使用ReactiveExtensions（Rx）解决异步编程难题

Rx+HOL+.NET.pdf

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"Rx+HOL+.NET.pdf 是一个关于使用.NET Reactive Extensions（Rx）的手动实践实验室，旨在帮助读者理解和掌握如何利用Rx编写异步应用程序。实验室通过一系列逐步增加的示例，介绍Rx的组合能力，这些能力基于可观察集合的概念来处理异步编程的复杂性。" 在深入探讨Rx之前，先来了解一下必要的先决条件： 1. 对.NET框架和C#编程语言有活跃的认识。这是完成实验的基础，因为Rx是用C#实现的，并且通常与.NET开发环境一起使用。 2. 理解异步编程的概念以及其相关的复杂性。异步编程在现代软件开发中至关重要，尤其是在处理I/O密集型任务或需要优化性能的应用中。 3. 需要安装Visual Studio 2010及.NET Framework 4.虽然可以使用更早的版本，但实验内容是针对VS2010设计的。 4. 从MSDN DevLabs (http://msdn.microsoft.com/en-us/devlabs) 安装Rx for .NET 4。这是进行实验所必需的库。 Rx，全称为Reactive Extensions，是一个用于构建异步和事件驱动程序的库，它利用了可观察集合的概念。以下三个核心属性定义了Rx的核心价值： 1. **可观察集合**：在传统的编程模型中，数据通常以集合的形式存在，而Rx扩展了这一概念，使得事件流也可以像集合一样被处理、查询和操作。这意味着你可以使用LINQ（Language Integrated Query）对事件进行操作，就像对静态数据进行操作一样。 2. **异步编程**：Rx提供了一种结构化的方式来处理异步操作，通过将异步行为转化为可观察序列，使得代码更易于理解、测试和维护。这解决了传统异步回调和事件处理的“回调地狱”问题。 3. **组合能力**：Rx允许开发者通过组合多个可观察序列来创建复杂的异步逻辑。例如，你可以合并来自多个源的事件，过滤特定事件，或者在事件之间延迟操作。这种组合能力使得代码更加模块化和可重用。在本实验中，你将逐步学习如何使用Rx创建和操作可观察序列，包括创建基本的事件流、订阅事件、使用LINQ操作符处理事件、错误处理和取消订阅。此外，你还将了解如何使用Rx来管理复杂的异步流程，比如合并多个异步操作，处理并发和竞态条件。通过这些实践，你将更好地理解Rx如何简化异步编程，提高代码的可读性和可维护性，从而在实际项目中更好地应用和利用Rx的威力。

5. One final essential factory method or primitive constructor is called Return. Its role is to represent a single-

element sequence, just like a single-cell array would be in the world of IEnumerable sequences. The behavior

observed by subscribed observers is two messages: OnNext with the value and OnCompleted signaling the end

of the sequence has been received:

IObservable<int> source = Observable.Return(42);

Running this code produces the following output:

OnNext: 42

OnCompleted

Background: Return plays an essential role in the theory behind LINQ, known as monads. Together with an

operator called SelectMany (which we’ll learn about more later on), they form the primitive functions needed to

leverage the power of monadic computation. More information can be found by searching the web using monad

and functional as the keywords.

6. At this point, we’ve seen the most trivial observable sequence constructors that are intimately related to an

observer’s triplet of methods. While those are of interest in certain cases, more meaty sequences are worth to

explore as well. The Range operator is just one operator that generates such a sequence. Symmetric to the same

operator on Enumerable, Range does return a sequence with 32-bit integer values given a starting value and a

length:

IObservable<int> source = Observable.Range(5, 3);

Running this code produces the following output:

OnNext: 5

OnNext: 6

OnNext: 7

OnCompleted

Note: As with all the sequences mentioned in this exercise, Range is a cold observable. To recap, this simply

means that it starts producing its results to an observer upon subscription. Another property of cold observable

sequence is that every subscription will cause such reevaluation. Thus, if two calls to Subscribe are made, both

of the observers passed to those calls will receive the messages from the observable. It’s not because the data

observation has run to completion for one observer that other observers won’t run anymore. Whether or not

the produced data is the same for every observer depends on the characteristics of the sequence that’s being

generated. For deterministic and “purist functional” operators like Return and Range, the messages delivered to

every observer will be the same. However, one could imagine other kinds of observable sequences that depend

on side-effects and thus deliver different results for every observer that subscribes to them.

7. To generalize the notion of sequence creation in terms of a generator function, the Generate constructor

function was added to Rx. It closely resembles the structure of a for-loop as one would use to generate an

enumerable sequence using C# iterators (cf. the “yield return” and “yield break” statements). To do so, it takes a

number of delegate-typed parameters that expect function to check for termination, to iterate one step and to

emit a result that becomes part of the sequence and is sent to the observer:

IObservable<int> source = Observable.Generate(0, i => i < 5, i => i + 1, i => i * i);

Running this code produces the following output:

OnNext: 0

OnNext: 1

OnNext: 4

OnNext: 9

OnNext: 16

OnCompleted

Note: A sister function called GenerateWithTime exists that waits a computed amount of time before moving

on to the next iteration. We’ll look at this one in just a moment.

8. One operator that may seem interesting from a curiosity point of view only is called Never. It creates an

observable sequence that will never signal any notification to a subscribed observer:

IObservable<int> source = Observable.Never<int>();

Running this code shouldn’t produce any output till the end of mankind.

Background: One reason this operator has a role is to reason about composition with other sequences, e.g.

what would it mean to concatenate a finite and an infinite sequence? Should the result also exhibit an infinite

characteristic? Those answers are essential to ensuring all of the Rx semantics make sense and are consistent

throughout various operators. Besides the operator’s theoretical use, there are real use cases for it as well. For

example, you may use it to ensure a sequence never terminates by avoiding an OnCompleted handler getting

triggered. Another use is to test whether an application does not hang in the presence of non-termination.

9. Where the Subscribe method has an asynchronous characteristic, other blocking operators exist to observe a

sequence in a synchronous manner. One such operator is called Run.

First of all, let’s formalize “asynchronous”. It’s near to impossible to say something is asynchronous in itself

without mentioning two parties. In particular, the thread calling the Subscribe method on an observable

sequence is not blocked till the sequence runs to completion, which may happen on another thread. That is,

Subscribe is asynchronous in that the caller is not blocked till the observation of the sequence completes.

For demos and testing it’s often useful to perform a “blocking subscribe” such that the caller is blocked till the

observable sequence triggers OnError or OnCompleted to the observer. That’s what Run is all about:

IObservable<int> source = Observable.Range(0, 10);

source.Run(

x => Console.WriteLine("OnNext: {0}", x),

ex => Console.WriteLine("OnError: {0}", ex.Message),

() => Console.WriteLine("OnCompleted")

);

In here, we won’t get to the next statement till the sequence completes gracefully or exceptionally.

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