perf scripts python: exported-sql-viewer.py: Improve TreeModel abstraction

Instead of passing the tree root, get it from a method that can be
implemented in any derived class.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: https://lkml.kernel.org/n/tip-ovcv28bg4mt9swk36ypdyz14@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/scripts/python/exported-sql-viewer.py

@@ -167,9 +167,10 @@ class Thread(QThread):
 
 class TreeModel(QAbstractItemModel):
 
-	def __init__(self, root, parent=None):
+	def __init__(self, glb, parent=None):
 		super(TreeModel, self).__init__(parent)
-		self.root = root
+		self.glb = glb
+		self.root = self.GetRoot()
 		self.last_row_read = 0
 
 	def Item(self, parent):
@@ -562,8 +563,10 @@ class CallGraphRootItem(CallGraphLevelItemBase):
 class CallGraphModel(TreeModel):
 
 	def __init__(self, glb, parent=None):
-		super(CallGraphModel, self).__init__(CallGraphRootItem(glb), parent)
-		self.glb = glb
+		super(CallGraphModel, self).__init__(glb, parent)
+
+	def GetRoot(self):
+		return CallGraphRootItem(self.glb)
 
 	def columnCount(self, parent=None):
 		return 7
@@ -1339,8 +1342,7 @@ class BranchModel(TreeModel):
 	progress = Signal(object)
 
 	def __init__(self, glb, event_id, where_clause, parent=None):
-		super(BranchModel, self).__init__(BranchRootItem(), parent)
-		self.glb = glb
+		super(BranchModel, self).__init__(glb, parent)
 		self.event_id = event_id
 		self.more = True
 		self.populated = 0
@@ -1364,6 +1366,9 @@ class BranchModel(TreeModel):
 		self.fetcher.done.connect(self.Update)
 		self.fetcher.Fetch(glb_chunk_sz)
 
+	def GetRoot(self):
+		return BranchRootItem()
+
 	def columnCount(self, parent=None):
 		return 8