从父的静态方法返回子类

Question

I just learned about this fancy new feature of PHP 5.4. JsonSerializable! This is perfect for my app.

My app uses DateTime objects, and when I json_encode them, I get the following (by running json_encode([new DateTime])):

[{"date":"2013-09-11 15:39:22","timezone_type":3,"timezone":"UTC"}]

Depending on what timezone_type is, the timezone value may be different. I haven't found a good way to parse this object in JavaScript.

So, I decided to create my own DateTime class, and have it serialize to JSON how I wanted.

class SerialDateTime extends DateTime implements JsonSerializable{

    public function jsonSerialize(){
        return ['timestamp' => $this->getTimestamp()];
    }
}

When I now run json_encode([new SerialDateTime]), I get this:

[{"timestamp":1378914190}]

That's much easier to parse in JavaScript.

So, I figured this was a fine solution, but I discovered a problem. Static methods! SerialDateTime::createFromFormat returns a DateTime object!

If I do: json_encode([SerialDateTime::createFromFormat('m/d/Y', '10/31/2011')]), I get:

[{"date":"2011-10-31 15:46:07","timezone_type":3,"timezone":"UTC"}]

Why is this happening? Why doesn't SerialDateTime::createFromFormat return me a SerialDateTime object?!

How can I fix this, or do I need to override all the static methods from DateTime in SerialDateTime? If I do that, how would I even make a new SerialDateTime from the createFromFormat method? How can I "cast" a DateTime object to SerialDateTime?

I thought of a workaround, but there's got to be a better way:

public static function createFromFormat($f, $t, $tz=NULL){
    $dateTime = call_user_func(
        array('SerialDateTime', 'parent::createFromFormat'),
        $f, $t, $tz
    );
    $ret = new self();
    return $ret->setTimestamp($dateTime->getTimestamp());
}

Could I maybe use __callStatic and return call_user_func_array(array(__CLASS__ , 'parent::'.__FUNCTION__), func_get_args()); or something?

Too bad I can't magically convert DateTime to use late static bindings.

Answer 1

Like you already said & tried, override static method. Method createFromFormat by default returns the DateTime object, so you only need to fix returning part so it will return your object SerialDateTime instead of DateTime.

class SerialDateTime extends DateTime implements JsonSerializable {

    public function jsonSerialize()
    {
        return ['timestamp' => $this->getTimestamp()];
    }

    public static function createFromFormat($format, $time, $timezone = null)
    {
        if ($timezone) {
            $dt = parent::createFromFormat($format, $time, $timezone);
        } else {
            $dt = parent::createFromFormat($format, $time);
        }

        return new self($dt->format(self::W3C));
    }

}

echo json_encode(new SerialDateTime);
echo json_encode(SerialDateTime::createFromFormat('Y', '2013'));

It doesn't matter how you call static method createFromFormat, it will always return DateTime object; so all your ideas of automatically rewriting static methods will fail, because you need to modify method with new logic (return instance of other object), and this cannot be accomplished with auto-call-method-magic-or-something.

Late static bindings would be great if it were implemented in the DateTime::createFromFormat method like :

public static function createFromFormat($format, $time, $timezone = null)
{
    // logic of converting $time from format $format to some default format 
    return new static($newTime);
}

... but it is not ;( Source code