update docs

docs/extending.md

@@ -91,6 +91,219 @@ Always keep in mind that all extension files are merged together at runtime.
 That allows plugins overwriting the code from the main application or altering other plugins.
 </p>
 
+### Startup behaviour
+
+First, the root `app.extensions.json` is loaded by means of the special `Loader` service.
+The file can contain all the necessary declarations for an application to function,
+and having extra plugin files is fully optional.
+
+Next, the `Loader` traverses the `$references` metadata and loads additional files if provided.
+For the sake of speed the files are loaded in parallel,
+however, once everything is loaded, they are applied in the order of declaration.
+
+After all the external files are fetched, the `Loader` sorts them, removes the metadata properties
+and stacks the resulting JSON objects on top of each other.
+
+<p class="tip">
+Any top-level property name that starts with the `$` symbol is considered a metadata and does not participate in merge process.
+That allows a plugin to carry extra information for maintenance and visualisation purposes,
+for example: `$name`, `$version`, `$description`, `$license`, etc.
+</p>
+
+#### Merging properties
+
+There are no limits in the JSON structure and level of nesting.
+All objects are merged into a single set based on property keys and object IDs (for arrays).
+
+Before: Plugin 1
+
+```json
+{
+    "$name": "plugin1",
+    "plugin1.key": "value",
+    "plugin1.text": "string"
+}
+```
+
+Before: Plugin 2
+
+```json
+{
+    "$name": "plugin2",
+    "plugin2.key": "value",
+    "plugin1.text": "custom string"
+}
+```
+
+Final result:
+
+```json
+{
+    "plugin1.key": "value",
+    "plugin1.text": "custom string",
+    "plugin2.key": "value"
+}
+```
+
+Note that as a result we have two unique properties `plugin1.key` and `plugin2.key`,
+and also a `plugin1.text` that was first defined in the `Plugin 1`, but then overwritten by the `Plugin 2`.
+
+<p class="tip">
+JSON merging is very powerful concept as it gives you abilities to alter any base application settings,
+or toggle features in other plugins without rebuilding the application or corresponding plugin libraries.
+</p>
+
+#### Merging objects
+
+The complex objects are merged by properties. This process is recursive and has no limits for nesting levels.
+
+Before: Plugin 1
+
+```json
+{
+    "$name": "plugin1",
+    "features": {
+        "title": "some title",
+        "page1": {
+            "title": "page 1"
+        }
+    }
+}
+```
+
+Before: Plugin 2
+
+```json
+{
+    "$name": "plugin2",
+    "features": {
+        "page1": {
+            "title": "custom title"
+        },
+        "page2": {
+            "title": "page 2"
+        }
+    }
+}
+```
+
+Final result:
+
+```json
+{
+    "features": {
+        "title": "some title",
+        "page1": {
+            "title": "custom title"
+        },
+        "page2": {
+            "title": "page 2"
+        }
+    }
+}
+```
+
+As you can see, the unique properties get merged together in a single object.
+However the last non-unique property always wins and overwrites the previous value.
+
+As per current design it is not possible to delete any application property from the plugin.
+The loader engine supports only overwriting values on purpose.
+Many components however support the `disabled` property you can change from external definition:
+
+Before: Plugin 1
+
+```json
+{
+    "$name": "plugin1",
+    "feature1": {
+        "disabled": false,
+        "text": "some-feature",
+        "icon": "some-icon"
+    }
+}
+```
+
+Before: Plugin 2
+
+```json
+{
+    "$name": "plugin2",
+    "feature1": {
+        "disabled": true
+    }
+}
+```
+
+Final result:
+
+```json
+{
+    "feature1": {
+        "disabled": true,
+        "text": "some-feature",
+        "icon": "some-icon"
+    }
+}
+```
+
+You can find more details in the [Disabling Content](#disabling-content) section
+
+#### Merging arrays
+
+The extension `Loader` provides a special support for merging Arrays.
+By default, two collections will be merged into a single array unless objects have `id` properties.
+
+<p class="tip">
+If array contains two objects with the same `id` property, the objects will be merged rather than appended.
+</p>
+
+Before: Plugin 1
+
+```json
+{
+    "$name": "plugin1",
+    "features": [
+        { "text": "common 1" },
+        {
+            "id": "page1",
+            "text": "page 1"
+        }
+    ]
+}
+```
+
+Before: Plugin 2
+
+```json
+{
+    "$name": "plugin2",
+    "features": [
+        { "text": "common 2" },
+        {
+            "id": "page1",
+            "text": "custom page"
+        }
+    ]
+}
+```
+
+Final result:
+
+```json
+{
+    "features": [
+        { "text": "common 1" },
+        { "text": "common 2" },
+        {
+            "id": "page1",
+            "text": "custom page"
+        }
+    ]
+}
+```
+
+Note that objects with the same `page1` identifiers were merged while other unique entries were appended to the resulting array.
+
 ### Disabling content
 
 Most of the schema elements can be switched off by using the `disabled` property: