Add custom columns documentation

Author: knguyen1

extending/custom_columns_api.rst

@@ -0,0 +1,255 @@
+.. MusicBrainz Picard Documentation Project
+
+Custom Columns API
+==================
+
+Public API to define, register, sort, and persist custom columns for Picard's File and Album views.
+
+.. note::
+   This API was added in `PICARD-2103: Add API for custom columns (field ref, scripts, transforms, etc.) <https://github.com/metabrainz/picard/pull/2709>`_.
+
+Overview and Flow
+-----------------
+
+**Motivation**: Extend Picard's File/Album views with columns that compute values dynamically (from tags, scripts, or code) and integrate with sorting, sizing and visibility like built-ins.
+
+**Lifecycle at a glance**:
+
+- **Create**: Build a ``CustomColumn`` via factory helpers in ``picard.ui.itemviews.custom_columns.factory`` (e.g., ``make_field_column``, ``make_script_column``, ``make_callable_column``, ``make_transformed_column``) or construct ``CustomColumn`` with your own provider.
+- **Register**: Add the column to live views with ``registry.register(column, add_to_file_view=..., add_to_album_view=..., insert_after_key=...)``. This inserts into the mutable column collections used by the File/Album widgets.
+- **Persist** (optional): Store UI-defined columns as specs using ``picard.ui.itemviews.custom_columns.storage``. Use ``CustomColumnSpec`` + ``register_and_persist(spec)`` to save to config and auto-register; ``load_persisted_columns_once()`` restores saved columns on startup.
+- **Paint** (Qt): Once registered, the column participates like any other. Values come from the provider's ``evaluate(item)``. The header and sections are owned by the Qt views; width/resize hints are applied during registration (see ``registry._apply_column_width_to_headers``). Painting is handled by the standard header; only image columns overlay custom paint.
+
+**Key modules**:
+
+- ``custom_columns.__init__``: public API surface (factories, adapters, registry).
+- ``column.CustomColumn``: column type bridging providers to Picard's ``Column``.
+- ``factory``: helpers to create columns and infer sort behavior.
+- ``registry``: insertion/removal into File/Album views and live header updates.
+- ``providers``: reusable value providers and transforms.
+- ``sorting_adapters``: add ``.sort_key`` for ``ColumnSortType.SORTKEY`` sorting.
+- ``storage``: persist/load/register UI-defined column specs.
+
+Imports
+-------
+
+.. code-block:: python
+
+    from picard.ui.columns import ColumnAlign, ColumnSortType
+    from picard.ui.itemviews.custom_columns import (
+        CustomColumn,
+        make_field_column,
+        make_script_column,
+        make_transformed_column,
+        make_provider_column,
+        make_callable_column,
+        registry,
+        # Sorting adapters
+        CasefoldSortAdapter,
+        DescendingCasefoldSortAdapter,
+        NumericSortAdapter,
+        DescendingNumericSortAdapter,
+        LengthSortAdapter,
+        RandomSortAdapter,
+        ArticleInsensitiveAdapter,
+        CompositeSortAdapter,
+        NullsLastAdapter,
+        NullsFirstAdapter,
+        CachedSortAdapter,
+        ReverseAdapter,
+    )
+
+Quick Start
+-----------
+
+Field reference column:
+
+.. code-block:: python
+
+    col = make_field_column(
+        title="Bitrate",
+        key="~bitrate",  # same key you would pass to obj.column(key)
+        width=80,
+        align=ColumnAlign.RIGHT,
+    )
+    registry.register(col, add_to_file_view=True, add_to_album_view=False, insert_after_key="length")
+
+Script column:
+
+.. code-block:: python
+
+    script = "$if(%title%,$if2(%artist%,Unknown Artist) - $if2(%title%,Unknown Title),$if2(%albumartist%,Unknown Artist) - $if2(%album%,Unknown Album))"
+    col = make_script_column(
+        title="Artist – Title",
+        key="artist_title_script",
+        script=script,
+        width=280,
+        align=ColumnAlign.LEFT,
+    )
+    registry.register(col, add_to_album_view=True, insert_after_key="title")
+
+Transformed base field:
+
+.. code-block:: python
+
+    from picard.ui.itemviews.custom_columns.providers import FieldReferenceProvider
+
+    upper_title = make_transformed_column(
+        title="TITLE (UPPER)",
+        key="title_upper",
+        base=FieldReferenceProvider("title"),
+        transform=lambda s: s.upper(),
+    )
+    registry.register(upper_title)
+
+Callable-backed column:
+
+.. code-block:: python
+
+    from picard.item import Item
+
+    def file_ext(item: Item) -> str:
+        return item.column("~extension")
+
+    col = make_callable_column("Ext", key="ext", func=file_ext, sort_type=ColumnSortType.TEXT)
+    registry.register(col)
+
+Registration
+------------
+
+.. code-block:: python
+
+    registry.register(column,
+                      add_to_file_view=True,
+                      add_to_album_view=True,
+                      insert_after_key="title")
+
+- Inserts into live UI collections (``FILEVIEW_COLUMNS``, ``ALBUMVIEW_COLUMNS``).
+- ``insert_after_key`` places the column after an existing key; falls back to append if not found.
+- Idempotent per ``key`` (re-registration replaces existing instances). Use ``registry.unregister(key)`` to remove.
+
+Sorting
+-------
+
+- Default sort type is text. To supply a computed sort key, wrap the provider with an adapter that implements ``sort_key`` and use ``ColumnSortType.SORTKEY``.
+
+Case-insensitive sort for a script column:
+
+.. code-block:: python
+
+    base = make_script_column("Artist – Title", key="artist_title_script", script=script)
+    sorted_provider = CasefoldSortAdapter(base.provider)  # provides .sort_key
+    sorted_col = CustomColumn(
+        title=base.title,
+        key=base.key,
+        provider=sorted_provider,
+        width=base.width,
+        align=base.align,
+        sort_type=ColumnSortType.SORTKEY,
+    )
+    registry.register(sorted_col, insert_after_key="title")
+
+Available adapters (imported from ``picard.ui.itemviews.custom_columns``):
+
+- **CasefoldSortAdapter**: case-insensitive (str.casefold) text sort
+- **DescendingCasefoldSortAdapter**: descending case-insensitive text sort
+- **NumericSortAdapter**: numeric sort using parser (default float)
+- **DescendingNumericSortAdapter**: descending numeric (negated value)
+- **LengthSortAdapter**: sort by string length
+- **RandomSortAdapter**: deterministic pseudo-random by value and seed
+- **ArticleInsensitiveAdapter**: ignore leading articles (e.g. a, an, the)
+- **CompositeSortAdapter**: tuple sort from multiple key functions
+- **NullsFirstAdapter**: empty/whitespace values sort first
+- **NullsLastAdapter**: empty/whitespace values sort last
+- **CachedSortAdapter**: cache sort keys for performance
+- **ReverseAdapter**: invert existing sort key (numeric or string)
+
+You can also create a custom provider that implements ``sort_key`` to participate in ``SORTKEY`` sorting.
+
+Providers
+---------
+
+Protocols (typing only):
+
+.. code-block:: python
+
+    from picard.ui.itemviews.custom_columns import ColumnValueProvider, SortKeyProvider
+
+Built-ins:
+
+- **FieldReferenceProvider(key: str)**: returns ``obj.column(key)``; safe on missing keys.
+- **TransformProvider(base: ColumnValueProvider, transform: Callable[[str], str])**: applies a string transform.
+- **CallableProvider(func: Callable[[Item], str])**: wraps a Python callable.
+- Script provider is created via ``make_script_column(...)`` (do not instantiate directly).
+
+Factory helpers return a ``CustomColumn`` and infer a sane ``sort_type`` when possible:
+
+- ``make_field_column(...)``
+- ``make_script_column(...)`` (tunable: ``max_runtime_ms``, ``cache_size``, optional parser or factory)
+- ``make_transformed_column(...)``
+- ``make_callable_column(...)``
+- ``make_provider_column(...)``
+
+``CustomColumn`` signature:
+
+.. code-block:: python
+
+    CustomColumn(title, key, provider, width=None, align=ColumnAlign.LEFT,
+                 sort_type=ColumnSortType.TEXT, always_visible=False)
+
+Persistence Utilities
+---------------------
+
+Serialize specs to config and (optionally) auto-register columns.
+
+.. code-block:: python
+
+    from picard.ui.itemviews.custom_columns.storage import (
+        CustomColumnSpec, CustomColumnKind, TransformName,
+        build_column_from_spec,
+        load_specs_from_config, save_specs_to_config,
+        add_or_update_spec, delete_spec_by_key, get_spec_by_key,
+        register_and_persist, unregister_and_delete,
+        load_persisted_columns_once,
+    )
+
+    # Create and persist a script spec
+    spec = CustomColumnSpec(
+        title="Artist – Title",
+        key="artist_title_script",
+        kind=CustomColumnKind.SCRIPT,
+        expression=script,
+        width=280,
+        align="LEFT",
+        add_to_file_view=False,
+        add_to_album_view=True,
+        insert_after_key="title",
+    )
+    register_and_persist(spec)  # saves to config and registers in views
+
+    # Load and register all saved specs once (idempotent)
+    load_persisted_columns_once()
+
+    # Remove and delete
+    unregister_and_delete("artist_title_script")
+
+Notes:
+
+- ``CustomColumnSpec.align`` accepts "LEFT" or "RIGHT" (mapped to ``ColumnAlign``).
+- ``CustomColumnSpec.kind``: ``FIELD``, ``SCRIPT``, or ``TRANSFORM``.
+- ``TRANSFORM`` specs use ``expression`` as the base field and optional ``transform: TransformName``.
+- Registry insertion uses the spec's ``add_to_file_view``, ``add_to_album_view``, and ``insert_after_key``.
+
+Field Keys and Scripting
+------------------------
+
+- Field keys are the same strings used with ``obj.column(key)`` and Picard variables without percent signs (e.g. ``title``, ``albumartist``, ``~bitrate``).
+- Script expressions use the standard Picard scripting language (e.g. ``$if()``, ``$if2()``, ``%artist%``).
+- See ``picard.const.tags.ALL_TAGS`` for the authoritative list of variables.
+
+Runtime & Safety
+----------------
+
+- Script provider has configurable ``max_runtime_ms`` and internal caching; errors return empty strings rather than raising.
+- ``registry.register`` is UI-safe after the main window has initialized; re-entrant calls replace existing keys.
+- ``registry.unregister(key)`` removes from both views (if present).

extending/extending.rst

@@ -27,3 +27,4 @@ Scripts are stored within the user settings, and are managed from the :menuselec
    plugins
    scripts
    processing
+   custom_columns_api

index.rst

@@ -53,7 +53,9 @@ plugins and tutorials are provided when available rather than trying to reproduc
    /usage/other
    /usage/option_profiles
    /usage/command_processing
+   /usage/custom_columns
    /extending/extending
+   /extending/custom_columns_api
    /faq/faq