fix(client): combine & prefer leave* transitions of from route

#566

lib/app/client.js

@@ -14,12 +14,23 @@ let router
 <%= (store ? 'let store' : '') %>
 
 function mapTransitions(Components, to, from) {
+  const resolve = t => (typeof t.options.transition === 'function')
+    ? t.options.transition(to, from)
+    : Object.assign({}, t.options.transition)
+
+  const resolveRoute = r => resolve(r.matched[0].components.default)
+
   return Components.map((Component) => {
-    let transition = Component.options.transition
-    if (typeof transition === 'function') {
-      return transition(to, from)
-    }
-    return transition
+    let transitions = Object.assign({}, to ? resolveRoute(to) : resolve(Component))
+    // Combine transitions
+    // Prefer leave* transitions of from route
+    let from_transitions = from ? resolveRoute(from) : {}
+    Object.keys(from_transitions).forEach(key=> {
+      if(from_transitions[key] && key.toLowerCase().indexOf('leave') !== -1) {
+        transitions[key] = from_transitions[key]
+      }
+    })
+    return transitions
   })
 }