流接口模式（Fluent Interface）

## 目的

用来编写易于阅读的代码，就像自然语言一样（如英语）。

## 例子

- Doctrine2 的 QueryBuilder，就像下面例子中类似。
- PHPUnit 使用连贯接口来创建 mock 对象。
- Yii 框架：CDbCommand 与 CActiveRecord 也使用此模式。

## UML

![K6NHRBLVHw.png](/media/202203/2022-03-24_1344320.7505853177577538.png)

## 示例代码

### PHP

```php
class Sql
{
    /**
     * @var array
     */
    private $fields = [];

/**
     * @var array
     */
    private $from = [];

/**
     * @var array
     */
    private $where = [];

public function select(array $fields): Sql
    {
        $this->fields = $fields;

return $this;
    }

public function from(string $table, string $alias): Sql
    {
        $this->from[] = $table.' AS '.$alias;

return $this;
    }

public function where(string $condition): Sql
    {
        $this->where[] = $condition;

return $this;
    }

public function __toString(): string
    {
        return sprintf(
            'SELECT %s FROM %s WHERE %s',
            join(', ', $this->fields),
            join(', ', $this->from),
            join(' AND ', $this->where)
        );
    }
}

class FluentInterfaceTest extends TestCase
{
    public function testBuildSQL()
    {
        $query = (new Sql())
                ->select(['foo', 'bar'])
                ->from('foobar', 'f')
                ->where('f.bar = ?');

$this->assertEquals('SELECT foo, bar FROM foobar AS f WHERE f.bar = ?', (string) $query);
    }
}
```