iOS上的函数响应式编程：ReactiveCocoa入门

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"《Leanpub.Functional Reactive Programming on iOS.2014》是一本由Ash Furrow编写的关于在iOS平台上使用功能性响应式编程（Functional Reactive Programming, FRP）的书籍，主要介绍如何利用ReactiveCocoa框架进行实践。本书在2014年发布，并可在Leanpub上购买。书中内容涵盖了FRP的基本概念、功能性编程中的RXCollections、ReactiveCocoa的安装和使用，以及在实际项目中的应用等。在介绍功能性响应式编程时，作者首先阐述了FRP的基本理念，指出它是一种结合函数式编程和反应式编程思想的方法，能够帮助开发者更好地处理异步事件和数据流。书中还提到了FRP的优点，如代码的可读性和可维护性，以及通过反馈机制实现灵活的系统设计。接着，书中深入讲解了功能性编程与RXCollections的关系，特别是如何使用高阶函数来操作集合。如介绍了如何安装RXCollections库，以及如何运用map、filter和fold等核心函数进行数据转换和处理，同时关注性能优化。在ReactiveCocoa部分，作者逐步引导读者了解这个强大的框架。从安装开始，逐步揭示了流（Streams）和序列（Sequences）的概念，包括信号（Signals）的使用和订阅（Subscriptions）的管理。此外，书中还讨论了如何从信号中派生状态、命令（Commands）的运用、RACSubject的特性，以及热信号（Hot Signals）和冷信号（Cold Signals）的区别，以及多播（Multicasting）的概念。在实践中应用ReactiveCocoa的部分，作者给出了基本的功能性反应式编程技巧，帮助开发者将理论知识转化为实际开发中的解决方案。这部分内容可能包括信号组合、错误处理、UI绑定等多个方面，旨在让读者能更好地在iOS应用开发中运用ReactiveCocoa。《Leanpub.Functional Reactive Programming on iOS.2014》是一本面向iOS开发者的实用指南，通过ReactiveCocoa展示了如何在iOS环境中优雅地实施功能性响应式编程，提升软件开发的效率和质量。"

Functional Programming with RXCollections 9

Conclusion

We’ve seen over this past chapter how we can operate on lists without needing mutable variables.

While RXCollections may be using mutable variables under the hood, we don’t worry about that

because it’s abstracted away for us. It’s unimportant to the tasks at hand: mapping, filtering, and

folding.

(That’s not to say you shouldn’t become familiar with the source code of RXCollections, just that

you don’t need to in order to accomplish your tasks.)

We also saw, in the last example, how we can chain operations together to get a more complex result.

We’re going to be talking more about chaining operations together in the next chapter – in fact, it’s

one of the main principles of using ReactiveCocoa.

We’re going to be discussing more about map, filter, and fold in the next chapter. Not only will we

be using these higher-order functions on lists, but we’re also going to see them applied to streams,

which will be introduced next. We’ll also take a look at some other higher-level functions, now that

you’re familiar with the concepts introduced in this chapter.

Introduction to ReactiveCocoa 12

To recap, we’ve got an array, which is a list of values. Then we’re turning that array into a sequence,

which is a kind of stream. We’re mapping that sequence, which returns a new sequence, then turning

it back into an array.

Sequences are, by default, lazily-loaded. They’re pull-driven, providing values whenever they’re

asked for them. The array method forces evaluation of each of the members of the sequence.

Let’s take a look at filtering. In order to filter our array using ReactiveCocoa, we need to again turn

it into a sequence to filter it.

1 NSLog(@"%@", [[[array rac_sequence] filter:^BOOL(id value) {

2 return [value integerValue] % 2 == 0;

3 }] array]);

Finally, we can see how to to fold values from a sequence down to a single value.

1 NSLog(@"%@", [[[array rac_sequence] map:^id(id value) {

2 return [value stringValue];

3 }] foldLeftWithStart:@"" reduce:^id(id accumulator, id value) {

4 return [accumulator stringByAppendingString:value];

5 }]);

In this case, we’re chaining two operations on sequences together. This is going to become a key

concept when we discuss signals in the next section.

ReactiveCocoa has a concept of a left fold and a right fold. A left fold will traverse an array from its

beginning to its end, where a right fold will traverse it backward. The nomenclature is an allusion

to programming languages with list comprehension, which Objective-C does not have.

Make sure you understand what’s going on here. It’s really important before moving on.

Signals

A signal is another type of stream. In contrast to sequences, signals are push-driven. New values

are pushed through the pipe and cannot be pulled. They abstract data that will be delivered in the

future.

Signals send three different types of values. Next values are exactly what the appear to be: the next

value sent down the pipe. Error values indicate that a signal could not complete successfully. They

are relatively rare, and we’ll see in the next chapter how to use them. Completion values indicate that

a signal completed successfully. Again, we’ll see how to use them in the next chapter. It’s important

to note that once an error or completion value is sent via a signal, no other values will be sent.

Additionally, only one of either error or completion will be sent – never both.

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