流式接口

流式接口（fluent interface）是软件工程中面向对象API的一种实现方式，以提供更为可读的源代码。最早由Eric Evans与Martin Fowler于2005年提出。
通常采取方法瀑布调用 (具体说是方法链式调用)来转发一系列对象方法调用的上下文 。这个上下文（context）通常是指：- 通过被调方法的返回值定义- 自引用，新的上下文等于老的上下文。- 返回一个空的上下文来终止。
C++的iostream流式调用就是一个典型的例子。Smalltalk在1970年代就实现了方法瀑布调用。


**例子**

**JavaScript**
用于数据库查询的jQuery，例如https://github.com/Medium/dynamite date=20201106122008  :
// getting an item from a table
client.getItem('user-table')
.setHashKey('userId', 'userA')
.setRangeKey('column', '@')
.execute()
.then(function(data) {
// data.result: the resulting object
})
JavaScript使用原型继承与`this`.
// example from http://schier.co/post/method-chaining-in-javascript
// define the class
var Kitten = function() {
this.name = 'Garfield';
this.color = 'brown';
this.gender = 'male';
};
Kitten.prototype.setName = function(name) {
this.name = name;
return this;
};
Kitten.prototype.setColor = function(color) {
this.color = color;
return this;
};
Kitten.prototype.setGender = function(gender) {
this.gender = gender;
return this;
};
Kitten.prototype.save = function() {
console.log(
'saving ' + this.name + ', the ' +
this.color + ' ' + this.gender + ' kitten...'
);
// save to database here...
return this;
};
// use it
new Kitten()
.setName('Bob')
.setColor('black')
.setGender('male')
.save();

**Java**
jOOQ库模拟了SQL
Author author = AUTHOR.as("author");
create.selectFrom(author)
.where(exists(selectOne()
.from(BOOK)
.where(BOOK.STATUS.eq(BOOK_STATUS.SOLD_OUT))
.and(BOOK.AUTHOR_ID.eq(author.ID))));


**C#**
C#在LINQ中大量使用 standard query operators与扩展方法。
var translations = new Dictionary
{
{"cat", "chat"},
{"dog", "chien"},
{"fish", "poisson"},
{"bird", "oiseau"}
};
// Find translations for English words containing the letter "a",
// sorted by length and displayed in uppercase
IEnumerable query = translations
	.Where   (t => t.Key.Contains("a"))
	.OrderBy (t => t.Value.Length)
	.Select  (t => t.Value.ToUpper());
// The same query constructed progressively:
var filtered   = translations.Where (t => t.Key.Contains("a"));
var sorted     = filtered.OrderBy   (t => t.Value.Length);
var finalQuery = sorted.Select      (t => t.Value.ToUpper());
流式接口可用于一系列方法，他们运行在同一对象上。
// Defines the data context
class Context
{
public string FirstName { get; set; }
public string LastName { get; set; }
public string Sex { get; set; }
public string Address { get; set; }
}
class Customer
{
private Context _context = new Context(); // Initializes the context
// set the value for properties
public Customer FirstName(string firstName)
{
_context.FirstName = firstName;
return this;
}
public Customer LastName(string lastName)
{
_context.LastName = lastName;
return this;
}
public Customer Sex(string sex)
{
_context.Sex = sex;
return this;
}
public Customer Address(string address)
{
_context.Address = address;
return this;
}
// Prints the data to console
public void Print()
{
Console.WriteLine("First name: {0} \nLast name: {1} \nSex: {2} \nAddress: {3}", _context.FirstName, _context.LastName, _context.Sex, _context.Address);
}
}
class Program
{
static void Main(string args)
{
// Object creation
Customer c1 = new Customer();
// Using the method chaining to assign & print data with a single line
c1.FirstName("vinod").LastName("srivastav").Sex("male").Address("bangalore").Print();
}
}

**C++**
下述代码对比了传统的风格与流式接口的实现风格：
// Basic definition
class GlutApp {
private:
int w_, h_, x_, y_, argc_, display_mode_;
char **argv_;
char *title_;
public:
GlutApp(int argc, char** argv) {
argc_ = argc;
argv_ = argv;
}
void setDisplayMode(int mode) {
display_mode_ = mode;
}
int getDisplayMode() {
return display_mode_;
}
void setWindowSize(int w, int h) {
w_ = w;
h_ = h;
}
void setWindowPosition(int x, int y) {
x_ = x;
y_ = y;
}
void setTitle(const char *title) {
title_ = title;
}
void create(){;}
};
// Basic usage
int main(int argc, char **argv) {
GlutApp app(argc, argv);
app.setDisplayMode(GLUT_DOUBLE|GLUT_RGBA|GLUT_ALPHA|GLUT_DEPTH); // Set framebuffer params
app.setWindowSize(500, 500); // Set window params
app.setWindowPosition(200, 200);
app.setTitle("My OpenGL/GLUT App");
app.create();
}
// Fluent wrapper
class FluentGlutApp : private GlutApp {
public:
FluentGlutApp(int argc, char **argv) : GlutApp(argc, argv) {} // Inherit parent constructor
FluentGlutApp &withDoubleBuffer() {
setDisplayMode(getDisplayMode() | GLUT_DOUBLE);
return *this;
}
FluentGlutApp &withRGBA() {
setDisplayMode(getDisplayMode() | GLUT_RGBA);
return *this;
}
FluentGlutApp &withAlpha() {
setDisplayMode(getDisplayMode() | GLUT_ALPHA);
return *this;
}
FluentGlutApp &withDepth() {
setDisplayMode(getDisplayMode() | GLUT_DEPTH);
return *this;
}
FluentGlutApp &across(int w, int h) {
setWindowSize(w, h);
return *this;
}
FluentGlutApp &at(int x, int y) {
setWindowPosition(x, y);
return *this;
}
FluentGlutApp &named(const char *title) {
setTitle(title);
return *this;
}
// It doesn't make sense to chain after create(), so don't return *this
void create() {
GlutApp::create();
}
};
// Fluent usage
int main(int argc, char **argv) {
FluentGlutApp(argc, argv)
.withDoubleBuffer().withRGBA().withAlpha().withDepth()
.at(200, 200).across(500, 500)
.named("My OpenGL/GLUT App")
.create();
}

**Ruby**
Ruby语言允许修改核心类，这使得流式接口成为原生易于实现。 
# Add methods to String class
class String
def prefix(raw)
"#{raw} #{self}"
end
def suffix(raw)
"#{self} #{raw}"
end
def indent(raw)
raw = " " * raw if raw.kind_of? Fixnum
prefix(raw)
end
end

# Fluent interface
message = "there"
puts message.prefix("hello")
.suffix("world")
.indent(8)

**Scala**
Scala supports a fluent syntax for both method calls and class mixins, using traits and the with keyword.  For example:
class Color { def rgb(): Tuple3Decimal }
object Black extends Color { override def rgb(): Tuple3Decimal = ("0", "0", "0"); }
trait GUIWindow {
// Rendering methods that return this for fluent drawing
def set_pen_color(color: Color): this.type
def move_to(pos: Position): this.type
def line_to(pos: Position, end_pos: Position): this.type
def render(): this.type = this // Don't draw anything, just return this, for child implementations to use fluently
def top_left(): Position
def bottom_left(): Position
def top_right(): Position
def bottom_right(): Position
}
trait WindowBorder extends GUIWindow {
def render(): GUIWindow = {
super.render()
.move_to(top_left())
.set_pen_color(Black)
.line_to(top_right())
.line_to(bottom_right())
.line_to(bottom_left())
.line_to(top_left())
}
}
class SwingWindow extends GUIWindow { ... }
val appWin = new SwingWindow() with WindowBorder
appWin.render()

**Perl 6**
In Perl 6, there are many approaches, but one of the simplest is to declare attributes as read/write and use the given keyword. The type annotations are optional, but the native gradual typing makes it much safer to write directly to public attributes.
class Employee {
subset Salary         of Real where * > 0;
subset NonEmptyString of Str  where * ~~ /\S/; # at least one non-space character
has NonEmptyString $.name    is rw;
has NonEmptyString $.surname is rw;
has Salary         $.salary  is rw;
method gist {
return qq:toEND;
Name:    $.name
Surname: $.surname
Salary:  $.salary
END
}
}
my $employee = Employee.new();
given $employee {
.name    = 'Sally';
.surname = 'Ride';
.salary  = 200;
}
say $employee;
# Output:
# Name:    Sally
# Surname: Ride
# Salary:  200

**PHP**
在PHP中，可以使用表示实例的特殊变量$this返回当前对象。因此返回$this将使方法返回实例。下面的示例定义了一个Employee类和三个方法来设置它的名称、姓和薪水。每个Employee类的实例允许调用这些方法。
name = $name;
return $this;
}
public function setSurname($surname)
{
$this->surName = $surname;
return $this;
}
public function setSalary($salary)
{
$this->salary = $salary;
return $this;
}
public function __toString()
{
$employeeInfo = 'Name: ' . $this->name . PHP_EOL;
$employeeInfo .= 'Surname: ' . $this->surName . PHP_EOL;
$employeeInfo .= 'Salary: ' . $this->salary . PHP_EOL;
return $employeeInfo;
}
}
# Create a new instance of the Employee class, Tom Smith, with a salary of 100:
$employee = (new Employee())
->setName('Tom')
->setSurname('Smith')
->setSalary('100');
# Display the value of the Employee instance:
echo $employee;
# Display:
# Name: Tom
# Surname: Smith
# Salary: 100

**Python**
Python通过在实例方法中返回`self`：
class Poem(object):
def __init__(self, content):
self.content = content
def indent(self, spaces):
self.content = " " * spaces + self.content
return self
def suffix(self, content):
self.content = self.content + " - " + content
return self
>>> Poem("Road Not Travelled").indent(4).suffix("Robert Frost").content
'    Road Not Travelled - Robert Frost'


**Visual Basic.Net**

**参考文献**

**外部链接**
  Martin Fowler's original bliki entry coining the term date=20210308190652 -  A Delphi example of writing XML with a fluent interface date=20090405030147 -  A .NET fluent validation library written in C# date=20171223050720 -  A tutorial for creating formal Java fluent APIs from a BNF notation date=20130920165112