Operator that runs when a value input is changed

Question

So I need an operator (for creating a DECIMATE modifier in the selected object) that runs every time that the value input is changed and uses that value (for the Ratio of the modifier).

For now I've this alternative. The problem is that when the value is changed nothing happens until the operator button is clicked (see code below).

So basically I would like an interface in which just the value is shown and if its changed then the operator runs, changinge of the value of the decimate modifier.

import bpy
from bpy.props import (BoolProperty,
                       PointerProperty,
                       )
from bpy.types import (Panel,
                       Operator,
                       PropertyGroup,
                       )
class MyProperties(bpy.types.PropertyGroup):
my_float : bpy.props.FloatProperty (name = "Decimate Radio", soft_min = 0, soft_max= 1)




class ADDONNAME_PT_main_panel(bpy.types.Panel):
    bl_label= "Add-On"
    bl_idname = "PT_TestPanel"
    bl_space_type = 'VIEW_3D'
    bl_region_type= 'UI'
    bl_category= 'Add-On'
def draw(self,context):
    layout = self.layout
    scene= context.scene
    mytool= scene.my_tool


    i=0

    for obj in bpy.context.selected_objects:
           i =i+1

    if i==1 :

        layout.prop(mytool,"my_float")
        row= layout.row()
        row.operator("addonname.myop_operator_3")







class ADDONNAME_OT_my_op_3(bpy.types.Operator):
    bl_label = "Operator"
    bl_idname = "addonname.myop_operator_3"
def execute(self, context):

    scene = context.scene
    mytool= scene.my_tool
    obj_1 = bpy.context.selected_objects[0]
    mod = obj_1.modifiers.new("Decimate", 'DECIMATE')
    mod.ratio=mytool.my_float 






    return {'FINISHED'}


classes = [ ADDONNAME_PT_main_panel, ADDONNAME_OT_my_op_3, MyProperties]
def register():
    from bpy.utils import register_class
    for cls in classes:
        register_class(cls)
bpy.types.Scene.my_tool = PointerProperty(type=MyProperties)


def unregister():
    from bpy.utils import unregister_class
    for cls in reversed(classes):
        unregister_class(cls)
    del bpy.types.Scene.my_tool
if name == "main":
    register()

Answer 1

You can use an Application Handler.

import bpy
from bpy.app.handlers import persistent
@persistent
def decimate_addon_handler(scene):
    for o in scene.objects:
        if o.type=='MESH':
            if "Decimate" in o.modifiers:
                 o.modifiers["Decimate"].ratio = scene.my_tool.my_float
bpy.app.handlers.depsgraph_update_post.append(decimate_addon_handler)

It runs every time something is changed in the scene, so that will become an issue quite fast especially when updating decimate modifier for all objects in the scene. Be sure to check for conditions that you really need it in so it only runs when needed. For example you probably want to run it only when the value changes so you should probably think of a way to track if the value changes.