copyedit gui coding section

Author: Tom-Willemsen

doc/client/coding/Activators.md

@@ -0,0 +1,50 @@
+# Activators
+
+An Activator is the entry-point for a Plug-in - it gets created by the Eclipse framework when the Plug-in is first used.
+
+Activators are implemented using a singleton pattern; a simplified typical activator looks like:
+
+```java
+import org.osgi.framework.BundleActivator;
+import org.osgi.framework.BundleContext;
+
+public class Help implements BundleActivator {
+
+    private static BundleContext context;
+    private static Help instance;
+
+    static BundleContext getContext() {
+        return context;
+    }
+    
+    public Help() {
+        instance = this;
+    }
+    
+    @Override
+    public void start(BundleContext bundleContext) throws Exception {
+        Help.context = bundleContext;
+    }
+
+    public static Help getInstance() {
+        return instance;
+    }
+    
+    @Override
+    public void stop(BundleContext bundleContext) throws Exception {
+        Help.context = null;
+    }
+}
+```
+
+Activators may create and hold references to static data - for example models - so that they can later be referenced by
+other plugins that need access to those models (for example, views).
+
+:::{note}
+At first glance the above pattern may seem dangerous - in most cases a singleton would need to check whether an instance
+already exists and create one if not in `getInstance`, and would also need to use locking for thread safety.
+
+However, this pattern is safe for Activators, because the Eclipse framework guarantees that `start` will
+be called exactly once for a singleton plugin, before any other code can use the plugin, and that `stop` will only be
+called when no other plugin will be able to access it afterwards (for example, during platform shutdown).
+:::