move SkMessageBus::Get out of header

src/core/SkMessageBus.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkMessageBus_DEFINED
 #define SkMessageBus_DEFINED
 
@@ skipping 26 matching lines @@
 
 private:
     SkMessageBus();
     static SkMessageBus* Get();
     static void New(SkMessageBus**);
 
     SkTDArray<Inbox*> fInboxes;
     SkMutex           fInboxesMutex;
 };
 
+// This must go in a single .cpp file, not some .h, or we risk creating more than one global
+// SkMessageBus per type when using shared libraries.
+#define DECLARE_SKMESSAGEBUS_MESSAGE(Message)             \
+    template <>                                           \
+    SkMessageBus<Message>* SkMessageBus<Message>::Get() { \
+        static SkMessageBus<Message>* bus = NULL;         \
+        SK_DECLARE_STATIC_ONCE(once);                     \
+        SkOnce(&once, &New, &bus);                        \
+        SkASSERT(bus != NULL);                            \
+        return bus;                                       \
+    }
+
 //   ----------------------- Implementation of SkMessageBus::Inbox -----------------------
 
 template<typename Message>
 SkMessageBus<Message>::Inbox::Inbox() {
     // Register ourselves with the corresponding message bus.
     SkMessageBus<Message>* bus = SkMessageBus<Message>::Get();
     SkAutoMutexAcquire lock(bus->fInboxesMutex);
     bus->fInboxes.push(this);
 }
 
@@ skipping 29 matching lines @@
 
 template <typename Message>
 SkMessageBus<Message>::SkMessageBus() {}
 
 template <typename Message>
 /*static*/ void SkMessageBus<Message>::New(SkMessageBus<Message>** bus) {
     *bus = new SkMessageBus<Message>();
 }
 
 template <typename Message>
-/*static*/ SkMessageBus<Message>* SkMessageBus<Message>::Get() {
-    // The first time this method is called, create the singleton bus for this message type.
-    static SkMessageBus<Message>* bus = NULL;
-    SK_DECLARE_STATIC_ONCE(once);
-    SkOnce(&once, &New, &bus);
-
-    SkASSERT(bus != NULL);
-    return bus;
-}
-
-template <typename Message>
 /*static*/ void SkMessageBus<Message>::Post(const Message& m) {
     SkMessageBus<Message>* bus = SkMessageBus<Message>::Get();
     SkAutoMutexAcquire lock(bus->fInboxesMutex);
     for (int i = 0; i < bus->fInboxes.count(); i++) {
         bus->fInboxes[i]->receive(m);
     }
 }
 
 #endif  // SkMessageBus_DEFINED