Update StringProperty on object selection change?

Question

I have a StringProperty that I want to show the name of the selected object. Currently, I update the property by invoking an operator manually from a panel. However, I want the property to be updated automatically every time the selected object in the viewport changes.

What is the best way to achieve this?

I have considered running a modal operator, which checks the selection change all the time. However, that does not seem like an optimal solution. I have also tried playing around with handlers (bpy.app.handlers.scene_update_post as mentioned here), but it seems to have the same result as a modal operator (constant checking in infinite loop). Is it possible to trigger the update only when the selection changes by some callback? Or alternatively, when the right mouse button is clicked (assuming selection only happens this way)?

Edit, for clarification:

I specifically need a StringProperty to be updated, and not just a regular text label. This is because the StringProperty (and other properties) will display some specific information about the selected object, which the user then can change to change the selected object.

My current code, where the StringProperty is updated manually by click:

import bpy

# Define string property.
class MyGroup(bpy.types.PropertyGroup):
    bpy.types.Scene.my_string = bpy.props.StringProperty(name='my_string')

# Define operator to change string property.
last_selection = []
class UpdateStringOperator(bpy.types.Operator):
    bl_idname = "update.mystring" 
    bl_label = ""
    def invoke(self, context, event):
        global last_selection
        if bpy.context.selected_objects != last_selection:
            last_selection = bpy.context.selected_objects
            bpy.context.scene.my_string = bpy.context.selected_objects[0].name
        return {'FINISHED'}

# Draw property and operator.
class MyPanel(bpy.types.Panel):
    ...
    def draw(self, context):
        layout = self.layout
        layout.row().prop(context.scene, "my_string")
        layout.row().operator("update.mystring", text="Update")

# Register.
def register():  
    bpy.utils.register_module(__name__)
def unregister(): 
    bpy.utils.unregister_module(__name__)
if __name__ == "__main__": 
    register()

Answer 1

This is the full script for selection and update, where emped the update function in our custom selection tool, the last function replaces the shortcuts of the old select with this operator so no damage is done and when unregistering the addon the shortcuts are reset to the previous state. run the script and open the Tools panel ( T in viewport ) and select an objectto see the string update

---

import bpy
from bpy.props import BoolProperty, IntVectorProperty

class MyGroup(bpy.types.PropertyGroup):
    bpy.types.Scene.my_string = bpy.props.StringProperty(name='my_string')

# Define operator to change string property.
last_selection = []

# the selection operator with the update func :
class SelectionOperator(bpy.types.Operator):
    """ Costum selection
    """
    bl_idname = "view3d.select_costum" 
    bl_label = "costum selection"

    extend = BoolProperty(default = False)
    deselect = BoolProperty(default = False)
    toggle = BoolProperty(default = False)
    center = BoolProperty(default = False)
    enumerate = BoolProperty(default = False)
    object = BoolProperty(default = False)
    location = IntVectorProperty(default = (0,0),subtype ='XYZ', size = 2)

    def execute(self, context):
        #select the object
        bpy.ops.view3d.select(extend=self.extend, deselect=self.deselect, toggle=self.toggle, center=self.center, enumerate=self.enumerate, object=self.object, location=(self.location[0] , self.location[1] ))
        #change the property
        global last_selection
        if bpy.context.selected_objects != last_selection:
               last_selection = bpy.context.selected_objects
               bpy.context.scene.my_string = bpy.context.selected_objects[0].name

        return {'FINISHED'}

    def invoke(self, context, event):
        if context.space_data.type == 'VIEW_3D':
            self.location[0] = event.mouse_region_x
            self.location[1]  = event.mouse_region_y
            return self.execute(context)
        else:
                self.report({'WARNING'}, "Active space must be a View3d")
                return {'CANCELLED'}

def replace_shortkey( old_op_name, new_op_name) :

        wm = bpy.context.window_manager
        keyconfig = wm.keyconfigs.active
        keymap = keyconfig.keymaps['3D View']
        items = keymap.keymap_items

        item = items.get(old_op_name, None)
        while item :

                props = item.properties

                extend    = props.extend.real
                deselect  = props.deselect.real
                toggle    = props.toggle.real
                center    = props.center.real
                enumerate = props.enumerate.real
                object    = props.object.real

                item.idname = new_op_name

                props.extend    = extend 
                props.deselect  = deselect
                props.toggle    = toggle
                props.center    = center
                props.enumerate = enumerate
                props.object    = object

                item = items.get( old_op_name, None)


# Draw property and operator.
class HelloWorldPanel(bpy.types.Panel):
    """    """
    bl_label = "active object Panel"
    bl_idname = "OBJECT_PT_my_panel"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'TOOLS'

    def draw(self, context):
        layout = self.layout
        layout.row().prop(context.scene, "my_string")

# Register.
def register():
    bpy.utils.register_module(__name__)
    replace_shortkey( 'view3d.select', SelectionOperator.bl_idname )

def unregister():
    replace_shortkey(SelectionOperator.bl_idname, 'view3d.select')
    bpy.utils.unregister_module(__name__)

if __name__ == "__main__": 
    register()

Answer 2

Why not just access the selected object's name when you need it? Blender panels are updated when blender re-draws. This only happens with user interaction so blender will check the selected object at a rate that is appropriate and you never have to worry! That's what the whole "context" thing is for. Unless there is some other reason you need to monitor selection I think you might be over complicating things.

class MyPanel(bpy.types.Panel):
    def draw(self, context):
        layout = self.layout
        row = layout.row()
        if len(context.selected_objects):
            ob = context.selected_objects[0]
            row.prop(ob, "name")