简单地翻译官方文档的描述：
php7中，通过生成器委托（yield from），可以将其他生成器、可迭代的对象、数组委托给外层生成器。外层的生成器会先顺序 yield 委托出来的值，然后继续 yield 本身中定义的值。
利用 yield from 可以方便我们编写比较清晰生成器嵌套，而代码嵌套调用是编写复杂系统所必需的。
上例子：
<?phpfunction echotimes($msg, $max) { for ($i = 1; $i <= $max; ++$i) { echo "$msg iteration $i\n"; yield; }} function task() { yield from echotimes('foo', 10); // print foo ten times echo "---\n"; yield from echotimes('bar', 5); // print bar five times}foreach (task() as $item) { ;}
以上将输出：
foo iteration 1foo iteration 2foo iteration 3foo iteration 4foo iteration 5foo iteration 6foo iteration 7foo iteration 8foo iteration 9foo iteration 10---bar iteration 1bar iteration 2bar iteration 3bar iteration 4bar iteration 5
自然，内部生成器也可以接受它的父生成器发送的信息或者异常，因为 yield from 为父子生成器建立一个双向的通道。不多说，上例子：
<?phpfunction echomsg($msg) { while (true) { $i = yield; if($i === null){ break; } if(!is_numeric($i)){ throw new exception("hoo! must give me a number"); } echo "$msg iteration $i\n"; }}function task2() { yield from echomsg('foo'); echo "---\n"; yield from echomsg('bar');}$gen = task2();foreach (range(1,10) as $num) { $gen->send($num);}$gen->send(null);foreach (range(1,5) as $num) { $gen->send($num);}//$gen->send(hello world); //try it ,gay
输出和上个例子是一样的。
生成器返回值
如果生成器被迭代完成，或者运行到 return 关键字，是会给这个生成器返回值的。
可以有两种方法获取这个返回值：
使用 $ret = generator::getreturn() 方法。使用 $ret = yield from generator() 表达式。上例子：
<?phpfunction echotimes($msg, $max) { for ($i = 1; $i <= $max; ++$i) { echo "$msg iteration $i\n"; yield; } return "$msg the end value : $i\n";}function task() { $end = yield from echotimes('foo', 10); echo $end; $gen = echotimes('bar', 5); yield from $gen; echo $gen->getreturn();}foreach (task() as $item) { ;}
输出结果就不贴了，想必大家都猜到。
可以看到 yield from 和 return 结合使得 yield 的写法更像平时我们写的同步模式的代码了，毕竟，这就是 php 出生成器特性的原因之一呀。
一个非阻塞的web服务器
时间回到2015年，鸟哥博客上转载的一篇《 在php中使用协程实现多任务调度》。文章介绍了php5 的迭代生成器，协程，并实现了一个简单的非阻塞 web 服务器。（链接见文末引用）
现在我们利用 php7 中的这两个新特性重写这个 web 服务器，只需要 100 多行代码。
代码如下：
<?phpclass cosocket{ protected $mastercosocket = null; public $socket; protected $handlecallback; public $streampoolread = []; public $streampoolwrite = []; public function __construct($socket, cosocket $master = null) { $this->socket = $socket; $this->mastercosocket = $master $this; } public function accept() { $isselect = yield from $this->onread(); $accepts = null; if ($isselect && $as = stream_socket_accept($this->socket, 0)) { $accepts = new cosocket($as, $this); } return $accepts; } public function read($size) { yield from $this->onread(); yield ($data = fread($this->socket, $size)); return $data; } public function write($string) { yield from $this->onwriter(); yield fwrite($this->socket, $string); } public function close() { unset($this->mastercosocket->streampoolread[(int)$this->socket]); unset($this->mastercosocket->streampoolwrite[(int)$this->socket]); yield ($success = @fclose($this->socket)); return $success; } public function onread($timeout = null) { $this->mastercosocket->streampoolread[(int)$this->socket] = $this->socket; $pool = $this->mastercosocket->streampoolread; $rsocks = []; $wsocks = $esocks = null; foreach ($pool as $item) { $rsocks[] = $item; } yield ($num = stream_select($rsocks, $wsocks, $esocks, $timeout)); return $num; } public function onwriter($timeout = null) { $this->mastercosocket->streampoolwrite[(int)$this->socket] = $this->socket; $pool = $this->mastercosocket->streampoolread; $wsocks = []; $rsocks = $esocks = null; foreach ($pool as $item) { $wsocks[] = $item; } yield ($num = stream_select($rsocks, $wsocks, $esocks, $timeout)); return $num; } public function onrequest() { /** @var self $socket */ $socket = yield from $this->accept(); if (empty($socket)) { return false; } $data = yield from $socket->read(8192); $response = call_user_func($this->handlecallback, $data); yield from $socket->write($response); return yield from $socket->close(); } public static function start($port, callable $callback) { echo starting server at port $port...\n; $socket = @stream_socket_server(tcp://0.0.0.0:$port, $errno, $errstr); if (!$socket) throw new exception($errstr, $errno); stream_set_blocking($socket, 0); $cosocket = new self($socket); $cosocket->handlecallback = $callback; function gen($cosocket) { /** @var self $cosocket */ while (true) yield from $cosocket->onrequest(); } foreach (gen($cosocket) as $item){}; }}cosocket::start(8000, function ($data) { $response = <<