| push-pull |
| --------- |
| |
| Normally a source element will push data to the downstream element using |
| the gst_pad_push() method. The downstream peer pad will receive the |
| buffer in the Chain function. In the push mode, the source element is the |
| driving force in the pipeline as it initiates data transport. |
| |
| It is also possible for an element to pull data from an upstream element. |
| The downstream element does this by calling gst_pad_pull_range() on one |
| of its sinkpads. In this mode, the upstream element is the driving force |
| in the pipeline as it initiates data transfer. |
| |
| It is important that the elements are in the correct state to handle a |
| push() or a pull_range() from the peer element. For push() based elements |
| this means that all downstream elements should be in the correct state and |
| for pull_range() based elements this means the upstream elements should |
| be in the correct state. |
| |
| Most sinkpads implement a chain function. This is the most common case. |
| sinkpads implementing a loop function will be the exception. Likewise |
| srcpads implementing a getrange function will be the exception. |
| |
| |
| state changes |
| ------------- |
| |
| The GstBin sets the state of all the sink elements. These are the elements |
| without source pads. |
| |
| Setting the state on an element will first activate all the srcpads and then |
| the sinkpads. For each of the sinkpads, gst_pad_check_pull_range() is |
| performed. If the sinkpad supports a loopfunction and the peer pad returns TRUE |
| from the GstPadCheckPullRange function, then the peer pad is activated first as |
| it must be in the right state to handle a _pull_range(). Note that the |
| state change of the element is not yet performed, just the activate function |
| is called on the source pad. This means that elements that implement a |
| getrange function must be prepared to get their activate function called |
| before their state change function. |
| |
| Elements that have multiple sinkpads that require all of them to operate |
| in the same mode (push/pull) can use the _check_pull_range() on all |
| their pads and can then remove the loop functions if one of the pads does |
| not support pull based mode. |
| |
| |