[yamlcomposer] YAML Composer Add-on for Enhanced YAML Preprocessing

CODEOWNERS

@@ -473,6 +473,7 @@
 /bundles/org.openhab.io.metrics/ @pravussum
 /bundles/org.openhab.io.neeo/ @morph166955
 /bundles/org.openhab.io.openhabcloud/ @kaikreuzer
+/bundles/org.openhab.io.yamlcomposer/ @jimtng
 /bundles/org.openhab.persistence.dynamodb/ @ssalonen
 /bundles/org.openhab.persistence.influxdb/ @lujop
 /bundles/org.openhab.persistence.inmemory/ @J-N-K

bundles/org.openhab.io.yamlcomposer/NOTICE

@@ -0,0 +1,13 @@
+This content is produced and maintained by the openHAB project.
+
+* Project home: https://www.openhab.org
+
+== Declared Project Licenses
+
+This program and the accompanying materials are made available under the terms
+of the Eclipse Public License 2.0 which is available at
+https://www.eclipse.org/legal/epl-2.0/.
+
+== Source Code
+
+https://github.com/openhab/openhab-addons

bundles/org.openhab.io.yamlcomposer/README.md

@@ -0,0 +1,329 @@
+# YAML Composer
+
+YAML Composer introduces extended YAML features that make openHAB configuration more modular, reusable, and maintainable. These features let you structure configuration as composable building blocks rather than large, repetitive files.
+
+The add-on loads enhanced-syntax YAML files from `OPENHAB_CONF/yamlcomposer/` and compiles them into fully resolved plain YAML written to `OPENHAB_CONF/yaml/composed/`.
+
+[[toc]]
+
+## Feature Summary
+
+YAML Composer adds several enhancements on top of standard YAML.
+Each feature addresses a different kind of reuse, composition, or abstraction to help you build cleaner and more maintainable YAML.
+
+| Feature                                    | Purpose                                                   | Typical Use                                                                                                    |
+|--------------------------------------------|-----------------------------------------------------------|----------------------------------------------------------------------------------------------------------------|
+| **Variables and Substitution (`${..}`)**   | Insert dynamic values or evaluate expressions             | Build labels, topics, IDs, or computed values                                                                  |
+| **Conditionals (`!if`)**                   | Conditionally include or exclude YAML blocks              | Enable or disable features when using packages or template flags                                               |
+| **Include (`!include`)**                   | Insert the contents of another file                       | Reuse YAML across files; parameterize reusable blocks                                                          |
+| **Templates (`!insert`)**                  | Reuse YAML defined within the same file                   | Local parameterized blocks; reusable channel or item fragments                                                 |
+| **Packages**                               | Bundle multiple top-level sections into one reusable unit | Define reusable device structures containing things, items, metadata; sourced from external files or templates |
+| **Anchors and Aliases (`&name`, `*name`)** | Define small, reusable YAML fragments                     | Static defaults, shared fields                                                                                 |
+| **Merge Keys (`<<:`)**                     | Combine mappings from multiple sources                    | Layer defaults, override fields, compose structures                                                            |
+
+Each feature has a dedicated documentation page:
+
+- [Variables and Substitution](doc/variables.md)
+- [Conditionals](doc/conditionals.md)
+- [Include](doc/include.md)
+- [Templates](doc/templates.md)
+- [Packages](doc/packages.md)
+- [Anchors and Aliases](doc/anchors.md)
+- [Merge Keys](doc/merge-keys.md)
+
+These features can be used independently, but they become especially powerful when combined.
+
+For a general introduction to YAML, see [YAML Basics](doc/basics.md).
+
+## Packaging Example
+
+**CONF/yamlcomposer/LivingRoom.yaml:**
+
+```yaml
+version: 1
+
+variables:
+  # Captures "LivingRoom" from the filename automatically
+  location: ${__FILE_NAME__} # => LivingRoom
+
+packages:
+  Light1: !include
+    file: $pkg/zigbee_light.inc.yaml
+    vars: &LIGHT_VARS
+      color_temperature: {} # Include color temperature feature
+      power: # Customize the power item
+        groups:
+          - gInsideLights
+          - gSmartLights
+
+  Light2: !include
+    file: $pkg/zigbee_light.inc.yaml
+    vars:
+      <<: *LIGHT_VARS
+```
+
+**CONF/yamlcomposer/pkg/zigbee_light.inc.yaml:**
+
+```yaml
+# This is the package file, i.e. the template for a zigbee light
+variables:
+  id: ${location}_${package_id}
+  thingid: ${id | lower | replace("_", "-")}
+  equipment: ${id}_Equipment
+  label: ${id | label}
+
+  # Set defaults
+  power: &DEFAULTS
+    groups: []
+  dimmer:
+    <<: *DEFAULTS
+
+things:
+  mqtt:topic:${thingid}:
+    bridge: mqtt:broker:mosquitto
+    channels:
+      power:
+        type: switch
+        config:
+          stateTopic: zigbee2mqtt/${thingid}/state
+          commandTopic: zigbee2mqtt/${thingid}/set/state
+
+      dimmer:
+        type: dimmer
+        config:
+          stateTopic: zigbee2mqtt/${thingid}/brightness
+          commandTopic: zigbee2mqtt/${thingid}/set/brightness
+
+      <<: !if
+        if: VARS.containsKey('color_temperature')
+        then:
+          color-temperature: !sub
+            type: dimmer
+            config:
+              stateTopic: zigbee2mqtt/${thingid}/color_temp
+              commandTopic: zigbee2mqtt/${thingid}/set/color_temp
+
+items:
+  ${equipment}:
+    type: Group
+    label: ${label} Equipment
+    tags: [Lightbulb]
+    groups: ${[location]}
+
+  ${id}:
+    type: Switch
+    label: ${label}
+    tags: [Control, Light]
+    groups: ${[equipment] + power.groups}
+    channel: mqtt:topic:${thingid}:power
+
+  ${id}_Dimmer:
+    type: Dimmer
+    label: ${label} Brightness
+    tags: [Control, Level]
+    groups: ${[equipment] + dimmer.groups}
+    channel: mqtt:topic:${thingid}:dimmer
+
+  <<: !if
+    if: VARS.containsKey('color_temperature')
+    then:
+      ${id}_CT:
+        type: Dimmer
+        label: ${label} Color Temperature
+        tags: [Control, ColorTemperature]
+        groups: ${[equipment] + color_temperature.groups}
+        channel: mqtt:topic:${thingid}:color-temperature
+```
+
+<details>
+<summary><b>Output in CONF/yaml/composed/LivingRoom.yaml:</b></summary>
+
+```yaml
+version: 1
+
+things:
+  mqtt:topic:livingroom-light1:
+    bridge: mqtt:broker:mosquitto
+    channels:
+      power:
+        type: switch
+        config:
+          stateTopic: zigbee2mqtt/livingroom-light1/state
+          commandTopic: zigbee2mqtt/livingroom-light1/set/state
+      dimmer:
+        type: dimmer
+        config:
+          stateTopic: zigbee2mqtt/livingroom-light1/brightness
+          commandTopic: zigbee2mqtt/livingroom-light1/set/brightness
+      color-temperature:
+        type: dimmer
+        config:
+          stateTopic: zigbee2mqtt/livingroom-light1/color_temp
+          commandTopic: zigbee2mqtt/livingroom-light1/set/color_temp
+
+  mqtt:topic:livingroom-light2:
+    bridge: mqtt:broker:mosquitto
+    channels:
+      power:
+        type: switch
+        config:
+          stateTopic: zigbee2mqtt/livingroom-light2/state
+          commandTopic: zigbee2mqtt/livingroom-light2/set/state
+      dimmer:
+        type: dimmer
+        config:
+          stateTopic: zigbee2mqtt/livingroom-light2/brightness
+          commandTopic: zigbee2mqtt/livingroom-light2/set/brightness
+      color-temperature:
+        type: dimmer
+        config:
+          stateTopic: zigbee2mqtt/livingroom-light2/color_temp
+          commandTopic: zigbee2mqtt/livingroom-light2/set/color_temp
+
+items:
+  LivingRoom_Light1_Equipment:
+    type: Group
+    label: Living Room Light 1 Equipment
+    tags:
+      - Lightbulb
+    groups:
+      - LivingRoom
+
+  LivingRoom_Light1:
+    type: Switch
+    label: Living Room Light 1
+    tags:
+      - Control
+      - Light
+    groups:
+      - LivingRoom_Light1_Equipment
+      - gInsideLights
+      - gSmartLights
+    channel: mqtt:topic:livingroom-light1:power
+
+  LivingRoom_Light1_Dimmer:
+    type: Dimmer
+    label: Living Room Light 1 Brightness
+    tags:
+      - Control
+      - Level
+    groups:
+      - LivingRoom_Light1_Equipment
+    channel: mqtt:topic:livingroom-light1:dimmer
+
+  LivingRoom_Light1_CT:
+    type: Dimmer
+    label: Living Room Light 1 Color Temperature
+    tags:
+      - Control
+      - ColorTemperature
+    groups:
+      - LivingRoom_Light1_Equipment
+    channel: mqtt:topic:livingroom-light1:color-temperature
+
+  LivingRoom_Light2_Equipment:
+    type: Group
+    label: Living Room Light 2 Equipment
+    tags:
+      - Lightbulb
+    groups:
+      - LivingRoom
+
+  LivingRoom_Light2:
+    type: Switch
+    label: Living Room Light 2
+    tags:
+      - Control
+      - Light
+    groups:
+      - LivingRoom_Light2_Equipment
+      - gInsideLights
+      - gSmartLights
+    channel: mqtt:topic:livingroom-light2:power
+
+  LivingRoom_Light2_Dimmer:
+    type: Dimmer
+    label: Living Room Light 2 Brightness
+    tags:
+      - Control
+      - Level
+    groups:
+      - LivingRoom_Light2_Equipment
+    channel: mqtt:topic:livingroom-light2:dimmer
+
+  LivingRoom_Light2_CT:
+    type: Dimmer
+    label: Living Room Light 2 Color Temperature
+    tags:
+      - Control
+      - ColorTemperature
+    groups:
+      - LivingRoom_Light2_Equipment
+    channel: mqtt:topic:livingroom-light2:color-temperature
+```
+
+</details>
+
+## Processing Overview
+
+YAML Composer reads enhanced-syntax YAML files and performs a compilation pass that expands all extended features into a single, fully resolved YAML document that openHAB can load.
+
+During compilation, YAML Composer performs the following steps:
+
+1. **YAML Parsing**: The source file is parsed into an internal structure.
+1. **Variable Substitution (`${..}`)**: Expressions are evaluated and injected.
+1. **Conditionals (`!if`)**: Conditional logic determines which blocks remain.
+1. **Template and Include Expansion**: `!insert` and `!include` bring in referenced content, often using resolved variables.
+1. **Package Expansion**: External or local packages are loaded and expanded into their component sections.
+1. **Recursive Merging**: Merge keys and package structures are combined into the main document.
+1. **Hidden Key Removal**: Keys beginning with `.` are removed from the final output.
+
+The resulting YAML contains:
+
+- all variables resolved
+- all conditionals evaluated
+- all templates and includes expanded
+- all anchors and merges applied
+- all packages integrated
+- all hidden keys removed
+
+This final compiled YAML is written to `OPENHAB_CONF/yaml/composed/`, where openHAB loads it as Things, Items, Metadata, and other configuration elements as defined by the Core YAML Configuration structure.
+
+## Hidden Keys
+
+Keys beginning with a dot (`.`) are treated as hidden. They:
+
+- exist only during compilation
+- are ideal for storing anchors, templates, or shared structures
+- keep visible configuration clean
+- are removed from the final output
+
+**Example:**
+
+```yaml
+.base-switch: &BASE_SWITCH
+  type: Switch
+  autoupdate: false
+
+items:
+  Light1:
+    <<: *BASE_SWITCH
+    label: Light One
+```
+
+## File Structure and Conventions
+
+YAML files can be organized freely, but the following conventions improve clarity and maintainability:
+
+- Place `variables:` and `templates:` near the top of the file.
+- Group reusable structures under hidden keys.
+- Use anchors for static fragments and includes for parameterized ones.
+- Keep packages in separate files when they represent reusable device or feature definitions.
+
+These conventions are optional but help keep complex configurations predictable and easy to navigate.
+
+### File naming convention
+
+- Use `*.yaml` or `*.yml` for main source files that produce composed output.
+- Use `*.inc.yaml` or `*.inc.yml` for include fragments referenced by `!include`.
+- Mixed-role files (both main and include) are not supported.