23. Backup – more on task and async unwrapping

24. Nesting Async GetFileAsync .then return OpenAsync .then
task<String^> ReadStringTask(String^ fileName) {
StorageFolder^ item = KnownFolders::PicturesLibrary;
auto reader = std::make_shared<IDataReader^>(nullptr);
task<StorageFile^> getFileTask(item->GetFileAsync(fileName));
return getFileTask.then([](StorageFile^ storageFile) {
return storageFile->OpenAsync(FileAccessMode::Read);
}).then([reader](IRandomAccessStream^ istream) {
*reader = ref new DataReader(istream);
return (*reader)->LoadAsync(istream->Size);
}).then([reader](UINT bytesRead) {
return (*reader)->ReadString(bytesRead);
}); }
return OpenAsync
.then
return LoadAsync
.then
return <t>
.then(..task<t>) //catch

25. Sharing Objects (When converting to .then ladder)
“reader” would have to be explicitly shared to make it available to the “inner most” lambda
task<String^> ReadStringTask(String^ fileName) {
StorageFolder^ item = KnownFolders::PicturesLibrary;
auto reader = std::make_shared<IDataReader^>(nullptr);
task<StorageFile^> getFileTask(item->GetFileAsync(fileName));
return getFileTask.then([](StorageFile^ storageFile) {
return storageFile->OpenAsync(FileAccessMode::Read);
}).then([reader](IRandomAccessStream^ istream) {
*reader = ref new DataReader(istream);
return (*reader)->LoadAsync(istream->Size);
}).then([reader](UINT bytesRead) {
return (*reader)->ReadString(bytesRead);
}); }

26. Nesting PPL With Task Unwrapping
Preserves Nesting AND keeps .then ladder semantics
task<String^> ReadStringTask(String^ fileName) {
StorageFolder^ item = KnownFolders::PicturesLibrary;
auto reader = std::make_shared<IDataReader^>(nullptr);
task<StorageFile^> getFileTask(item->GetFileAsync(fileName));
return getFileTask.then([](StorageFile^ storageFile) {
return create_task(storageFile->OpenAsync(FileAccessMode::Read))
.then([](IRandomAccessStream^ istream) {
auto reader = ref new DataReader(istream);
return create_task(reader->LoadAsync(istream->Size))
.then([reader](UINT bytesRead) {
return reader->ReadString(bytesRead);
}); }