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Iterators
=========
Iterators allow you to traverse over collections of your resources in an
efficient and easy way. Currently there are two Iterators provided by
the SDK:
- **ResourceIterator**. The standard iterator class that implements
SPL's standard
`Iterator <http://php.net/manual/en/class.iterator.php>`__,
`ArrayAccess <http://www.php.net/manual/en/class.arrayaccess.php>`__
and `Countable <http://php.net/manual/en/class.countable.php>`__
interfaces. In short, this allows you to traverse this object (using
``foreach``), count its internal elements like an array (using
``count`` or ``sizeof``), and access its internal elements like an
array (using ``$iterator[1]``).
- **PaginatedIterator**. This is a child of ResourceIterator, and as
such inherits all of its functionality. The difference however is
that when it reaches the end of the current collection, it attempts
to construct a URL to access the API based on predictive paginated
collection templates.
Common behaviour
----------------
.. code-block:: php
$iterator = $computeService->flavorList();
There are two ways to traverse an iterator. The first is the longer,
more traditional way:
.. code-block:: php
while ($iterator->valid()) {
$flavor = $iterator->current();
// do stuff..
echo $flavor->id;
$iterator->next();
}
There is also a shorter and more intuitive version:
.. code-block:: php
foreach ($iterator as $flavor) {
// do stuff...
echo $flavor->id;
}
Because the iterator implements PHP's native ``Iterator`` interface, it
can inherit all the native functionality of traversible data structures
with ``foreach``.
Very important note
-------------------
Until now, users have been expected to do this:
.. code-block:: php
while ($flavor = $iterator->next()) {
// ...
}
which is **incorrect**. The single responsibility of ``next`` is to move
the internal pointer forward. It is the job of ``current`` to retrieve
the current element.
For your convenience, these two Iterator classes are fully backward
compatible: they exhibit all the functionality you'd expect from a
correctly implemented iterator, but they also allow previous behaviour.
Using paginated collections
---------------------------
For large collections, such as retrieving DataObjects from
CloudFiles/Swift, you need to use pagination. Each resource will have a
different limit per page; so once that page is traversed, there needs to
be another API call to retrieve to *next* page's resources.
There are two key concepts:
- **limit** is the amount of resources returned per page
- **marker** is the way you define a starting point. It is some form of
identifier that allows the collection to begin from a specific
resource
Resource classes
~~~~~~~~~~~~~~~~
When the iterator returns a current element in the internal list, it
populates the relevant resource class with all the data returned to the
API. In most cases, a ``stdClass`` object will become an instance of
``OpenCloud\Common\PersistentObject``.
In order for this instantiation to happen, the ``resourceClass`` option
must correspond to some method in the parent class that creates the
resource. For example, if we specify 'ScalingPolicy' as the
``resourceClass``, the parent object (in this case
``OpenCloud\Autoscale\Group``, needs to have some method will allows the
iterator to instantiate the child resource class. These are all valid:
1. ``Group::scalingGroup($data);``
2. ``Group::getScalingGroup($data);``
3. ``Group::resource('ScalingGroup', $data);``
where ``$data`` is the standard object. This list runs in order of
precedence.
Setting up a PaginatedIterator
------------------------------
.. code-block:: php
use OpenCloud\Common\Collection\PaginatedIterator;
$service = $client->computeService();
$flavors = PaginatedIterator::factory($service, array(
'resourceClass' => 'Flavor',
'baseUrl' => $service->getUrl('flavors')
'limit.total' => 350,
'limit.page' => 100,
'key.collection' => 'flavors'
));
foreach ($flavors as $flavor) {
echo $flavor->getId();
}
As you can see, there are a lot of configuration parameters to pass in -
and getting it right can be quite fiddly, involving a lot of API
research. For this reason, using the convenience methods like
``flavorList`` is recommended because it hides the complexity.
PaginatedIterator options
~~~~~~~~~~~~~~~~~~~~~~~~~
There are certain configuration options that the paginated iterator
needs to work. These are:
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| Name | Description | Type | Required | Default |
+=========================+===================================================================================================================================================================================================================================================+==============================+============+===============+
| resourceClass | The resource class that is instantiated when the current element is retrieved. This is relative to the parent/service which called the iterator. | string | Yes | - |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| baseUrl | The base URL that is used for making new calls to the API for new pages | ``Guzzle\Http\Url`` | Yes | - |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| limit.total | The total amount of resources you want to traverse in your collection. The iterator will stop as this limit is reached, regardless if there are more items in the list | int | No | 10000 |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| limit.page | The amount of resources each page contains | int | No | 100 |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| key.links | Often, API responses will contain "links" that allow easy access to the next page of a resource collection. This option specifies what that JSON element is called (its key). For example, for Rackspace Compute images it is ``images_links``. | string | No | links |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| key.collection | The top-level key for the array of resources. For example, servers are returned with this data structure: ``{"servers": [...]}``. The **key.collection** value in this case would be ``servers``. | string | No | ``null`` |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| key.collectionElement | Rarely used. But it indicates the key name for each nested resource element. KeyPairs, for example, are listed like this: ``{"keypairs": [ {"keypair": {...}} ] }``. So in this case the collectionElement key would be ``keypair``. | string | No | ``null`` |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| key.marker | The value used as the marker. It needs to represent a valid property in the JSON resource objects. Often it is ``id`` or ``name``. | string | No | name |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| request.method | The HTTP method used when making API calls for new pages | string | No | GET |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| request.headers | The HTTP headers to send when making API calls for new pages | array | No | ``array()`` |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| request.body | The HTTP entity body to send when making API calls for new pages | ``Guzzle\Http\EntityBody`` | No | ``null`` |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+
| request.curlOptions | Additional cURL options to use when making API calls for new pages | array | No | ``array()`` |
+-------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------+------------+---------------+