[ACA-1814] Documentation on extending the Content Metadata (#677)

docs/extending.md

@@ -30,6 +30,7 @@ You can create plugins that change, toggle or extend the following areas:
   - toolbar entries
     - buttons
     - "More actions" buttons
+- Content metadata presets (used on `Properties` tab)
 
 Extensions can also:
 
@@ -847,6 +848,7 @@ The ACA supports the following set of extension points:
 - Context Menu
 - Viewer
 - Sidebar (aka Info Drawer)
+- Content metadata presets (for `Properties` tab)
 
 All the customisations are stored in the `features` section of the configuration file:
 
@@ -867,7 +869,8 @@ All the customisations are stored in the `features` section of the configuration
       "openWith": [],
       "content": []
     },
-    "sidebar": []
+    "sidebar": [],
+    "content-metadata-presets": []
   }
 }
 ```
@@ -1361,6 +1364,112 @@ and invoke it from the custom `Open With` menu entry called `Snackbar`.
 
 As with other content actions, custom plugins can disable, update or extend `Open With` actions.
 
+### Content metadata presets
+
+The content metadata presets are needed by the [Content Metadata Component](https://alfresco.github.io/adf-component-catalog/components/ContentMetadataComponent.html#readme) to render the properties of metadata aspects for a given node. 
+The different aspects and their properties are configured in the `app.config.json` file, but they can also be set on runtime through extension files.
+
+Configuring these presets from `app.extensions.json` will overwrite the default application setting. 
+Settings them from custom plugins allows user to disable, update or extend these presets.
+Check out more info about merging extensions [here](#merging-properties).
+
+The `content-metadata-presets` elements can be switched off by setting the `disabled` property.
+This can be applied also for nested items, allowing disabling down to aspect level. 
+
+<p class="tip">
+In order to modify or disable existing entries, you need to know the id of the target element, along with its parents ids.
+</p>
+
+Your extensions can perform the following actions at runtime:
+* Add new presets items
+* Add new items to existing presets at any level
+* Disable specific items down to the aspect level
+* Modify any existing item based on id.  
+
+Regarding properties, you can either:
+ * add new properties to existing aspect, or 
+ * redefine the properties of an aspect. 
+ 
+Check out this code snippet to see how you can overwrite the properties for `exif:exif` aspect from external plugin:
+```json
+  {
+     "$schema": "../../../extension.schema.json",
+     "$version": "1.0.0",
+     "$name": "plugin1",
+
+     "features": {
+       "content-metadata-presets": [
+         {
+           "id": "app.content.metadata.custom",
+           "custom": [
+             {
+               "id": "app.content.metadata.customGroup",
+               "items": [
+                 {
+                   "id": "app.content.metadata.exifAspect",
+                   "disabled": true
+                 },
+                 {
+                   "id": "app.content.metadata.exifAspect2",
+                   "aspect": "exif:exif",
+                   "properties": [
+                     "exif:orientation",
+                     "exif:manufacturer",
+                     "exif:model",
+                     "exif:software"
+                   ]
+                 }
+               ]
+             }
+           ]
+         }
+       ]
+     }
+   }
+``` 
+This external plugin disables the initial `exif:exif` aspect already defined in the `app.extensions.json` and defines other properties for the `exif:exif` aspect. 
+Here is the initial setting from `app.extension.json`:
+```json
+...
+    "content-metadata-presets": [
+      {
+        "id": "app.content.metadata.custom",
+        "custom": [
+          {
+            "id": "app.content.metadata.customGroup",
+            "title": "APP.CONTENT_METADATA.EXIF_GROUP_TITLE",
+            "items": [
+              {
+                "id": "app.content.metadata.exifAspect",
+                "aspect": "exif:exif",
+                "properties": [
+                  "exif:pixelXDimension",
+                  "exif:pixelYDimension",
+                  "exif:dateTimeOriginal",
+                  "exif:exposureTime",
+                  "exif:fNumber",
+                  "exif:flash",
+                  "exif:focalLength",
+                  "exif:isoSpeedRatings",
+                  "exif:orientation",
+                  "exif:manufacturer",
+                  "exif:model",
+                  "exif:software"
+                ]
+              }
+            ]
+          }
+        ]
+      }
+    ]
+...
+
+``` 
+<p class="tip">
+In order to allow extending the content-metadata presets, the settings from `app.config.json` must be copied to the `app.extensions.json` and ids must be added to all its items.
+Having ids would allow external plugins to extend the current setting.
+</p>
+
 ## Registration
 
 You can use `ExtensionService` to register custom components, authentication guards,