Observer not triggered for declared attribute

[cryzed]

Hello, I am trying to minimally wrap PyQt5's QWebEngineView:

```from PyQt5.QtWebEngineWidgets import QWebEngineView

from atom.api import Unicode, observe, set_default

from enaml.core.declarative import d_

from enaml.widgets.api import RawWidget

class WebEngineViewWidget(RawWidget):

    __slots__ = '__weakref__'

    url = d_(Unicode())

    hug_width = set_default('ignore')

    hug_height = set_default('ignore')

    def create_widget(self, parent):

        return QWebEngineView(parent)

    @observe('url')

    def _update_proxy(self, change):

        print('change', change)

        super()._update_proxy(change)

```

In my *.enaml-file I have this line

```

        WebEngineViewWidget:

            url << folder.image_path

```

My model (folder-)model looks like this:

```import os

import magic

import natsort

from atom.api import Atom, Bytes, Int, List, Str, observe

from enaml.image import Image

class ImageFolder(Atom):

    path = Str()

    image_data = Bytes()

    image_path = Str()

    items = List(str)

    index = Int(default=0)

    _supported_formats = set(Image.format.items).union({'jpeg'}) - {'auto'}

    def _load_image(self):

        if self.items:

            self.image_path = self.items[self.index]

            with open(self.image_path, 'rb') as file:

                self.image_data = file.read()

    @observe('path')

    def _on_path(self, change):

        # Clear image, items, and reset index

        self.image_data = b''

        self.items.clear()

        self.index = 0

        # If the path is empty or doesn't exist just return

        path = change['value']

        if not os.path.isdir(path):

            return

        # Collect all images recursively

        for root, folders, file_names in os.walk(path):

            for file_name in file_names:

                path = os.path.join(root, file_name)

                mime = magic.from_file(path, mime=True)

                kind, format = mime.split('/', 1)

                if kind == 'image' and format in self._supported_formats:

                    self.items.append(path)

        self.items = natsort.natsorted(self.items)

        self._load_image()

    def next(self):

        self.index += 1

        if self.index >= len(self.items):

            self.index = 0

        self._load_image()

    def previous(self):

        self.index -= 1

        if self.index < 0:

            self.index = len(self.items) - 1

        self._load_image()

```

I know that image_path definitely changes, but it never seems to trigger the observer method. The observer method successfully works for other (default attributes):

```change {'type': 'create', 'object': <widgets.WebEngineViewWidget object at 0x7f7812e273a8>, 'name': 'background', 'value': None}

change {'type': 'create', 'object': <widgets.WebEngineViewWidget object at 0x7f7812e273a8>, 'name': 'foreground', 'value': None}

change {'type': 'create', 'object': <widgets.WebEngineViewWidget object at 0x7f7812e273a8>, 'name': 'font', 'value': None}

change {'type': 'create', 'object': <widgets.WebEngineViewWidget object at 0x7f7812e273a8>, 'name': 'minimum_size', 'value': Size(width=-1, height=-1)}

change {'type': 'create', 'object': <widgets.WebEngineViewWidget object at 0x7f7812e273a8>, 'name': 'maximum_size', 'value': Size(width=-1, height=-1)}

change {'type': 'create', 'object': <widgets.WebEngineViewWidget object at 0x7f7812e273a8>, 'name': 'tool_tip', 'value': ''}

change {'type': 'create', 'object': <widgets.WebEngineViewWidget object at 0x7f7812e273a8>, 'name': 'status_tip', 'value': ''}

change {'type': 'create', 'object': <widgets.WebEngineViewWidget object at 0x7f7812e273a8>, 'name': 'enabled', 'value': True}

change {'type': 'create', 'object': <widgets.WebEngineViewWidget object at 0x7f7812e273a8>, 'name': 'visible', 'value': True}```

What am I doing wrong here?

[frmdstryr]

Is there a reason you're not using enaml's WebView which uses Qt's web engine (unlike the Html widget)? 

The code is not working because `_update_proxy` attempts to trigger `set_<name>` on the proxy. If it is not defined (as is the case here) it does nothing.  For RawWidgets just call the methods on the proxy directly. For example

  @observe('url')

    def _update_url(self, change):

          self.proxy.widget.setUrl(QUrl(change['value']))

[cryzed]

I think I must have missed that widget and will probably use it. But this still hasn't fixed my problem, even after adjusting my code:

```from PyQt5.QtCore import QUrl

from PyQt5.QtWebEngineWidgets import QWebEngineView

from atom.api import Unicode, observe, set_default

from enaml.core.declarative import d_

from enaml.widgets.api import RawWidget

class WebEngineViewWidget(RawWidget):

    __slots__ = '__weakref__'

    url = d_(Unicode())

    hug_width = set_default('ignore')

    hug_height = set_default('ignore')

    def create_widget(self, parent):

        return QWebEngineView(parent)

    @observe('url')

    def _update_url(self, change):

        print('update', change)

        self.proxy.widget.setUrl(QUrl(change['value']))

```

The print (what I was really after) is never triggered, nor is the URL of the proxy changed. To be complete:

https://gist.github.com/cryzed/5b7cff02e8921620d784912c9f9d31ad (widgets.py)

https://gist.github.com/cryzed/fffe75aa1bca3cc42f9d94a83dcf61f6 (view.enaml)

https://gist.github.com/cryzed/7d1524b36cb534b97d8dd3559b6fb34d (main.py)

https://gist.github.com/cryzed/32db30e2de04e34dea5de59f76b16dfe (models.py)

Do you maybe know now what's going wrong? The update method is never called once, and I know for a fact that image_path/image_data do change (so observers bound to it should be triggered).

[cryzed]

Ah yes, and the reason why I wasn't using the WebView widget is because it uses the outdated PyQt5.QtWebKitWidgets, instead of the still developed and more modern WebEngine.

[cryzed]

Nevermind, I'm blind, just saw this line:

```

# Only needed for Qt 5.10+

from enaml.qt import QtWebEngineWidgets

```

[Matthieu Dartiailh]

Testing your code by adding a stupid button to manually change the url show that the observer is properly triggered. So you probably have another issue (but I cannot test quickly because I don’t have magic nor natsort).

Best 

Matthieu

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[cryzed]

Thank you for Matthieu & frmdstryr. I'll figure out the issue with the observer some other time, when I'm less frustrated, and use the WebEngine widget for now (which luckily works). Thanks!

[frmdstryr]

There is no initial print because the url is not accessed so the initial read never occurs.

Typically widgets will sync the initial state in the init_widget method.