Syscall与Context

按照nikic的思路引入与调度器内部交互的Syscall，将需要执行的函数打包成Syscall，通过yield返回迭代器，可以从Syscall参数获取到当前迭代器对象，这里提供了一个外界与AsyncTask交互的扩展点。

我们借此演示如何添加跨生成器上下文，在嵌套生成器共享数据，解耦生成器之间依赖。

<?php
final class AsyncTask implements Async
{
    public $gen;
    public $continuation;
    public $parent;
    // 我们在构造器添加$parent参数， 把父子生成器链接起来，使其可以进行回溯
    public function __construct(\Generator $gen, AsyncTask $parent = null)
    {
        $this->gen = new Gen($gen);
        $this->parent = $parent;
    }
    public function begin(callable $continuation)
    {
        $this->continuation = $continuation;
        $this->next();
    }
    public function next($result = null, $ex = null)
    {
        try {
            if ($ex) {
                $value = $this->gen->throw_($ex);
            } else {
                $value = $this->gen->send($result);
            }
            if ($this->gen->valid()) {
                // 这里注意优先级， Syscall 可能返回\Generator 或者 Async
                if ($value instanceof Syscall) { // Syscall 签名见下方
                    $value = $value($this);
                }
                if ($value instanceof \Generator) {
                    $value = new self($value, $this);
                }
                if ($value instanceof Async) {
                    $cc = [$this, "next"];
                    $value->begin($cc);
                } else {
                    $this->next($value, null);
                }
            } else {
                $cc = $this->continuation;
                $cc($result, null);
            }
        } catch (\Exception $ex) {
            if ($this->gen->valid()) {
                $this->next(null, $ex);
            } else {
                $cc = $this->continuation;
                $cc($result, $ex);
            }
        }
    }
}

Syscall将 (callable :: AsyncTask $task -> mixed) 包装成单独类型：

<?php
class Syscall
{
    private $fun;
    public function __construct(callable $fun)
    {
        $this->fun = $fun;
    }
    public function __invoke(AsyncTask $task)
    {
        $cb = $this->fun;
        return $cb($task);
    }
}

因为PHP对象属性为Hashtable实现，而生成器对象本身无任何属性，我们这里把Context的KV数据附加到根生成器对象上，然后得到的Context的Get与Set函数：

<?php
function getCtx($key, $default = null)
{
    return new Syscall(function(AsyncTask $task) use($key, $default) {
        while($task->parent && $task = $task->parent);
        if (isset($task->gen->generator->$key)) {
            return $task->gen->generator->$key;
        } else {
            return $default;
        }
    });
}
function setCtx($key, $val)
{
    return new Syscall(function(AsyncTask $task) use($key, $val) {
        while($task->parent && $task = $task->parent);
        $task->gen->generator->$key = $val;
    });
}

<?php
function setTask()
{
    yield setCtx("foo", "bar");
}
function ctxTest()
{
    yield setTask();
    $foo = (yield getCtx("foo"));
    echo $foo;
}
$task = new AsyncTask(ctxTest());
$task->begin($trace); // output: bar