An in-depth analysis of the iterator pattern in PHP design patterns_PHP tutorial

The Iterator pattern provides an abstraction over a very common process: iteration over a collection of objects (or scalars) located in an unknown part of the object graph. Iteration can be performed in several different ways: iterating over array properties, collection objects, arrays, or even a query result set.

In the world of objects, the iterator pattern should maintain array-like functions and be regarded as a non-intrusive object facet. The Client class is often separated from the real object implementation and refers to the iterator interface. Whenever possible, we can pass a reference to the iterator instead of a concrete or abstract class that may change in the future.

Participant:
◆Client: Methods referencing the Iterator pattern perform a loop over a set of values ​​or objects .
◆Iterator: Abstraction on the iteration process, including next(), isFinished(), current() and other methods.
◆ConcreteIterators: Implement iteration on a specific set of objects, such as arrays, trees, combinations, collections, etc.
Through the Traversable interface, PHP natively supports the iterator mode. This interface is extended by Iterator and IteratorAggregate. These two sub-interfaces not only define a set of standard methods, but each Traversable object can be passed intact Foreach(), foreach is the main client of iterators, the Iterator implementation is the real iterator, and the IteratorAggregate is a Traversable object with other responsibilities, which returns an Iterator through the getIterator() method.

The Standard PHP Library is the only general-purpose object-oriented library bundled in PHP, defining additional interfaces and public classes. The OuterIterator implementation decorates an Iterator. CachingIterator and LimitIterator are two examples of this interface.

RecursiveIterator is an extension of the Iterator interface implemented for tree structures. It defines a set of additional methods to check whether the sub-object of the current element in the iteration exists. RecursiveArrayIterator and RecursiveDirectoryIterator are examples of implementations of this interface. These types of iterators can be used as-is, or bridged to a regular iterator contract using a RecursiveIteratorIterator. This OuterIterator implementation will perform depth-first or breadth-first traversal depending on the construction parameters.

When using RecursiveIteratorIterator, you can pass it to foreach, please see the following code examples to understand the different uses of RecursiveIterators and their superset Iterator. Finally, SeekableIterators adds a seek() method to the contract, which can be used to move the Iterator's internal state to a specific iteration point.

Note that iterator is a better abstraction than object set, because we can make InfiniteIterators, NoRewindIterators, etc., without being consistent with ordinary array arrays, therefore, Iterator Functions such as count() function are missing.

A complete list of SPL iterators can be found in the official PHP manual. Thanks to the strong support for PHP, most of the work of using the iterator pattern is included in the standard implementation. The following code example takes advantage of the functionality of the standard Iterator and RecursiveIterators.

Copy code The code is as follows:

        /** 
     * Collection that wraps a numeric array. 
     * All five public methods are needed to implement 
     * the Iterator interface. 
    */ 
    class Collection implements Iterator 
    { 
 private $_content; 
 private $_index = 0; 

 public function __construct(array $content) 
 { 
     $this->_content = $content; 
 } 

 public function rewind() 
 { 
     $this->_index = 0; 
 } 

 public function valid() 
 { 
     return isset($this->_content[$this->_index]); 
 } 

 public function current() 
 { 
     return $this->_content[$this->_index]; 
 } 

 public function key() 
 { 
     return $this->_index; 
 } 

 public function next() 
 { 
     $this->_index++; 
 } 
    } 

    $array = array('A', 'B', 'C', 'D'); 
    echo "Collection: "; 
    foreach (new Collection($array) as $key => $value) { 
 echo "$key => $value. "; 
    } 
    echo "n";
    /** 
     * Usually IteratorAggregate is the interface to implement. 
     * It has only one method, which must return an Iterator 
     * already defined as another class (e.g. ArrayIterator) 
     * Iterator gives a finer control over the algorithm, 
     * because all the hook points of Iterator' contract 
     * are available for implementation. 
    */ 
    class NumbersSet implements IteratorAggregate 
    { 
 private $_content; 

 public function __construct(array $content) 
 { 
     $this->_content = $content; 
 } 

 public function contains($number) 
 { 
     return in_array($number, $this->_content); 
 } 

 /** 
  * Only this method is necessary to implement IteratorAggregate. 
  * @return Iterator 
 */ 
 public function getIterator() 
 { 
     return new ArrayIterator($this->_content); 
 } 
    } 

    echo "NumbersSet: "; 
    foreach (new NumbersSet($array) as $key => $value) { 
 echo "$key => $value. "; 
    } 
    echo "n";
    // let's play with RecursiveIterator implementations 
    $it = new RecursiveArrayIterator(array( 
 'A', 
 'B', 
 array( 
     'C', 
     'D' 
 ), 
 array( 
     array( 
  'E', 
  'F' 
     ), 
     array( 
  'G', 
  'H', 
  'I' 
     ) 
 ) 
    )); 
    // $it is a RecursiveIterator but also an Iterator, 
    // so it loops normally over the four elements 
    // of the array. 
    echo "Foreach over a RecursiveIterator: "; 
    foreach ($it as $value) { 
 echo $value; 
 // but RecursiveIterators specify additional 
 // methods to explore children nodes 
 $children = $it->hasChildren() ? '{Yes}' : '{No}'; 
 echo $children, ' '; 
    }
echo "n";
// we can bridge it to a different contract via
// a RecursiveIteratorIterator, whose cryptic name
// should be read as 'an Iterator that spans over
// a RecursiveIterator'.
echo "Foreach over a RecursiveIteratorIterator: ";
foreach (new RecursiveIteratorIterator($it) as $value) {
echo $value;
}
echo "n";

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