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< frp is awsome! >
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         \  (oo)\_______
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Reactive Stream (flyd fork)

A modular, KISS, functional reactive programming library for JavaScript.

Original Document

用来取代传统的观察者模式及事件响应回调模式
它具有高组合性，可以模块化事件驱动的相关逻辑
有别于传统的处理方式：构建出来的程序是被动的响应发生的事件流，或者一串数据流
它能对程序的状态量管理地更高效
实际上在最初使用观察者模式或者事件监听模式处理问题后，最终都会归到使用FRP的方式来从根本上解决问题

以上是比较正式的frp介绍，实际开发中我们不需要死扣概念，能拿它解决问题就达到目的了。大概总结一下，frp能够优雅地处理复杂事件驱动应用场景，能够模块化，可以像Promise一样有高组合能力，为处理事件模型提供了一个通用的抽象类型。

现阶段的流行js的frp库都比较大，不太适合移动前端开发，flyd是为数不多的比较小巧，实现也比较易懂的一个(同时性能也不低)。

它的核心代码提拱有限的接口，很容易掌握，同时在理解核心概念之后对其扩展功能也相对较容易。

这里的实现去掉了原有库里的一些函数式变成标准的支持，把一些依赖换成我们自己库里已有的依赖，添加了一些工具方法，便于调试，同时开发了一些常用工具模块配合核心库使用。

Features

Main features

Simple but powerful. Less is more! Flyd provides combinable observable streams as the basic building block. This minimal core is less than 200 SLOC which makes the library transparent – end users can realistically get a full understanding of how the library works.
More functional in style. Flyd is more functional than existing FRP libraries. Instead of methods it gives you curried functions with arguments in the order suitable for partial application. This gives more expressive power and modularity.
Modularity. The core of the Flyd is powerful and documented. This makes it easy for users of the library to create new FRP abstractions if existing ones do not exist. This in turn makes it viable to capture more patterns than otherwise because they can exist as separate modules. List of existing modules.

Other features

Elegant support for promises.
Atomic updates.

Examples

待更新

Tutorial

This is not general introduction to functional reactive programming. For that take a look at The introduction to Reactive Programming you've been missing and/or this Elm tutorial if you are comfortable with reading Haskell-like code.

This is not a demonstration of how you would write code with Flyd on a day to day basis. For that take a look at the examples.

This tutorial will however introduce you to the minimal but powerful core that Flyd provides and show you how it can be used to build FRP abstractions.

Creating streams

Flyd gives you streams as the building block for creating reactive dataflows. They serve the same purpose as what other FRP libraries call Signals, Observables, Properties and EventEmitters.

The function stream creates a representation of a value that changes over time. The resulting stream is a function. At first sight it works a bit like a getter-setter:

这里的stream是frp核心模块暴露出的唯一接口，其本身是一个function，以下介绍的其他接口都使用它作为全局对象绑在其身上，比如combine方法实际指的是stream.combine.

// Create a stream with initial value 5.
var number = stream(5);
// Get the current value of the stream.
console.log(number()); // logs 5
// Update the value of the stream.
console.log(number(7));
// The stream now returns the new value.
console.log(number()); // logs 7

Top level streams, that is streams without dependencies, should typically depend on the external world, like user input or fetched data.

Since streams are just functions you can easily plug them in whenever a function is expected.

var clicks = stream();
document.getElementById('button').addEventListener('click', clicks);
var messages = stream();
webSocket.onmessage = messages;

Clicks events will now flow down the clicks stream and WebSockets messages down the messages stream.

Dependent streams

Streams can depend on other streams. Use var combined = combine(combineFn, [a, b, c, ...]). The combineFn function will be called as (a, b, c, ..., self, changed) => v, where a, b, c, ... is a spread of each dependency, self is a reference to the combine stream itself, and changed is an array of streams that were atomically updated.

Flyd automatically updates the stream whenever a dependency changes. This means that the sum function below will be called whenever x and y changes. You can think of dependent stream as streams that automatically listens to or subscribes to their dependencies.

// Create two streams of numbers
var x = stream(4);
var y = stream(6);
// Create a stream that depends on the two previous streams
// and with its value given by the two added together.
var sum = combine(function(x, y) {
  return x() + y();
}, [x, y]);
// `sum` is automatically recalculated whenever the streams it depends on changes.
x(12);
console.log(sum()); // logs 18
y(8);
console.log(sum()); // logs 20

Naturally, a stream with dependencies can depend on other streams with dependencies.

// Create two streams of numbers
var x = stream(4);
var y = stream(6);
var squareX = combine(function(x) {
  return x() * x();
}, [x]);
var squareXPlusY = combine(function(y, squareX) {
  return y() + squareX();
}, [y, squareX]);
console.log(squareXPlusY()); // logs 22
x(2);
console.log(squareXPlusY()); // logs 10

The body of a dependent stream is called with the spread of: each dependency, itself, and a list of the dependencies that have changed since its last invocation (due to atomic updates several streams could have changed).

// Create two streams of numbers
var x = stream(1);
var y = stream(2);
var sum = combine(function(x, y, self, changed) {
  // The stream can read from itself
  console.log('Last sum was ' + self());
  // On the initial call no streams has changed and `changed` will be []
  changed.map(function(s) {
    var changedName = (s === y ? 'y' : 'x');
    console.log(changedName + ' changed to ' + s());
  });
  return x() + y();
}, [x, y]);

Note Returning undefined in the combineFn will not trigger an update to the stream. To trigger on undefined, update directly:

combine((_, self, changed) => { self(undefined); }, [depStream]);

Using callback APIs for asynchronous operations

Instead of returning a value a stream can update itself by calling itself. This is handy when working with APIs that takes callbacks.

var urls = stream('/something.json');
var responses = combine(function(urls, self) {
  makeRequest(urls(), self);
}, [urls]);
combine(function(responses) {
  console.log('Received response!');
  console.log(responses());
}, [responses]);

Note that the stream that logs the responses from the server should only be called after an actual response has been received (otherwise responses() would return undefined). Fortunately a stream's body will not be called before all of its declared streams has received a value (this behaviour can be circumvented with immediate).

Using promises for asynchronous operations

Flyd has inbuilt support for promises. Similarly to how a promise can never be resolved with a promise, a promise can never flow down a stream. Instead the fulfilled value of the promise will be sent down the stream.

由于原始库没有对promise的error做处理，这里加上的对promise的error也捕获的逻辑，所以以下的例子中如果 responses又错误发生，错误也会作为事件值传给on方法的handleri, 所以处理promise的值时需要注意区分。

var urls = stream('/something.json');
var responses = stream(requestPromise(urls()));
on(function(responses) {
  console.log('Received response!');
  console.log(responses());
}, responses);

Mapping over a stream

You've now seen most of the basic building block which Flyd provides. Let's see what we can do with them. Let's write a function that takes a function and a stream and returns a new stream with the function applied to every value emitted by the stream. In short, a map function.

var mapStream = function(f, s) {
  return combine(function(s) {
    return f(s());
  }, [s]);
};

We simply create a new stream dependent on the first stream. We declare the stream as a dependency so that our stream won't return values before the original stream produces its first value.

Scanning a stream

Lets try something else: a scan function for accumulating a stream! It could look like this:

var scanStream = function(f, acc, s) {
  return combine(function(s) {
    acc = f(acc, s());
    return acc;
  }, [s]);
};

Our scan function takes an accumulator function, an initial value and a stream. Every time the original stream emits a value we pass it to the accumulator function along with the accumulated value.

Stream endings

When you create a stream with stream it will have an end property which is also a stream. That is an end stream:

var s = stream();
console.log(isStream(s.end)); // logs `true`

You can end a stream by pushing true into its end stream:

var s = stream();
s.end(true); // this ends `s`

When you create a dependent stream its end stream will initially depend on all the end streams of its dependencies:

var n1 = stream();
var n2 = stream();
var sum = combine(function(n1, n2) {
  return n1() + n2();
}, [n1, n2]);

sum.end now depends on n1.end and n2.end. This means that whenever one of the sums dependencies end sum will end as well.

You can change what a stream's end stream depends on with endsOn:

var number = stream(2);
var killer = stream();
var square = endsOn(merge(number.end, killer), combine(function(number) {
  return number() * number();
}, [number]));

Now square will end if either number ends or if killer emits a value.

The fact that a stream's ending is itself a stream is a very powerful concept. It means that we can use the full expressiveness of Flyd to control when a stream ends.

Fin

You're done! To learn more check out the API, the examples and the source of the modules.

API

var numbers = stream(0);
on(function(n) { console.log('numbers changed to', n); }, numbers);

instance methods

stream()

Returns the last value of the stream.

Signature

a

Example

var names = stream('Turing');
names(); // 'Turing'

stream(val)

Pushes a value down the stream.

Signature

a -> Stream a

Example

names('Bohr');
names(); // 'Bohr'

stream.end

A stream that emits true when the stream ends. If true is pushed down the stream the parent stream ends.

stream.on(fn)

Listen to stream events

Signature: (a -> result) -> Stream a -> Stream undefined

Example

var n = stream();
n.on(console.log.bind(console));
n(1);
//1

stream.log(msg)

Print log info according to stream events, including end event

Signature

Called bound to Stream (a): m -> undefined

Example

var n = stream(1);
n.log();
//1

Modules

从其他的源构建事件流(build r$)：

[from/promise] 从promise实例构建一个事件流
[from/sequence] 从数组构建一个事件流，可以指定事件发生的时间间隔

从其他事件流构建新的事件流(compose r$):

[composer/apply]
[composer/debounce]
[composer/throttle]
[composer/filter]
[composer/flatmap]
[composer/lift]
[composer/map]
[composer/merge]
[composer/scan]
[composer/switchlatest]

Misc

Atomic updates

Consider the following example:

var a = stream(1);
var b = combine(function(a) { return a() * 2; }, [a]);
var c = combine(function(a) { return a() + 4; }, [a]);
var d = combine(function(b, c, self, ch) {
  result.push(b() + c());
}, [b, c]);

The dependency graph looks like this.

    a
  /   \
 b     c
  \   /
    d

Now, when a value flows down a, both b and c will change because they depend on a. If you merely consider streams as being event emitters you'd expect d to be updated twice. Because a triggers b triggers d after which a also triggers c which again triggers d.

But Flyd handles such cases optimally. Since only one value entered the system d will only be updated once with the changed values of b and c.

Flyd guarantees that when a single value enters the system every stream will only be updated once, along with their dependencies in their most recent state.

This avoids superfluous updates of your streams and intermediate states when several streams change at the same time.

Flyd implements atomic updates with a O(n) topological sort where n is number of streams that directly or indirectly depends on the updated stream.

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frp.md

frp.md

Reactive Stream (flyd fork)

Table of contents

Introduction

Features

Examples

Tutorial

Creating streams

Dependent streams

Using callback APIs for asynchronous operations

Using promises for asynchronous operations

Mapping over a stream

Scanning a stream

Stream endings

Fin

API

static methods

Stream()

combine(body, dependencies)

isStream(stream)

immediate(stream)

endsOn(endStream, s)

on(fn, s)

instance methods

stream()

stream(val)

stream.end

stream.on(fn)

stream.log(msg)

Modules

Misc

Atomic updates

Files

frp.md

Latest commit

History

frp.md

File metadata and controls

Reactive Stream (flyd fork)

Table of contents

Introduction

Features

Examples

Tutorial

Creating streams

Dependent streams

Using callback APIs for asynchronous operations

Using promises for asynchronous operations

Mapping over a stream

Scanning a stream

Stream endings

Fin

API

static methods

Stream()

combine(body, dependencies)

isStream(stream)

immediate(stream)

endsOn(endStream, s)

on(fn, s)

instance methods

stream()

stream(val)

stream.end

stream.on(fn)

stream.log(msg)

Modules

Misc

Atomic updates