php - 魔法 setter/getter / setter/getter 未被调用

Question

在阅读Zend PHP Certification学习指南5.5中的OOP章节时，我发现了一个问题，其答案让我震惊。这个问题是:

class Magic
{
    public $a = "A";
    protected $b = array( "a" => "A" , "b" => "B" , "c" => "C" );
    protected $c = array( 1 , 2 , 3 );

    public function __get( $v )
    {
        echo "$v";
        return $this->b[$v];
    }

    public function __set( $var , $val )
    {
        echo "$var: $val,";
        $this->$var = $val;
    }
}

$m = new Magic();
echo $m->a . ", " . $m->b . ", " . $m->c . ", ";
$m->c = "CC";
echo $m->a . ", " . $m->b . ", " . $m->c . ", ";

这段代码的输出是:

b, c, A, B, C, c: CC, b, c, A, B, C

为什么这段代码不打印a，它是如何工作的？

Answer 1

__get 仅针对不存在或不可见的属性调用。换句话说，当你写

$obj->prop

如果定义了 prop 并且在当前上下文中可见，它将“按原样”返回，而不调用 __get。

例子:

class X {

    public   $pub = 1;
    private  $pri = 2;

    function __get($v) {
        echo "[get $v]\n";
        return 42;
    }

    function test() {
        echo $this->foo, "\n";  // __get invoked
        echo $this->pri, "\n";  // no __get
        echo $this->pub, "\n";  // no __get

    }
}

$x = new X;
$x->test();

echo $x->foo, "\n"; // __get invoked
echo $x->pri, "\n"; // __get invoked (property not visible)
echo $x->pub, "\n"; // no __get

这解释了为什么 magic->a 不调用 getter。现在，由于您还定义了 setter，magic->c = CC 实际上更改了类的 protected 成员，因此，当您回显 magic-> c 之后，this 仍然调用 getter(由于 c 的不可见性)，并且 getter 返回 this->b[c]，而不是this->c 的实际值。

这是您的代码，为清晰起见略微重写:

class Magic
{
    public $a = "publicA";
    protected $values = array( "a" => "valA" , "b" => "valB" , "c" => "valC" );
    protected $c = "oldC";

    public function __get( $v )
    {
        echo "[get $v]\n";
        return $this->values[$v];
    }

    public function __set( $var , $val )
    {
        echo "[set $var=$val]\n";
        $this->$var = $val;
    }
}

$m = new Magic();
echo $m->a . ", " . $m->b . ", " . $m->c . "\n";
$m->c = "newC";
echo $m->a . ", " . $m->b . ", " . $m->c . "\n";

结果:

[get b]      
[get c]
publicA, valB, valC  # no getter for `a` 
[set c=newC]
[get b]
[get c]              # getter still invoked for `c`
publicA, valB, valC  # no getter for `a`

给读者的练习:

如果将 echo 语句中的点 . 替换为逗号，为什么输出会不同:

$m = new Magic();
echo $m->a , ", " , $m->b , ", " , $m->c , "\n";
$m->c = "newC";
echo $m->a . ", " , $m->b , ", " , $m->c , "\n";