chore(modeling/BpmnLayouter): simplify manhattan layout logic

lib/features/modeling/BpmnLayouter.js

@@ -61,41 +61,8 @@ BpmnLayouter.prototype.layoutConnection = function(connection, hints) {
 
   // manhattan layout sequence / message flows
   if (is(connection, 'bpmn:MessageFlow')) {
-    manhattanOptions = {
-      preferredLayouts: [ 'v:v' ]
-    };
+    manhattanOptions = getMessageFlowManhattanOptions(source, target);
 
-    if (is(target, 'bpmn:Participant')) {
-      manhattanOptions = {
-        preferredLayouts: [ 'straight', 'v:v' ]
-      };
-    }
-
-    if (isExpandedSubProcess(target)) {
-      manhattanOptions = {
-        preferredLayouts: [ 'straight', 'v:v' ]
-      };
-    }
-
-    if (isExpandedSubProcess(source) && is(target, 'bpmn:FlowNode')) {
-      manhattanOptions = {
-        preferredLayouts: [ 'straight', 'v:v' ],
-        preserveDocking: isExpandedSubProcess(target) ? 'source' : 'target'
-      };
-    }
-
-    if (is(source, 'bpmn:Participant') && is(target, 'bpmn:FlowNode')) {
-      manhattanOptions = {
-        preferredLayouts: [ 'straight', 'v:v' ],
-        preserveDocking: 'target'
-      };
-    }
-
-    if (is(target, 'bpmn:Event')) {
-      manhattanOptions = {
-        preferredLayouts: [ 'v:v' ]
-      };
-    }
   } else
 
 
@@ -174,6 +141,50 @@ function getAttachOrientation(attachedElement) {
 }
 
 
+function getMessageFlowManhattanOptions(source, target) {
+  return {
+    preferredLayouts: [ 'straight', 'v:v' ],
+    preserveDocking: getMessageFlowPreserveDocking(source, target)
+  };
+}
+
+
+function getMessageFlowPreserveDocking(source, target) {
+
+  // (1) docking element connected to participant has precedence
+
+  if (is(target, 'bpmn:Participant')) {
+    return 'source';
+  }
+
+  if (is(source, 'bpmn:Participant')) {
+    return 'target';
+  }
+
+  // (2) docking element connected to expanded sub-process has precedence
+
+  if (isExpandedSubProcess(target)) {
+    return 'source';
+  }
+
+  if (isExpandedSubProcess(source)) {
+    return 'target';
+  }
+
+  // (3) docking event has precedence
+
+  if (is(target, 'bpmn:Event')) {
+    return 'target';
+  }
+
+  if (is(source, 'bpmn:Event')) {
+    return 'source';
+  }
+
+  return null;
+}
+
+
 function getConnectionDocking(point, shape) {
   return point ? (point.original || point) : getMid(shape);
 }