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Properties
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==========
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In PHP, class member variables are also called *properties*. They are part of a class definition and are used
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to represent the state of a class instance. In practice, you may often want to do some special handling when
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a property is being read or modified. For example, you may want to trim a string when it is being assigned
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to a `label` property. You could use the following code to achieve this task:
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```php
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$object->label = trim($label);
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```
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The drawback of the above code is that you have to call `trim()` everywhere whenever you modify the `label`
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property. And if in future, the `label` property has a new requirement, such as the first letter must be turned
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into upper case, you would have to modify all those places - a practice you want to avoid as much as possible.
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To solve this problem, Yii introduces a base class called [[yii\base\Object]] to support defining properties
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based on *getter* and *setter* class methods. If a class needs such a support, it should extend from
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[[yii\base\Object]] or its child class.
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> Info: Nearly every core class in the Yii framework extends from [[yii\base\Object]] or its child class.
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This means whenever you see a getter or setter in a core class, you can use it like a property.
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A getter method is a method whose name starts with the word `get`, while a setter method starts with `set`.
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The name after the `get` or `set` prefix defines the name of a property. For example, a getter `getLabel()` and/or
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a setter `setLabel()` defines a property named `label`, as shown in the following code:
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```php
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namespace app\components;
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use yii\base\Object;
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class Foo extend Object
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{
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private $_label;
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public function getLabel()
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{
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return $this->_label;
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}
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public function setLabel($value)
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{
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$this->_label = trim($value);
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}
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}
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```
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Properties defined by getters/setters can be used like class member variables. The main difference is that
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when such a property is being read, the corresponding getter method will be called; and when the property is
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being assigned, the corresponding setter method will be called. For example,
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```php
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// equivalent to $label = $object->getLabel();
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$label = $object->label;
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// equivalent to $object->setLabel('abc');
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$object->label = 'abc';
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```
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A property defined by a getter without a setter is *read only*. Trying to assign a value to such a property will cause
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an [[yii\base\InvalidCallException|InvalidCallException]]. Similarly, a property defined by a setter without a getter
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is *write only*, and trying to read such a property will also cause an exception. It is not common to have write-only
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properties.
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There are several special rules or limitations of the properties defined based on getters and setters.
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* The names of such properties are *case-insensitive*. For example, `$object->label` and `$object->Label` are the same.
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This is because PHP method names are case-insensitive.
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* If the name of such a property is the same as a class member variable, the latter will take precedence.
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For example, if the above `Foo` class has a member variable `label`, then the assignment `$object->label = 'abc'`
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will happen to the member variable instead of the setter `setLabel()`.
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* The properties do not support visibility. It makes no difference for the visibility of a property
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if the defining getter or setter method is public, protected or private.
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* The properties can only be defined by *non-static* getters and/or setters. Static methods do not count.
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Back to the problem we described at the very beginning, instead of calling `trim()` everywhere, we are calling it
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only within the setter `setLabel()`. And if a new requirement comes that the first letter of the label should
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be turned into upper case, we just need to modify the `setLabel()` method without touching any other code.
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