Add "Link Cels" command in Timeline popup

From a comment in issue #746

data/gui.xml

@@ -748,6 +748,7 @@
       <separator />
       <item command="ClearCel" text="&amp;Clear" />
       <item command="UnlinkCel" text="&amp;Unlink" />
+      <item command="LinkCels" text="&amp;Link Cels" />
     </menu>
 
     <menu id="cel_movement_popup">

src/app/CMakeLists.txt

@@ -185,6 +185,7 @@ add_library(app-lib
   commands/cmd_layer_from_background.cpp
   commands/cmd_layer_properties.cpp
   commands/cmd_layer_visibility.cpp
+  commands/cmd_link_cels.cpp
   commands/cmd_load_mask.cpp
   commands/cmd_load_palette.cpp
   commands/cmd_mask_all.cpp

src/app/cmd/copy_cel.cpp

@@ -33,11 +33,12 @@ using namespace doc;
 
 CopyCel::CopyCel(
   LayerImage* srcLayer, frame_t srcFrame,
-  LayerImage* dstLayer, frame_t dstFrame)
+  LayerImage* dstLayer, frame_t dstFrame, bool continuous)
   : m_srcLayer(srcLayer)
   , m_dstLayer(dstLayer)
   , m_srcFrame(srcFrame)
   , m_dstFrame(dstFrame)
+  , m_continuous(continuous)
 {
 }
 
@@ -78,7 +79,7 @@ void CopyCel::onExecute()
     dstImage = dstCel->imageRef();
 
   bool createLink =
-    (srcLayer == dstLayer && dstLayer->isContinuous());
+    (srcLayer == dstLayer && m_continuous);
 
   // For background layer
   if (dstLayer->isBackground()) {

src/app/cmd/copy_cel.h

@@ -26,7 +26,7 @@ namespace cmd {
   public:
     CopyCel(
       LayerImage* srcLayer, frame_t srcFrame,
-      LayerImage* dstLayer, frame_t dstFrame);
+      LayerImage* dstLayer, frame_t dstFrame, bool continuous);
 
   protected:
     void onExecute() override;
@@ -35,6 +35,7 @@ namespace cmd {
   private:
     WithLayer m_srcLayer, m_dstLayer;
     frame_t m_srcFrame, m_dstFrame;
+    bool m_continuous;
   };
 
 } // namespace cmd

src/app/cmd/copy_frame.cpp

@@ -46,7 +46,7 @@ void CopyFrame::onExecute()
     if (layer->isImage())  {
       executeAndAdd(new cmd::CopyCel(
           static_cast<LayerImage*>(layer), fromFrame,
-          static_cast<LayerImage*>(layer), m_newFrame));
+          static_cast<LayerImage*>(layer), m_newFrame, layer->isContinuous()));
     }
   }
 }

src/app/cmd/move_cel.cpp

@@ -35,11 +35,12 @@ using namespace doc;
 
 MoveCel::MoveCel(
   LayerImage* srcLayer, frame_t srcFrame,
-  LayerImage* dstLayer, frame_t dstFrame)
+  LayerImage* dstLayer, frame_t dstFrame, bool continuous)
   : m_srcLayer(srcLayer)
   , m_dstLayer(dstLayer)
   , m_srcFrame(srcFrame)
   , m_dstFrame(dstFrame)
+  , m_continuous(continuous)
 {
 }
 
@@ -80,7 +81,7 @@ void MoveCel::onExecute()
     dstImage = dstCel->imageRef();
 
   bool createLink =
-    (srcLayer == dstLayer && dstLayer->isContinuous());
+    (srcLayer == dstLayer && m_continuous);
 
   // For background layer
   if (dstLayer->isBackground()) {

src/app/cmd/move_cel.h

@@ -26,7 +26,7 @@ namespace cmd {
   public:
     MoveCel(
       LayerImage* srcLayer, frame_t srcFrame,
-      LayerImage* dstLayer, frame_t dstFrame);
+      LayerImage* dstLayer, frame_t dstFrame, bool continuous);
 
   protected:
     void onExecute() override;
@@ -35,6 +35,7 @@ namespace cmd {
   private:
     WithLayer m_srcLayer, m_dstLayer;
     frame_t m_srcFrame, m_dstFrame;
+    bool m_continuous;
   };
 
 } // namespace cmd

src/app/commands/cmd_link_cels.cpp

@@ -0,0 +1,113 @@
+// Aseprite
+// Copyright (C) 2001-2015  David Capello
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License version 2 as
+// published by the Free Software Foundation.
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include "app/app.h"
+#include "app/cmd/copy_cel.h"
+#include "app/commands/command.h"
+#include "app/context_access.h"
+#include "app/modules/gui.h"
+#include "app/transaction.h"
+#include "app/ui/main_window.h"
+#include "app/ui/status_bar.h"
+#include "app/ui/timeline.h"
+#include "doc/cel.h"
+#include "doc/layer.h"
+#include "doc/sprite.h"
+
+namespace app {
+
+class LinkCelsCommand : public Command {
+public:
+  LinkCelsCommand();
+  Command* clone() const override { return new LinkCelsCommand(*this); }
+
+protected:
+  bool onEnabled(Context* context) override;
+  void onExecute(Context* context) override;
+};
+
+LinkCelsCommand::LinkCelsCommand()
+  : Command("LinkCels",
+            "Links Cels",
+            CmdRecordableFlag)
+{
+}
+
+bool LinkCelsCommand::onEnabled(Context* context)
+{
+  if (context->checkFlags(ContextFlags::ActiveDocumentIsWritable)) {
+    // TODO the range of selected frames should be in doc::Site.
+    Timeline::Range range = App::instance()->getMainWindow()->getTimeline()->range();
+    return (range.enabled() && range.frames() > 1);
+  }
+  else
+    return false;
+}
+
+void LinkCelsCommand::onExecute(Context* context)
+{
+  ContextWriter writer(context);
+  Document* document(writer.document());
+  bool nonEditableLayers = false;
+  {
+    // TODO the range of selected frames should be in doc::Site.
+    Timeline::Range range = App::instance()->getMainWindow()->getTimeline()->range();
+    if (!range.enabled())
+      return;
+
+    Transaction transaction(writer.context(), friendlyName());
+    Sprite* sprite = writer.sprite();
+    frame_t begin = range.frameBegin();
+    frame_t end = range.frameEnd();
+
+    for (LayerIndex layerIdx = range.layerBegin(); layerIdx <= range.layerEnd(); ++layerIdx) {
+      Layer* layer = sprite->indexToLayer(layerIdx);
+      if (!layer->isImage())
+        continue;
+
+      if (!layer->isEditable()) {
+        nonEditableLayers = true;
+        continue;
+      }
+
+      LayerImage* layerImage = static_cast<LayerImage*>(layer);
+      for (frame_t frame=begin; frame < end+1; ++frame) {
+        Cel* cel = layerImage->cel(frame);
+        if (cel) {
+          for (frame = cel->frame()+1;
+               frame < end+1; ++frame) {
+            transaction.execute(
+              new cmd::CopyCel(
+                layerImage, cel->frame(),
+                layerImage, frame,
+                true));         // true = force links
+          }
+          break;
+        }
+      }
+    }
+
+    transaction.commit();
+  }
+
+  if (nonEditableLayers)
+    StatusBar::instance()->showTip(1000,
+      "There are locked layers");
+
+  update_screen_for_document(document);
+}
+
+Command* CommandFactory::createLinkCelsCommand()
+{
+  return new LinkCelsCommand;
+}
+
+} // namespace app

src/app/commands/commands_list.h

@@ -60,6 +60,7 @@ FOR_EACH_COMMAND(Launch)
 FOR_EACH_COMMAND(LayerFromBackground)
 FOR_EACH_COMMAND(LayerProperties)
 FOR_EACH_COMMAND(LayerVisibility)
+FOR_EACH_COMMAND(LinkCels)
 FOR_EACH_COMMAND(LoadMask)
 FOR_EACH_COMMAND(LoadPalette)
 FOR_EACH_COMMAND(MaskAll)

src/app/document_api.cpp

@@ -351,7 +351,7 @@ void DocumentApi::moveCel(
   ASSERT(srcLayer != dstLayer || srcFrame != dstFrame);
   m_transaction.execute(new cmd::MoveCel(
       srcLayer, srcFrame,
-      dstLayer, dstFrame));
+      dstLayer, dstFrame, dstLayer->isContinuous()));
 }
 
 void DocumentApi::copyCel(
@@ -361,7 +361,7 @@ void DocumentApi::copyCel(
   ASSERT(srcLayer != dstLayer || srcFrame != dstFrame);
   m_transaction.execute(new cmd::CopyCel(
       srcLayer, srcFrame,
-      dstLayer, dstFrame));
+      dstLayer, dstFrame, dstLayer->isContinuous()));
 }
 
 void DocumentApi::swapCel(