Preserve/Restore e-mail reading position

* Introduce the "MAIL:read_pos" attribute of e-mail file node to
  store the latest scroll position of the mail text view;
* Fixes #4302 (Mail should store and restore scroll offset);
* This feature was implemented during completing GCI2011 task.

Signed-off-by: Siarzhuk Zharski <zharik@gmx.li>

src/apps/mail/Content.cpp

@@ -2622,6 +2622,9 @@ TTextView::Reader::Run(void *_this)
 	}
 
 done:
+	// restore the reading position if available
+	view->Window()->PostMessage(M_READ_POS);
+
 	reader->Unlock();
 
 	delete reader;

src/apps/mail/MailWindow.cpp

@@ -794,6 +794,31 @@ TMailWindow::SetTrackerSelectionToCurrent()
 }
 
 
+void
+TMailWindow::PreserveReadingPos(bool save)
+{
+	BScrollBar *scroll = fContentView->fTextView->ScrollBar(B_VERTICAL);
+	if (scroll == NULL || fRef == NULL)
+		return;
+
+	BNode node(fRef);
+	float pos = scroll->Value();
+
+	const char* name = "MAIL:read_pos";
+	if (save) {
+		node.WriteAttr(name, B_FLOAT_TYPE, 0, &pos, sizeof(pos));
+		syslog(0, "save: %f", pos);
+		return;
+	}
+
+	if (node.ReadAttr(name, B_FLOAT_TYPE, 0, &pos, sizeof(pos)) == sizeof(pos)) {
+		Lock();
+		scroll->SetValue(pos);
+		Unlock();
+	}
+}
+
+
 void
 TMailWindow::MarkMessageRead(entry_ref* message, read_flags flag)
 {
@@ -810,6 +835,9 @@ TMailWindow::MarkMessageRead(entry_ref* message, read_flags flag)
 	// don't wait for the server write the attribute directly
 	write_read_attr(node, flag);
 
+	// preserve the read position in the node attribute
+	PreserveReadingPos(true);
+
 	BMailDaemon::MarkAsRead(account, *message, flag);
 }
 
@@ -1337,6 +1365,10 @@ TMailWindow::MessageReceived(BMessage *msg)
 			break;
 		}
 
+		case M_READ_POS:
+			PreserveReadingPos(false);
+			break;
+
 		case M_PRINT_SETUP:
 			PrintSetup();
 			break;
@@ -2848,6 +2880,10 @@ TMailWindow::OpenMessage(const entry_ref *ref, uint32 characterSetForDecoding)
 			}
 			AddEnclosure(&msg);
 		}
+		
+		// restore the reading position if available
+		PostMessage(M_READ_POS);
+
 		PostMessage(RESET_BUTTONS);
 		fIncoming = false;
 		fDraft = true;

src/apps/mail/MailWindow.h

@@ -108,6 +108,7 @@ class TMailWindow : public BWindow {
 				void			SaveTrackerPosition(entry_ref*);
 				void			SetOriginatingWindow(BWindow* window);
 
+				void			PreserveReadingPos(bool save);
 				void			MarkMessageRead(entry_ref* message,
 									read_flags flag);
 				void			SetTrackerSelectionToCurrent();

src/apps/mail/Messages.h

@@ -86,6 +86,7 @@ enum MENUS {
 	M_CLOSE_SAVED,
 	M_CLOSE_CUSTOM,
 	M_STATUS,
+	M_READ_POS,
 
 	// edit
 	M_SELECT,