create_async is an API which is used to produce a WinRT asynchronous operation (IAsyncAction / IAsyncOperation<T> with or without progress) from C++ which can be consumed in
a cross-language way from either C++ or JavaScript. .then
schedules a continuation which runs (often on the calling thread) when the task on the left side of the
.then call is completed.
When you call create_async and provide a void returning lambda such as shown below:
IAsyncAction^ asyncAction = create_async( [](){
//
// BODY OF CODE
//
});
the "BODY OF CODE" will be run on a background thread and the
create_async call will return an IAsyncAction^ representing that asynchronous body of code. The returned IAsyncAction^ can be passed between languages and consumed via constructs such as await in C#.
If a lambda provided to create_async happens to return a task, the returned task itself is the body of the asynchronous work and the lambda runs synchronously. This can allow you to compose multiple operations together.
For instance (please excuse the pseudo-code here):
IAsyncOperation<T>^ asyncOperation = create_async( []() {
return create_task(FirstAsync(...))
.then( [](X val){
return SecondAsync(val, ...);
}).then( [](Y val)
return ThirdAsync(val, ...);
});
});
When you make this call, the lambda will run synchronously -- creating a task to encapsulate FirstAsync and then returning a cotinuation off that chain. Despite the fact that the lambda here runs synchronously, it's purpose is simply to define the
asynchronous operation which is going to occur (in this case a continuation chain of FirstAsync->SecondAsync->ThirdAsync). The resulting IAsyncOperation<T>^ returned from this
create_async call represents the asynchronous return value of ThirdAsync.
I hope that helps.
what the different form create_async return task and void
