Handles Technical Documentation

Phabricator Technical Documentation (handles)

Technical overview of Handles.

Overview

Most objects in Phabricator have PHIDs, which are globally unique identifiers that look like PHID-USER-2zw4hwdt4i5b5ypikv6x. If you know the PHID for an object, you can load a handle for that object to get more information about it.

Handles are lightweight reference objects which provide some basic information common across all objects (like their type, icons, names, monograms, URIs, and whether they are open or closed). Applications don't need to know anything about other types of objects in order to load and use handles. There are uniform mechanisms available to load and work with handles which work across all types of objects in every application.

Loading Handles

To load handles, you'll usually call loadHandles(...) on the viewer:

$handles = $viewer->loadHandles($phids);

This returns a PhabricatorHandleList. This object behaves like an array, and you can access handle objects by using their PHIDs as indexes:

$handle = $handles[$phid];

Handles will always load, even if the PHID is invalid or the object it identifies is restricted or broken. In these cases, the handle will accurately represent the state of the associated object. This means that you generally do not need to check if a handle loaded.

Rendering Handles

After loading handles, you'll usually call renderHandle($phid) to render a link to an object:

$view = $handles->renderHandle($phid);

This returns a PHUIHandleView. The class exposes some methods which can adjust how the handle renders.

If you want to render a list of handles, you can use renderList():

$list_view = $handles->renderList();

This returns a PHUIHandleListView. This class also exposes some methods to adjust how the list renders.

Convenience methods for these operations are also available on the viewer object itself:

$view = $viewer->renderHandle($phid);
$list_view = $viewer->renderHandleList($phids);

When you only need to render a handle once, these methods make it easier.

Fetch Semantics

When you load and render handles through the viewer, the actual data fetching occurs just-in-time. Specifically, all of the required PHIDs are queued up until a concrete representation needs to be produced. Handles are then bulk loaded.

This means that, unlike most other types of data fetching, it's OK to single-fetch handles, because they won't really single-fetch. This code is correct and desirable:

$list->addProperty(pht('Pilot'), $viewer->renderHandle($pilot_phid));
$list->addProperty(pht('Copilot'), $viewer->renderHandle($copilot_phid));

If you're rendering a very large number of handles (for example, 100+ handles in a result list view) it's slightly more efficient to render them through a PhabricatorHandleList:

$handles = $viewer->loadHandles($phids);
foreach ($items as $item) {
  // ...
  $view = $handles->renderHandle($item->getPHID());
  // ...
}

This shaves off a tiny bit of internal bookkeeping overhead. This does not change the underlying semantics of the data fetch.

Handles are particularly well suited to use this just-in-time fetch pattern because they're ubiquitous and code essentially never makes decisions based on handles, so it's very rare that they need to be made concrete until final page rendering. Most other kinds of data do not have the same sort of application-level semantics. This generally makes other objects much less suitable to be fetched just-in-time.