plugin_services.md: add flowgraph

buildUML/basics/getting-service.puml

@@ -0,0 +1,53 @@
+@startuml
+:getService;
+note right
+  In any thread.
+  Get on demand only.
+  Do not cache result.
+  Do not request in constructor unless needed.
+end note
+
+if (Is Light Service) then (yes)
+else (no)
+  if (Is Service Declaration Found) then (yes)
+  else (no)
+    :Return ""null"";
+    detach
+  endif
+endif
+
+if (Is Created and Initialized?) then (yes)
+else (no)
+  if (Is Container Active?) then (active)
+    partition "synchronized on service class" {
+      if (Is Created and Initialized?) then (yes)
+      else (no)
+        if (Is Initializing?) then (yes)
+          :Throw ""PluginException""
+          Cyclic Service Initialization;
+          detach
+        else (no)
+          partition "non cancelable" {
+            :Create Instance]
+            note right
+              Avoid getting other services to reduce initialization tree.
+              As less dependencies, as more faster and reliable.
+            end note
+
+            :Register to be Disposed on Container Dispose
+            if Implements ""Disposable""]
+            :Load Persistent State
+            if Implements ""PersistentStateComponent""]
+          }
+        endif
+      endif
+    }
+  else (disposed or dispose in progress)
+    :Throw ""ProcessCanceledException"";
+    detach
+  endif
+endif
+
+:Return Instance;
+
+@enduml

topics/basics/plugin_structure/plugin_services.md

@@ -99,6 +99,12 @@ val projectService = project.service<MyProjectService>()
 
 </tabs>
 
+<procedure title="Getting Service Flow" initial-collapse-state="collapsed">
+
+![Getting Service](getting_service.svg)
+
+</procedure>
+
 ## Project Service Sample
 This minimal sample shows [Light Service](#light-services) `ProjectService` interacting with another project level service `AnotherService` (not shown here).