How can I add custom icon for my operators in a specific addon?

Question

According to this question and this example I am trying to add some custom icons to my addon's operators. But I have an "issue".

In both examples, functions draw, register and unregister are in the same file. In my case, register and unregister functions located into my __init__.py file and looks totally different and the draw function located into an other file I have create by the name fu3dm_main.py. Can anyone help me on how I'll apply this example in my case?

Here is how my __init__.py file looks:

bl_info = {
    "name" : "Blah blah blah",
    "author" : "Simonetos The Greek <simonetos.the.greek@gmail.com>",
    "description" : "Blah blah blah",
    "blender" : (2, 90, 1),
    "version" : (1, 0, 0),
    "location" : "View3D",
    "warning" : "",
    "category" : "Generic"
}
import bpy
from . fu3dm      import *
from . fu3dm_main import *
ops = fu3dm.ops
classes = (
    fu3dm.ops.view.front,
    fu3dm.ops.view.back,
    fu3dm.ops.view.right,
    fu3dm.ops.view.left,
    fu3dm.ops.view.top,
    fu3dm.ops.view.bottom,
    fu3dm.ops.preferences.model,
    fu3dm_main.panel
)
register, unregister = bpy.utils.register_classes_factory(classes)

And here is how my fu3dm_main.py file looks:

import bpy
from . fu3dm import *
class fu3dm_main():
    """Main class"""
class panel(bpy.types.Panel):
    """Main panel class"""

    bl_idname = "fu3dm.main_PT_Panel"
    bl_label = "Blah blah blah"
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
    bl_category = "FU3DM"

    @classmethod
    def poll(cls, context):
        return bpy.data.collections.get("FU3DM v20.0")

    def draw(self, context):
            layout = self.layout
        #   ┌────────┐
        #   │ Box #1 │
        #   └┬───────┘
            box1 = layout.box()
            box1_col1 = box1.column()

            box1_row1 = box1_col1.split()
            box1_row1.label(text="Preferences:")
            box1_row1.operator("fu3dm.model_prefs")
        #   ┌┴────┐
        #   │ End │
        #   └─────┘

Answer 1

Well I found the solution from this attached file here, and here is a code example...

import os
import bpy
import bpy.utils.previews
from bpy.types import Panel
preview_collections = {}
If you work with a simple script.
dir = os.path.dirname(bpy.data.filepath)
If you work with an addon.
dir = os.path.join(os.path.dirname(file), "icons")
pcoll = bpy.utils.previews.new()
for entry in os.scandir(dir):
    if entry.name.endswith(".png"):
        name = os.path.splitext(entry.name)[0]
        pcoll.load(name.upper(), entry.path, "IMAGE")
class VIEW3D_PT_test(Panel):
    bl_label = "TEST"
    bl_category = "TEST"
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
def draw(self, context):
    layout = self.layout
    scene = context.scene

    layout.prop(scene, "use_gravity", text="Enable Layout")

    col = layout.column()
    col.enabled = scene.use_gravity
    col.operator("object.duplicate_move", text="Custom Icon", icon_value=pcoll["ICON"].icon_id)
    col.operator("object.duplicate_move", text="Built-in Icon", icon="OUTPUT")



bpy.utils.register_class(VIEW3D_PT_test)

Just note, that If you work on an addon, the dir should be like this dir = os.path.join(os.path.dirname(__file__), "icons") and icons should be saved in .png format into folder icons into the same place where your .py file is.

And if you work on a simple script, dir should be like this dir = os.path.dirname(bpy.data.filepath) and icons should be into the same place where your .py and .blend file is.