Trigger background processing update when switching to a preview tab.

Implements "Go Direct to the preview screen after slicing #152"

src/libslic3r/Print.hpp

@@ -297,7 +297,10 @@ public:
 
     // methods for handling state
     bool                is_step_done(PrintStep step) const { return Inherited::is_step_done(step); }
+    // Returns true if an object step is done on all objects and there's at least one object.    
     bool                is_step_done(PrintObjectStep step) const;
+    // Returns true if the last step was finished with success.
+    bool                finished() const override { return this->is_step_done(psGCodeExport); }
 
     bool                has_infinite_skirt() const;
     bool                has_skirt() const;

src/libslic3r/PrintBase.hpp

@@ -285,6 +285,8 @@ public:
 	void                       cancel_internal() { m_cancel_status = CANCELED_INTERNAL; }
     // Cancel the running computation. Stop execution of all the background threads.
 	void                       restart() { m_cancel_status = NOT_CANCELED; }
+    // Returns true if the last step was finished with success.
+    virtual bool               finished() const = 0;
 
     const PlaceholderParser&   placeholder_parser() const { return m_placeholder_parser; }
     PlaceholderParser&         placeholder_parser() { return m_placeholder_parser; }

src/libslic3r/SLAPrint.cpp

@@ -936,6 +936,18 @@ bool SLAPrint::invalidate_state_by_config_options(const std::vector<t_config_opt
     return invalidated;
 }
 
+// Returns true if an object step is done on all objects and there's at least one object.
+bool SLAPrint::is_step_done(SLAPrintObjectStep step) const
+{
+    if (m_objects.empty())
+        return false;
+    tbb::mutex::scoped_lock lock(this->state_mutex());
+    for (const SLAPrintObject *object : m_objects)
+        if (! object->m_state.is_done_unguarded(step))
+            return false;
+    return true;
+}
+
 SLAPrintObject::SLAPrintObject(SLAPrint *print, ModelObject *model_object):
     Inherited(print, model_object),
     m_stepmask(slaposCount, true),

src/libslic3r/SLAPrint.hpp

@@ -185,6 +185,9 @@ public:
     bool                empty() const override { return m_objects.empty(); }
     ApplyStatus         apply(const Model &model, const DynamicPrintConfig &config) override;
     void                process() override;
+    bool                is_step_done(SLAPrintObjectStep step) const;
+    // Returns true if the last step was finished with success.
+    bool                finished() const override { return this->is_step_done(slaposIndexSlices); }
 
     template<class Fmt> void export_raster(const std::string& fname) {
         if(m_printer) m_printer->save<Fmt>(fname);

src/slic3r/GUI/BackgroundSlicingProcess.hpp

@@ -110,6 +110,11 @@ public:
 	State 	state() 	const { return m_state; }
 	bool    idle() 		const { return m_state == STATE_IDLE; }
 	bool    running() 	const { return m_state == STATE_STARTED || m_state == STATE_RUNNING || m_state == STATE_FINISHED || m_state == STATE_CANCELED; }
+    // Returns true if the last step of the active print was finished with success.
+    // The "finished" flag is reset by the apply() method, if it changes the state of the print.
+    // This "finished" flag does not account for the final export of the output file (.gcode or zipped PNGs),
+    // and it does not account for the OctoPrint scheduling.
+    bool    finished() const { return m_print->finished(); }
 
 private:
 	void 	thread_proc();

src/slic3r/GUI/Plater.cpp

@@ -2212,6 +2212,7 @@ void Plater::priv::set_current_panel(wxPanel* panel)
     }
     else if (current_panel == preview)
     {
+        this->q->reslice();        
         preview->reload_print();
         preview->set_canvas_as_dirty();
     }
@@ -3064,7 +3065,7 @@ void Plater::reslice()
 #else
         this->p->canvas3D->reload_scene(false);
 #endif // ENABLE_REMOVE_TABS_FROM_PLATER
-    if ((state & priv::UPDATE_BACKGROUND_PROCESS_INVALID) == 0 && !this->p->background_process.running()) {
+    if ((state & priv::UPDATE_BACKGROUND_PROCESS_INVALID) == 0 && !this->p->background_process.running() && !this->p->background_process.finished()) {
         // The print is valid and it can be started.
         if (this->p->background_process.start())
 			this->p->statusbar()->set_cancel_callback([this]() {