The original design of the filter base class required several specialized
base classes to control what information was pulled from the input
`DataSet` and provided to the derived class. Since the filter base class was
redesigned, the derived classes all get a `DataSet` and pull their own
information from it. Thus, most specialized filter base classes became
unnecessary and removed.
The one substantial exception was the `FilterField`. This filter base class
managed input and output arrays. This was kept separate from the base
`Filter` because not all filters need the ability to select this
information.
That said, this separation has not been particularly helpful. There are
several other features of `Filter` that does not apply to all subclasses.
Furthermore, there are several derived filters that are using `FilterField`
merely to pick a single part, like selecting a coordinate system, and
ignoring the rest of the abilities.
Thus, it makes more sense to deprecate `FilterField` and have these classes
inherit directly from `Filter`.
During the VTK-m 1.8 and 1.9 development, the filter infrastructure was
overhauled. Part of this created a completely new set of base classes. To
avoid confusion with the original filter base classes and ease transition,
the new filter base classes were named `NewFilter*`. Eventually after all
filters were transitioned, the old filter base classes were deprecated.
With the release of VTK-m 2.0, the old filter base classes are removed. The
"new" filter base classes are no longer new. Thus, they have been renamed
simply `Filter` (and `FilterField`).
With the major revision 2.0 of VTK-m, many items previously marked as
deprecated were removed. If updating to a new version of VTK-m, it is
recommended to first update to VTK-m 1.9, which will include the deprecated
features but provide warnings (with the right compiler) that will point to
the replacement code. Once the deprecations have been fixed, updating to
2.0 should be smoother.
VTK-m has a deprecation method that supports API changes in minor
releases. When an API change is made, the old API is marked with the
VTKM_DEPRECATED macro. If code attempts to use the old API, it still
works, but the compiler issues a warning that the thing is deprecated
and where to find the new API.
We have recently run into an issue when the API changes have a header
file renamed or moved. We still keep the old header file with the old
API, so code including that file will still work. However, sometimes
code expected the contents of that header file without directly
including that header file. In these cases, the code could get an error
about missing classes.
As an example, consider the change from `DynamicCellSet` to
`UnknownCellSet`/`UncertainCellSet`. The deprecated `DynamicCellSet` is
still around. But there is a lot of code that did not directly include
DynamicCellSet.h. This header file was necessarily included by
DataSet.h. Now, when this code uses `vtkm::cont::DynamicCellSet`, you
get a confusing error that the class does not exist. Backward
compatibility broken.
In response to this, we should be more careful about where we put the
deprecated API. Instead of containing the deprecated API, moved headers
should be empty except for a warning and an inclusion of the new header
file. The deprecated API should be moved to the new header file. For
example, in the case of `DynamicCellSet`, the implementation for the
deprecated `DynamicCellSet` is moved to UnknownCellSet.h, which is
included by anything that was including DynamicCellSet.h before.
