## GstIterator

GstIterator — Object to retrieve multiple elements in a threadsafe way.

## Synopsis

    #include <gst/gst.h>

                        GstIterator;
    enum                GstIteratorItem;
    enum                GstIteratorResult;
    void                (*GstIteratorDisposeFunction)       (gpointer owner);
    GstIteratorResult   (*GstIteratorNextFunction)          (GstIterator *it,
                                                             gpointer *result);
    GstIteratorItem     (*GstIteratorItemFunction)          (GstIterator *it,
                                                             gpointer item);
    void                (*GstIteratorResyncFunction)        (GstIterator *it);
    void                (*GstIteratorFreeFunction)          (GstIterator *it);
    gboolean            (*GstIteratorFoldFunction)          (gpointer item,
                                                             GValue *ret,
                                                             gpointer user_data);
    #define             GST_ITERATOR                        (it)
    #define             GST_ITERATOR_LOCK                   (it)
    #define             GST_ITERATOR_COOKIE                 (it)
    #define             GST_ITERATOR_ORIG_COOKIE            (it)
    GstIterator*        gst_iterator_new                    (guint size,
                                                             GType type,
                                                             GMutex *lock,
                                                             guint32 *master_cookie,
                                                             GstIteratorNextFunction next,
                                                             GstIteratorItemFunction item,
                                                             GstIteratorResyncFunction resync,
                                                             GstIteratorFreeFunction free);
    GstIterator*        gst_iterator_new_list               (GType type,
                                                             GMutex *lock,
                                                             guint32 *master_cookie,
                                                             GList **list,
                                                             gpointer owner,
                                                             GstIteratorItemFunction item,
                                                             GstIteratorDisposeFunction free);
    GstIteratorResult   gst_iterator_next                   (GstIterator *it,
                                                             gpointer *elem);
    void                gst_iterator_resync                 (GstIterator *it);
    void                gst_iterator_free                   (GstIterator *it);
    void                gst_iterator_push                   (GstIterator *it,
                                                             GstIterator *other);
    GstIterator*        gst_iterator_filter                 (GstIterator *it,
                                                             GCompareFunc func,
                                                             gpointer user_data);
    GstIteratorResult   gst_iterator_fold                   (GstIterator *it,
                                                             GstIteratorFoldFunction func,
                                                             GValue *ret,
                                                             gpointer user_data);
    GstIteratorResult   gst_iterator_foreach                (GstIterator *it,
                                                             GFunc func,
                                                             gpointer user_data);
    gpointer            gst_iterator_find_custom            (GstIterator *it,
                                                             GCompareFunc func,
                                                             gpointer user_data);

## Description

A GstIterator is used to retrieve multiple objects from another object in a threadsafe way.

Various GStreamer objects provide access to their internal structures using an iterator.

The basic use pattern of an iterator is as follows:

Example 10. Using an iterator

       it = _get_iterator(object);
       done = FALSE;
       while (!done) {
         switch (gst_iterator_next (it, &item)) {
           case GST_ITERATOR_OK:
             ... use/change item here...
             gst_object_unref (item);
             break;
           case GST_ITERATOR_RESYNC:
             ...rollback changes to items...
             gst_iterator_resync (it);
             break;
           case GST_ITERATOR_ERROR:
             ...wrong parameter were given...
             done = TRUE;
             break;
           case GST_ITERATOR_DONE:
             done = TRUE;
             break;
         }
       }
       gst_iterator_free (it);

Last reviewed on 2005-11-09 (0.9.4)

## Details

### GstIterator

    typedef struct {
    } GstIterator;

GstIterator base structure. The values of this structure are protected for subclasses, use the methods to use the GstIterator.

### enum GstIteratorItem

    typedef enum {
      GST_ITERATOR_ITEM_SKIP	= 0,
      GST_ITERATOR_ITEM_PASS	= 1,
      GST_ITERATOR_ITEM_END		= 2
    } GstIteratorItem;

The result of a GstIteratorItemFunction.

`GST_ITERATOR_ITEM_SKIP` | Skip this item
`GST_ITERATOR_ITEM_PASS` | Return item
`GST_ITERATOR_ITEM_END` | Stop after this item.

### enum GstIteratorResult

    typedef enum {
      GST_ITERATOR_DONE	= 0,
      GST_ITERATOR_OK	= 1,
      GST_ITERATOR_RESYNC	= 2,
      GST_ITERATOR_ERROR	= 3
    } GstIteratorResult;

The result of `gst_iterator_next()`.

`GST_ITERATOR_DONE` | No more items in the iterator
`GST_ITERATOR_OK` | An item was retrieved
`GST_ITERATOR_RESYNC` | Datastructure changed while iterating
`GST_ITERATOR_ERROR` | An error happened

### GstIteratorDisposeFunction ()

    void                (*GstIteratorDisposeFunction)       (gpointer owner);

The function that will be called when a GList iterator is freed. The owner of the GList iterator can then clean up its resources.

`owner` : | the owner of the iterator

### GstIteratorNextFunction ()

    GstIteratorResult   (*GstIteratorNextFunction)          (GstIterator *it,
                                                             gpointer *result);

The function that will be called when the next element of the iterator should be retrieved.

Implementors of a GstIterator should implement this function and pass it to the constructor of the custom iterator. The function will be called with the iterator lock held.

`it` : | the iterator
`result` : | a pointer to hold the next item
Returns : | the result of the operation.

### GstIteratorItemFunction ()

    GstIteratorItem     (*GstIteratorItemFunction)          (GstIterator *it,
                                                             gpointer item);

The function that will be called after the next item of the iterator has been retrieved. This function will typically increase the refcount of the item or make a copy.

Implementors of a GstIterator should implement this function and pass it to the constructor of the custom iterator. The function will be called with the iterator lock held.

`it` : | the iterator
`item` : | the item being retrieved.
Returns : | the result of the operation.

### GstIteratorResyncFunction ()

    void                (*GstIteratorResyncFunction)        (GstIterator *it);

This function will be called whenever a concurrent update happened to the iterated datastructure. The implementor of the iterator should restart the iterator from the beginning and clean up any state it might have.

Implementors of a GstIterator should implement this function and pass it to the constructor of the custom iterator. The function will be called with the iterator lock held.

`it` : | the iterator

### GstIteratorFreeFunction ()

    void                (*GstIteratorFreeFunction)          (GstIterator *it);

This function will be called when the iterator is freed.

Implementors of a GstIterator should implement this function and pass it to the constructor of the custom iterator. The function will be called with the iterator lock held.

`it` : | the iterator

### GstIteratorFoldFunction ()

    gboolean            (*GstIteratorFoldFunction)          (gpointer item,
                                                             GValue *ret,
                                                             gpointer user_data);

A function to be passed to `gst_iterator_fold()`.

`item` : | the item to fold
`ret` : | a GValue collecting the result
`user_data` : | data passed to `gst_iterator_fold()`
Returns : | TRUE if the fold should continue, FALSE if it should stop.

### GST_ITERATOR()

    #define GST_ITERATOR(it)		((GstIterator*)(it))

Macro to cast to a GstIterator

`it` : | the GstIterator value

### GST_ITERATOR_LOCK()

    #define GST_ITERATOR_LOCK(it)		(GST_ITERATOR(it)->lock)

Macro to get the lock protecting the datastructure being iterated.

`it` : | the GstIterator to get the lock of

### GST_ITERATOR_COOKIE()

    #define GST_ITERATOR_COOKIE(it)		(GST_ITERATOR(it)->cookie)

Macro to get the cookie of a GstIterator. The cookie of the iterator is the value of the master cookie when the iterator was created. Whenever the iterator is iterated, the value is compared to the value of the master cookie. If they are different, a concurrent modification happened to the iterator and a resync is needed.

`it` : | the GstIterator to get the cookie of

### GST_ITERATOR_ORIG_COOKIE()

    #define GST_ITERATOR_ORIG_COOKIE(it)	(GST_ITERATOR(it)->master_cookie)

Macro to get a pointer to where the master cookie is stored. The master cookie protects the structure being iterated and gets updated whenever the datastructure changes.

`it` : | the GstIterator to get the master cookie of

### gst_iterator_new ()

    GstIterator*        gst_iterator_new                    (guint size,
                                                             GType type,
                                                             GMutex *lock,
                                                             guint32 *master_cookie,
                                                             GstIteratorNextFunction next,
                                                             GstIteratorItemFunction item,
                                                             GstIteratorResyncFunction resync,
                                                             GstIteratorFreeFunction free);

Create a new iterator. This function is mainly used for objects implementing the next/resync/free function to iterate a data structure.

For each item retrieved, the `item` function is called with the lock held. The `free` function is called when the iterator is freed.

`size` : | the size of the iterator structure
`type` : | GType of children
`lock` : | pointer to a GMutex.
`master_cookie` : | pointer to a guint32 to protect the iterated object.
`next` : | function to get next item
`item` : | function to call on each item retrieved
`resync` : | function to resync the iterator
`free` : | function to free the iterator
Returns : | the new GstIterator. MT safe.

### gst_iterator_new_list ()

    GstIterator*        gst_iterator_new_list               (GType type,
                                                             GMutex *lock,
                                                             guint32 *master_cookie,
                                                             GList **list,
                                                             gpointer owner,
                                                             GstIteratorItemFunction item,
                                                             GstIteratorDisposeFunction free);

Create a new iterator designed for iterating `list`.

`type` : | GType of elements
`lock` : | pointer to a GMutex protecting the list.
`master_cookie` : | pointer to a guint32 to protect the list.
`list` : | pointer to the list
`owner` : | object owning the list
`item` : | function to call for each item
`free` : | function to call when the iterator is freed
Returns : | the new GstIterator for `list`. MT safe.

### gst_iterator_next ()

    GstIteratorResult   gst_iterator_next                   (GstIterator *it,
                                                             gpointer *elem);

Get the next item from the iterator. For iterators that return refcounted objects, the returned object will have its refcount increased and should therefore be unreffed after usage.

`it` : | The GstIterator to iterate
`elem` : | pointer to hold next element
Returns : | The result of the iteration. Unref after usage if this is a refcounted object. MT safe.

### gst_iterator_resync ()

    void                gst_iterator_resync                 (GstIterator *it);

Resync the iterator. this function is mostly called after `gst_iterator_next()` returned `GST_ITERATOR_RESYNC`.

MT safe.

`it` : | The GstIterator to resync

### gst_iterator_free ()

    void                gst_iterator_free                   (GstIterator *it);

Free the iterator.

MT safe.

`it` : | The GstIterator to free

### gst_iterator_push ()

    void                gst_iterator_push                   (GstIterator *it,
                                                             GstIterator *other);

Pushes `other` iterator onto `it`. All calls performed on `it` are forwarded tot `other`. If `other` returns GST_ITERATOR_DONE, it is popped again and calls are handled by `it` again.

This function is mainly used by objects implementing the iterator next function to recurse into substructures.

MT safe.

`it` : | The GstIterator to use
`other` : | The GstIterator to push

### gst_iterator_filter ()

    GstIterator*        gst_iterator_filter                 (GstIterator *it,
                                                             GCompareFunc func,
                                                             gpointer user_data);

Create a new iterator from an existing iterator. The new iterator will only return those elements that match the given compare function `func`. `func` should return 0 for elements that should be included in the iterator.

When this iterator is freed, `it` will also be freed.

`it` : | The GstIterator to filter
`func` : | the compare function to select elements
`user_data` : | user data passed to the compare function
Returns : | a new GstIterator. MT safe.

### gst_iterator_fold ()

    GstIteratorResult   gst_iterator_fold                   (GstIterator *it,
                                                             GstIteratorFoldFunction func,
                                                             GValue *ret,
                                                             gpointer user_data);

Folds `func` over the elements of `iter`. That is to say, `proc` will be called as `proc` (object, `ret`, `user_data`) for each object in `iter`. The normal use of this procedure is to accumulate the results of operating on the objects in `ret`.

This procedure can be used (and is used internally) to implement the foreach and find_custom operations.

The fold will proceed as long as `func` returns TRUE. When the iterator has no more arguments, GST_ITERATOR_DONE will be returned. If `func` returns FALSE, the fold will stop, and GST_ITERATOR_OK will be returned. Errors or resyncs will cause fold to return GST_ITERATOR_ERROR or GST_ITERATOR_RESYNC as appropriate.

The iterator will not be freed.

`it` : | The GstIterator to fold over
`func` : | the fold function
`ret` : | the seed value passed to the fold function
`user_data` : | user data passed to the fold function
Returns : | A GstIteratorResult, as described above. MT safe.

### gst_iterator_foreach ()

    GstIteratorResult   gst_iterator_foreach                (GstIterator *it,
                                                             GFunc func,
                                                             gpointer user_data);

Iterate over all element of `it` and call the given function `func` for each element.

`it` : | The GstIterator to iterate
`func` : | the function to call for each element.
`user_data` : | user data passed to the function
Returns : | the result call to `gst_iterator_fold()`. The iterator will not be freed. MT safe.

### gst_iterator_find_custom ()

    gpointer            gst_iterator_find_custom            (GstIterator *it,
                                                             GCompareFunc func,
                                                             gpointer user_data);

Find the first element in `it` that matches the compare function `func`. `func` should return 0 when the element is found.

The iterator will not be freed.

This function will return NULL if an error or resync happened to the iterator.

`it` : | The GstIterator to iterate
`func` : | the compare function to use
`user_data` : | user data passed to the compare function
Returns : | The element in the iterator that matches the compare function or NULL when no element matched. MT safe.

## See Also

GstElement, GstBin