iOS上使用Functional Reactive Programming实战教程

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《Functional Reactive Programming on iOS》是一本由Ash Furrow撰写并在2014年1月5日发布的电子书，它通过Leanpub平台提供，该平台注重作者与读者的互动过程，鼓励在书籍创作过程中收集反馈并不断迭代以优化内容。这本书专为iOS开发人员设计，介绍如何使用Functional Reactive Programming (FRP)技术，特别是通过ReactiveCocoa库来实现高效、响应式的编程。书中的内容包括： 1. **引言**：介绍了FRP的概念，强调其在iOS开发中的应用价值，以及为何选择ReactiveCocoa作为实现工具。 2. **函数式编程与RAC Collections**：讲解了如何利用函数式编程原则，如高阶函数，与RACCollections协作，提升代码的可读性和可维护性。 3. **映射（Map）、过滤（Filter）和折叠（Fold）操作**：展示了在处理数据流时，如何使用这些基本的FRP操作来处理和转换数据。 4. **性能优化**：探讨了在实际应用中如何保持FRP的性能，避免不必要的计算和资源消耗。 5. **ReactiveCocoa入门**：详细解释了ReactiveCocoa框架的安装与使用，涉及stream和sequence的概念，以及如何创建和操作signals（信号）。 6. **订阅机制（Subscriptions）**：讨论了如何管理数据流，确保订阅的正确取消以避免内存泄漏。 7. **状态衍生（Deriving State）**：阐述了如何通过FRP动态地根据输入数据变化来更新状态。 8. **命令（Commands）**：介绍了如何使用ReactiveCocoa的命令模式来处理用户交互，使得行为更加响应和可预测。 9. **RACSubject**：深入理解这个基础的ReactiveCocoa类，它是冷（cold）和热（hot）信号的核心区别所在，以及它们各自的应用场景。 10. **多播（Multicasting）**：探讨如何有效地将一个信号的值广播给多个订阅者，提高代码的复用性。 11. **实践篇：Functional Reactive Cocoa** - 提供了一些实际项目的例子，帮助读者将所学理论应用到实际开发中，掌握FRP在iOS开发中的实战技巧。通过阅读这本书，iOS开发者可以深入了解如何将FRP的思想和技术融入到iOS应用的开发中，提升应用程序的响应性和用户体验。同时，Leanpub的实时反馈机制也确保了内容的实时更新和完善，使学习过程更具互动性和效率。

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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