PHP下的异步尝试四：PHP版的Promise

PHP下的异步尝试系列

如果你还不太了解PHP下的生成器和协程，你可以根据下面目录翻阅

1. PHP下的异步尝试一：初识生成器
2. PHP下的异步尝试二：初识协程
3. PHP下的异步尝试三：协程的PHP版thunkify自动执行器
4. PHP下的异步尝试四：PHP版的Promise
5. PHP下的异步尝试五：PHP版的Promise的继续完善

Promise 实现

代码结构

│  │  autoload.php
│  │  promise1.php
│  │  promise2.php
│  │  promise3.php
│  │  promise4.php
│  │  promise5.php
│  │
│  └─classes
│          Promise1.php
│          Promise2.php
│          Promise3.php
│          Promise4.php
│          Promise5.php
│          PromiseState.php

尝试一 (Promise基础)

classess/PromiseState.php

final class PromiseState
{
    const PENDING = 'pending';
    const FULFILLED = 'fulfilled';
    const REJECTED = 'rejected';
}

classess/Promise1.php

// 尝试一
class Promise1
{
    private $value;
    private $reason;
    private $state;

    public function __construct(\Closure $func = null)
    {
        $this->state = PromiseState::PENDING;

        $func([$this, 'resolve'], [$this, 'reject']);
    }

    /**
     * 执行回调方法里的resolve绑定的方法
     * @param null $value
     */
    public function resolve($value = null)
    {
        // 回调执行resolve传参的值，赋值给result
        $this->value = $value;

        if ($this->state == PromiseState::PENDING) {
            $this->state = PromiseState::FULFILLED;
        }
    }

    public function reject($reason = null)
    {
        // 回调执行resolve传参的值，赋值给result
        $this->reason = $reason;

        if ($this->state == PromiseState::PENDING) {
            $this->state = PromiseState::REJECTED;
        }
    }

    public function getState()
    {
        return $this->state;
    }

    public function getValue()
    {
        return $this->value;
    }

    public function getReason()
    {
        return $this->reason;
    }
}

promise1.php

require "autoload.php";

$promise = new Promise1(function($resolve, $reject) {
    $resolve("打印我");
});

var_dump($promise->getState());
var_dump($promise->getValue());

结果：

string(9) "fulfilled"
string(9) "打印我"

结论或问题：

我们在这里建构了最基础的Promise模型

尝试二 (增加链式then)

classess/Promise2.php

<?php
// 尝试二 （增加链式then）
class Promise2
{
    private $value;
    private $reason;
    private $state;

    public function __construct(\Closure $func = null)
    {
        $this->state = PromiseState::PENDING;

        $func([$this, 'resolve'], [$this, 'reject']);
    }

    public function then(\Closure $onFulfilled = null, \Closure $onRejected = null)
    {
        // 如果状态是fulfilled，直接回调执行并传参value
        if ($this->state == PromiseState::FULFILLED) {
            $onFulfilled($this->value);
        }

        // 如果状态是rejected，直接回调执行并传参reason
        if ($this->state == PromiseState::REJECTED) {
            $onRejected($this->reason);
        }

        // 返回对象自身，实现链式调用
        return $this;

    }

    /**
     * 执行回调方法里的resolve绑定的方法
     * 本状态只能从pending->fulfilled
     * @param null $value
     */
    public function resolve($value = null)
    {
        if ($this->state == PromiseState::PENDING) {
            $this->state = PromiseState::FULFILLED;
            $this->value = $value;
        }
    }

    /**
     * 执行回调方法里的rejected绑定的方法
     * 本状态只能从pending->rejected
     * @param null $reason
     */
    public function reject($reason = null)
    {
        if ($this->state == PromiseState::PENDING) {
            $this->state = PromiseState::REJECTED;
            $this->reason = $reason;
        }
    }

    public function getState()
    {
        return $this->state;
    }

    public function getValue()
    {
        return $this->value;
    }

    public function getReason()
    {
        return $this->reason;
    }
}

promise2.php

<?php

require "autoload.php";

$promise = new Promise2(function($resolve, $reject) {
    $resolve("打印我");
});

$promise->then(function ($value) {
    var_dump($value);
}, function ($reason) {
    var_dump($reason);
})->then(function ($value) {
    var_dump($value);
}, function ($reason) {
    var_dump($reason);
});

结果：

string(9) "打印我"
string(9) "打印我"

结论或问题：

我们实现了链式then方法

如果我们的构造里的回调是异步执行的话，那么状态在没有变成fulfilled之前，我们then里的回调方法就永远没法执行

尝试三（真正的链式then）

classess/Promise3.php

// 解决思路：我们肯定要把then传入的回调，放到Promise构造里回调代码执行完后resolve调用后改变了state状态后再调用，所以我们必须存储到一个地方并方便后续调用
// 我们需要改造then、resolve和reject方法
class Promise3
{
    private $value;
    private $reason;
    private $state;
    private $fulfilledCallbacks = [];
    private $rejectedCallbacks = [];

    public function __construct(\Closure $func = null)
    {
        $this->state = PromiseState::PENDING;

        $func([$this, 'resolve'], [$this, 'reject']);
    }

    public function then(\Closure $onFulfilled = null, \Closure $onRejected = null)
    {
        // 如果是异步回调，状态未变化之前，then的回调方法压入相应的数组方便后续调用
        if ($this->state == PromiseState::PENDING) {
            $this->fulfilledCallbacks[] = static function() use ($onFulfilled, $value){
                $onFulfilled($this->value);
            };

            $this->rejectedCallbacks[] = static function() use ($onRejected, $reason){
                $onRejected($this->reason);
            };
        }

        // 如果状态是fulfilled，直接回调执行并传参value
        if ($this->state == PromiseState::FULFILLED) {
            $onFulfilled($this->value);
        }

        // 如果状态是rejected，直接回调执行并传参reason
        if ($this->state == PromiseState::REJECTED) {
            $onRejected($this->reason);
        }

        // 返回对象自身，实现链式调用
        return $this;

    }

    /**
     * 执行回调方法里的resolve绑定的方法
     * 本状态只能从pending->fulfilled
     * @param null $value
     */
    public function resolve($value = null)
    {
        if ($this->state == PromiseState::PENDING) {
            $this->state = PromiseState::FULFILLED;
            $this->value = $value;

            array_walk($this->fulfilledCallbacks, function ($callback) {
                $callback();
            });
        }
    }

    /**
     * 执行回调方法里的rejected绑定的方法
     * 本状态只能从pending->rejected
     * @param null $reason
     */
    public function reject($reason = null)
    {
        if ($this->state == PromiseState::PENDING) {
            $this->state = PromiseState::REJECTED;
            $this->reason = $reason;
        }
    }

    public function getState()
    {
        return $this->state;
    }

    public function getValue()
    {
        return $this->value;
    }

    public function getReason()
    {
        return $this->reason;
    }
}

promise3.php

require "autoload.php";

$promise = new Promise3(function($resolve, $reject) {
    $resolve("打印我");
});

$promise->then(function ($value) {
    var_dump($value);
}, function ($reason) {
    var_dump($reason);
})->then(function ($value) {
    var_dump($value);
}, function ($reason) {
    var_dump($reason);
});

结果：

string(9) "打印我"
string(9) "打印我"

结论或问题：

我们这次基本实现了真正的链式then方法

不过在Promise/A+里规范，要求then返回每次都要求是一个新的Promise对象
then方法成功执行，相当于返回一个实例一个Promise回调里执行resolve方法，resolve值为then里return的值
then方法执行失败或出错，相当于返回一个实例一个Promise回调里执行rejected方法，rejected值为then里return的值

尝试四（then返回pormise对象, 并传递上一次的结果给下一个Promise对象）

classess/Promise4.php

class Promise4
{
    private $value;
    private $reason;
    private $state;
    private $fulfilledCallbacks = [];
    private $rejectedCallbacks = [];

    public function __construct(\Closure $func = null)
    {
        $this->state = PromiseState::PENDING;

        $func([$this, 'resolve'], [$this, 'reject']);
    }

    public function then(\Closure $onFulfilled = null, \Closure $onRejected = null)
    {
        $thenPromise = new Promise4(function ($reslove, $reject) use (&$thenPromise, $onFulfilled, $onRejected) {

            //$this 代表的当前的Promise对象，不要混淆了

            // 如果是异步回调，状态未变化之前，then的回调方法压入相应的数组方便后续调用
            if ($this->state == PromiseState::PENDING) {
                $this->fulfilledCallbacks[] = static function() use ($thenPromise, $onFulfilled, $reslove, $reject){
                    $value = $onFulfilled($this->value);
                    $this->resolvePromise($thenPromise, $value, $reslove, $reject);
                };

                $this->rejectedCallbacks[] = static function() use ($thenPromise, $onRejected, $reslove, $reject){
                    $reason = $onRejected($this->reason);
                    $this->resolvePromise($thenPromise, $reason, $reslove, $reject);
                };
            }

            // 如果状态是fulfilled，直接回调执行并传参value
            if ($this->state == PromiseState::FULFILLED) {
                $value = $onFulfilled($this->value);
                $this->resolvePromise($thenPromise, $value, $reslove, $reject);
            }

            // 如果状态是rejected，直接回调执行并传参reason
            if ($this->state == PromiseState::REJECTED) {
                $reason = $onRejected($this->reason);
                $this->resolvePromise($thenPromise, $reason, $reslove, $reject);
            }

        });

        // 返回对象自身，实现链式调用
        return $thenPromise;

    }

    /**
     * 解决Pormise链式then传递
     * 可参考 [Promises/A+]2.3 [https://promisesaplus.com/#the-promise-resolution-procedure]
     * @param $thenPromise
     * @param $x            $x为thenable对象
     * @param $resolve
     * @param $reject
     */
    private function resolvePromise($thenPromise, $x, $resolve, $reject)
    {
        $called = false;

        if ($thenPromise === $x) {
            return $reject(new \Exception('循环引用'));
        }

        if ( is_object($x) && method_exists($x, 'then')) {

            $resolveCb = function ($value) use($thenPromise, $resolve, $reject, $called) {
                if ($called) return ;
                $called = true;
                // 成功值y有可能还是promise或者是具有then方法等，再次resolvePromise，直到成功值为基本类型或者非thenable
                $this->resolvePromise($thenPromise, $value, $resolve, $reject);
            };

            $rejectCb = function($reason) use($thenPromise, $resolve, $reject, $called) {
                if ($called) return ;
                $called = true;
                $reject($reason);
            };

            call_user_func_array([$x, 'then'], [$resolveCb, $rejectCb]);

        } else {
            if ($called) return ;
            $called = true;
            $resolve($x);
        }
    }

    /**
     * 执行回调方法里的resolve绑定的方法
     * 本状态只能从pending->fulfilled
     * @param null $value
     */
    public function resolve($value = null)
    {
        if ($this->state == PromiseState::PENDING) {
            $this->state = PromiseState::FULFILLED;
            $this->value = $value;

            array_walk($this->fulfilledCallbacks, function ($callback) {
                $callback();
            });
        }
    }

    /**
     * 执行回调方法里的rejected绑定的方法
     * 本状态只能从pending->rejected
     * @param null $reason
     */
    public function reject($reason = null)
    {
        if ($this->state == PromiseState::PENDING) {
            $this->state = PromiseState::REJECTED;
            $this->reason = $reason;
        }
    }

    public function getState()
    {
        return $this->state;
    }

    public function getValue()
    {
        return $this->value;
    }

    public function getReason()
    {
        return $this->reason;
    }
}

promise4.php

require "autoload.php";

$promise1 = new Promise4(function($resolve, $reject) {
    $resolve("打印我");
});

$promise2 = $promise1->then(function ($value) {
    var_dump($value);
    return "promise2";
}, function ($reason) {
    var_dump($reason);
});

$promise3 = $promise2->then(function ($value) {
    var_dump($value);
    return new Promise4(function($resolve, $reject) {
        $resolve("promise3");
    });
}, function ($reason) {
    var_dump($reason);
});

$promise4 = $promise3->then(function ($value) {
    var_dump($value);
    return "promise4";
}, function ($reason) {
    var_dump($reason);
});

var_dump($promise4);

结果：

string(9) "打印我"
string(8) "promise2"
string(8) "promise3"
object(Promise4)#15 (5) {
  ["value":"Promise4":private]=>
  string(8) "promise4"
  ["reason":"Promise4":private]=>
  NULL
  ["state":"Promise4":private]=>
  string(9) "fulfilled"
  ["fulfilledCallbacks":"Promise4":private]=>
  array(0) {
  }
  ["rejectedCallbacks":"Promise4":private]=>
  array(0) {
  }
}

结论或问题：

一个基本的Pormise，不过我们上面都是基于成功fulfilled状态的实现
下面我们来增加错误捕获

尝试五（错误捕获）

classess/Promise5.php

class Promise5
{
    private $value;
    private $reason;
    private $state;
    private $fulfilledCallbacks = [];
    private $rejectedCallbacks = [];

    public function __construct(\Closure $func = null)
    {
        $this->state = PromiseState::PENDING;

        $func([$this, 'resolve'], [$this, 'reject']);
    }

    public function then(\Closure $onFulfilled = null, \Closure $onRejected = null)
    {
        // 此处作用是兼容then方法的以下四种参数变化，catchError就是第二种情况
        // 1. then($onFulfilled, null)
        // 2. then(null, $onRejected)
        // 3. then(null, null)
        // 4. then($onFulfilled, $onRejected)
        $onFulfilled = is_callable($onFulfilled) ? $onFulfilled :  function ($value) {return $value;};
        $onRejected = is_callable($onRejected) ? $onRejected :  function ($reason) {throw $reason;};

        $thenPromise = new Promise5(function ($reslove, $reject) use (&$thenPromise, $onFulfilled, $onRejected) {

            //$this 代表的当前的Promise对象，不要混淆了

            // 如果是异步回调，状态未变化之前，then的回调方法压入相应的数组方便后续调用
            if ($this->state == PromiseState::PENDING) {
                $this->fulfilledCallbacks[] = static function() use ($thenPromise, $onFulfilled, $reslove, $reject){
                    try {
                        $value = $onFulfilled($this->value);
                        $this->resolvePromise($thenPromise, $value, $reslove, $reject);
                    } catch (\Exception $e) {
                        $reject($e);
                    }
                };

                $this->rejectedCallbacks[] = static function() use ($thenPromise, $onRejected, $reslove, $reject){
                    try {
                        $reason = $onRejected($this->reason);
                        $this->resolvePromise($thenPromise, $reason, $reslove, $reject);
                    } catch (\Exception $e) {
                        $reject($e);
                    }
                };
            }

            // 如果状态是fulfilled，直接回调执行并传参value
            if ($this->state == PromiseState::FULFILLED) {
                try {
                    $value = $onFulfilled($this->value);
                    $this->resolvePromise($thenPromise, $value, $reslove, $reject);
                } catch (\Exception $e) {
                    $reject($e);
                }
            }

            // 如果状态是rejected，直接回调执行并传参reason
            if ($this->state == PromiseState::REJECTED) {
                try {
                    $reason = $onRejected($this->reason);
                    $this->resolvePromise($thenPromise, $reason, $reslove, $reject);
                } catch (\Exception $e) {
                    $reject($e);
                }
            }

        });

        // 返回对象自身，实现链式调用
        return $thenPromise;

    }

    public function catchError($onRejected)
    {
        return $this->then(null, $onRejected);
    }

    /**
     * 解决Pormise链式then传递
     * 可参考 [Promises/A+]2.3 [https://promisesaplus.com/#the-promise-resolution-procedure]
     * @param $thenPromise
     * @param $x            $x为thenable对象
     * @param $resolve
     * @param $reject
     */
    private function resolvePromise($thenPromise, $x, $resolve, $reject)
    {
        $called = false;

        if ($thenPromise === $x) {
            return $reject(new \Exception('循环引用'));
        }

        if ( is_object($x) && method_exists($x, 'then')) {
            try {
                $resolveCb = function ($value) use ($thenPromise, $resolve, $reject, $called) {
                    if ($called) return;
                    $called = true;
                    // 成功值y有可能还是promise或者是具有then方法等，再次resolvePromise，直到成功值为基本类型或者非thenable
                    $this->resolvePromise($thenPromise, $value, $resolve, $reject);
                };

                $rejectCb = function ($reason) use ($thenPromise, $resolve, $reject, $called) {
                    if ($called) return;
                    $called = true;
                    $reject($reason);
                };

                call_user_func_array([$x, 'then'], [$resolveCb, $rejectCb]);
            } catch (\Exception $e) {
                if ($called) return ;
                $called = true;
                $reject($e);
            }

        } else {
            if ($called) return ;
            $called = true;
            $resolve($x);
        }
    }

    /**
     * 执行回调方法里的resolve绑定的方法
     * 本状态只能从pending->fulfilled
     * @param null $value
     */
    public function resolve($value = null)
    {
        if ($this->state == PromiseState::PENDING) {
            $this->state = PromiseState::FULFILLED;
            $this->value = $value;

            array_walk($this->fulfilledCallbacks, function ($callback) {
                $callback(); //因为回调本身携带了作用于，所以直接调用，无法参数
            });
        }
    }

    /**
     * 执行回调方法里的rejected绑定的方法
     * 本状态只能从pending->rejected
     * @param null $reason
     */
    public function reject($reason = null)
    {
        if ($this->state == PromiseState::PENDING) {
            $this->state = PromiseState::REJECTED;
            $this->reason = $reason;

            array_walk($this->rejectedCallbacks, function ($callback) {
                $callback(); //因为回调本身携带了作用于，所以直接调用，无法参数
            });
        }
    }

    public function getState()
    {
        return $this->state;
    }

    public function getValue()
    {
        return $this->value;
    }

    public function getReason()
    {
        return $this->reason;
    }
}

promise5.php

require "autoload.php";

$promise1 = new Promise5(function($resolve, $reject) {
    $resolve("打印我");
});

$promise2 = $promise1->then(function ($value) {
    var_dump($value);
    throw new \Exception("promise2 error");
    return "promise2";
}, function ($reason) {
    var_dump($reason->getMessage());
    return "promise3 error return";
});

//我们可以简写then方法，只传入$onFulfilled方法，然后错误会自己冒泡方式到下一个catchError或then里处理。
//$promise3 = $promise2->then(function ($value) {
//    var_dump($value);
//    return new Promise5(function($resolve, $reject) {
//        $resolve("promise3");
//    });
//})->catchError(function ($reason) {
//    var_dump($reason->getMessage());
//    return "promise3 error return";
//});

$promise3 = $promise2->then(function ($value) {
    var_dump($value);
    return new Promise5(function($resolve, $reject) {
        $resolve("promise3");
    });
}, function ($reason) {
    var_dump($reason->getMessage());
    return "promise3 error return";
});

$promise4 = $promise3->then(function ($value) {
    var_dump($value);
    return "promise4";
}, function ($reason) {
    echo $reason->getMessage();
});

var_dump($promise4);

结果：

string(9) "打印我"
string(14) "promise2 error"
string(21) "promise3 error return"
object(Promise4)#10 (5) {
  ["value":"Promise4":private]=>
  string(8) "promise4"
  ["reason":"Promise4":private]=>
  NULL
  ["state":"Promise4":private]=>
  string(9) "fulfilled"
  ["fulfilledCallbacks":"Promise4":private]=>
  array(0) {
  }
  ["rejectedCallbacks":"Promise4":private]=>
  array(0) {
  }
}

结论或问题：

这里我们基础实现了一个可以用于生产环境的Promise
后续我们会接续完善这个Promise的特有方法，比如：finally, all, race, resolve, reject等
后续再介绍用Promise实现的自动执行器等

附录参考

Promises/A+
Promises/A+ 中文
Promise 对象 - ECMAScript 6 入门 阮一峰