__get 魔术方法 - 如何抛出和捕获属性重载的错误？

Question

I define what happens when a non-existent property is being accessed by taking advantage of the __get magic method.

So if $property->bla does not exist I will get null .

return (isset($this->$name)) ? $this->$name : null;

But I want to throw and catch the error for $property->bla->bla when I know that $property->bla does not exist.

with return (isset($this->$name)) ? $this->$name : null; return (isset($this->$name)) ? $this->$name : null; I will get this error below,

<b>Notice</b>:  Trying to get property of non-object in...

So I use throw and catch error in my class,

class property {

public function __get($name)
{
    //return (isset($this->$name)) ? $this->$name : null;

    try {
        if (!isset($this->$name)) {
          throw new Exception("Property $name is not defined");
        }
        return $this->$name;
      }
      catch (Exception $e) {

        return $e->getMessage(); 
      }
}

}

But the result is not what I want because it throw the error message ("Property $name is not defined") instead of null for $property->bla .

How can I make it to throw error message only for $property->bla->bla , $property->bla->bla->bla , and so on?

Answer 1

You can't.

Within the scope of the __get() function, you only know about $property->blah . You have no clue what comes after it, because that's the functionality of the language.

Note the order of evaluations on $foo = $property->blah->blah2->blah3 :

1. $temp1 = $property->blah;
2. $temp1 = $temp1->blah2;
3. $foo = $temp1->blah3;

Of course, $temp1 is fictitious, but this is essentially what happens in the execution of that statement. Your __get() call is only aware of the first call in that list, and nothing more. What you can do is handle the error on the calling side, with property_exists() or check for null :

$temp = $property->blah;
if( $temp === null || !property_exists( $temp, 'blah2')) {
    throw new Exception("Bad things!");
}

Note that property_exists() will fail if the object returned in $temp also relies on __get() , but that was not clear in the OP.

Answer 2

Actually, when you calling $property->foo->bar and $property class have magic __get method, you can verify only foo property and throw error only for it.

But if your foo is also an instance of the same class (or similar class with the same magic __get method) you can also verify bar property and throw the next error.

For example:

class magicLeaf {
    protected $data;
    public function __construct($hashTree) {
        $this->data = $hashTree;
    }
    // Notice that we returning another instance of the same class with subtree
    public function __get($k) {
        if (array_key_exists($k, $this->data)) {
             throw new Exception ("Property $name is not defined");
        }
        if (is_array($this->data[$k])) { // lazy convert arrays to magicLeaf classes
            $this->data[$k] = new self($this->data[$k]);
        }
        return $this->data[$k];
    }
 }

And now just example of using it:

$property = new magicLeaf(array(
    'foo' => 'root property',
    'bar' => array(
        'yo' => 'Hi!',
        'baz' => array(
            // dummy
        ),
    ),
));
$property->foo; // should contain 'root property' string
$property->bar->yo; // "Hi!"
$property->bar->baz->wut; // should throw an Exception with "Property wut is not defined"

So now you can see how to made it almost like you want.

Now just modify a little your __construct and magic __get methods and add new param to know where we are at each level:

...
    private $crumbs;
    public function __construct($hashTree, $crumbs = array()) {
        $this->data = $hashTree;
        $this->crumbs = $crumbs;
    }
    public function __get($k) {
        if (array_key_exists($k, $this->data)) {
             $crumbs = join('->', $this->crumbs);
             throw new Exception ("Property {$crumbs}->{$name} is not defined");
        }
        if (is_array($this->data[$k])) { // lazy convert arrays to magicLeaf classes
            $this->data[$k] = new self($this->data[$k], $this->crumbs + array($k));
        }
        return $this->data[$k];
    }
...

I think it should works like you want.