Drupal 7 Module Development

Chapter 11
A read-only stream wrapper like ours needs to perform two main functions. First, it
needs to translate a URI like twitpic://y6vvv/thumb to a URL or path that can be
opened and read. Second, it needs to be able to open a file handle to this resource so
that developers can get the necessary data.
To manage the first requirement, we have implemented getExternalURL(). Any
class implementing DrupalStreamWrapperInterface is required to override this
function with their own implementation. This code is pretty straightforward; we just
parse the object's URI, set some appropriate defaults, and return an appropriately
structured Twitpic API URL:
function getExternalUrl() {
// Get image_id and size from the URI into an array.
$target = file_uri_target($this->uri);
$options = array_combine(
array('image_id', 'size'),
explode('/', $target)
);
Note the use of file_uri_target() to retrieve the target information from the URI.
This is a helper function provided by the Drupal File API to make it easier to parse
stream wrapper URIs. You can also call file_uri_scheme() to retrieve the scheme
from a URI.
The stream_open() function is similarly straightforward. This will get called
when a developer tries to open a resource handled by our stream wrapper using
PHP functions like fopen() or file_get_contents().This function takes four
arguments, and needs to return FALSE or a handle to our resource.
The first argument is our wrapper's URI. The second argument, $mode, indicates
whether the stream should be opened for reading and/or writing, as well as
other flags. Any mode can have b appended to it, to indicate that the file should be
opened in binary mode. (So where r indicates read-only, rb indicates read-only in
binary mode.)
$allowed_modes = array('r', 'rb');
if (!in_array($mode, $allowed_modes)) {
return FALSE;
}
We are implementing a read-only scheme, so if we get any mode other than r or rb
we return FALSE.
[ 323 ]