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Async/await

1、"async"的含义是"asynchronous"，"await"的含义是"等待"。

2、感觉它也是基于"promise-future communication channel"的

wikipedia Async/await

In computer programming, the async/await pattern is a syntactic feature of many programming languages that allows an asynchronous, non-blocking function to be structured in a way similar to an ordinary synchronous function.

NOTE: async/await其实是syntax sugar。

asynchronous function的结果will be binded to a promise，参见下面的"Example C#"章节

It is semantically related to the concept of a coroutine (携程) and is often implemented using similar techniques, and is primarily intended to provide opportunities for the program to execute other code while waiting for a long-running, asynchronous task to complete, usually represented by promises or similar data structures.

NOTE: 提高并发性

The feature is found in C# 5.0, Python 3.5, Hack, Dart, Kotlin 1.1, Rust 1.39,[1] Nim 0.9.4[2] and JavaScript ES2017, with some experimental work in extensions, beta versions, and particular implementations of Scala[3] and C++.

NOTE: 现代主流语言都开始考虑支持它了

Example C

The C# function below, which downloads a resource from a URI and returns the resource's length, uses this async/await pattern:

public async Task<int> FindPageSize(Uri uri) 
{
    byte[] data = await new WebClient().DownloadDataTaskAsync(uri);
    return data.Length;
}

1) First, the async keyword indicates to C# that the method is asynchronous, meaning that it may use an arbitrary number of await expressions and will bind the result to a promise.

NOTE: 这段话的意思是 the async keyword 表面: 这个method是asynchronous的，意味着 这个method 可以使用 await expression。

2) The return type, Task<T>, is C#'s analogue(类似物) to the concept of a promise, and here is indicated to have a result value of type int.

3) The first expression to execute when this method is called will be new WebClient().DownloadDataTaskAsync(uri), which is another asynchronous method returning a Task<byte[]>. Because this method is asynchronous, it will not download the entire batch of data before returning. Instead, it will begin the download process using a non-blocking mechanism (such as a background thread), and immediately return an unresolved, unrejected Task<byte[]> to this function.

NOTE: 立即返回而不阻塞

4) With the await keyword attached to the Task, this function will immediately proceed to return a Task<int> to its caller, who may then continue on with other processing as needed.

NOTE: int Length = await FindPageSize(Uri)，从上面这段话我们可以推测: await的作用对象是promise即Task，它从promise取得结果给用户。

5) Once DownloadDataTaskAsync() finishes its download, it will resolve the Task it returned with the downloaded data. This will trigger a callback and cause FindPageSize() to continue execution by assigning that value to data.

NOTE: 既然是asynchronous的，那么它必须要提供一种机制来保证当asynchronous operation完成时进行通知，通过上述描述来看，它也是通过callback来实现的。通过上述描述还可以看出，C#的实现其实保证了一定的顺序性的。

6) Finally, the method returns data.Length, a simple integer indicating the length of the array. The compiler re-interprets this as resolving the Task it returned earlier, triggering a callback in the method's caller to do something with that length value.

A function using async/await can use as many await expressions as it wants, and each will be handled in the same way (though(虽然) a promise will only be returned to the caller for the first await, while every other await will utilize(利用) internal callbacks). A function can also hold a promise object directly and do other processing first (including starting other asynchronous tasks), delaying awaiting the promise until its result is needed. Functions with promises also have promise aggregation methods that allow you to await multiple promises at once or in some special pattern (such as C#'s Task.WhenAll(), which returns a valueless Task that resolves when all of the tasks in the arguments have resolved). Many promise types also have additional features beyond what the async/await pattern normally uses, such as being able to set up more than one result callback or inspect the progress(进展) of an especially long-running task.

In C++

In C++, await (named co_await in C++) has been officially merged into C++20 draft, so it is on course to be formally accepted as a part of official C++20;[11] also MSVC and Clang compilers are already supporting at least some form of co_await (GCC still has no support for it).

#include <future>
#include <iostream>

using namespace std;

future<int> add(int a, int b)
{
    int c = a + b;
    co_return c;
}

future<void> test()
{
    int ret = co_await add(1, 2);
    cout << "return " << ret << endl;
}

int main()
{
    auto fut = test();
    fut.wait();

    return 0;
}

TODO

raywenderlich 5Async/Await

lankydan Async/await in coroutines