Howto reference a common data source to update a value across the 'app'?

[tfcroft4]

It seems to be tricky (for me) to access and update values of widgets from different parts of an app.
#1325 suggests the way to update a value across a Toga app is:
"preferably, with a reference to a common data source that, when updated, is reflected in the app's GUI"
I have a 'feedback' Text box which is a child of the main_window and I would like to update it from actions in a separate class that defines a box in an option container which is also a child of the main_window.
In the example below I want to report the outcome of a file upload process - Success or Failure.
Is the treesource example the best starting point to create a shared datasource for a Text Box?
Or perhaps there a simple example to get started with?
I hope that this makes sense.
Thanks

[freakboy3742]

There's two answers to your question.

The first is the pragmatic, "works right now" answer. The sibling relationship between widgets in a layout doesn't matter. All that matters is that you can get a reference to another widget. So - you create the text input as self.my_input = toga.TextInput(...), so it's a property of the app; the button has an on_press handler def my_press_handler(self, widget, **kwargs) which is a method on the app; and that method talks about self.my_input.value = "new value" - accessing the text input as a property of the app - and you're done. The button has referenced and updated the value in the text input.

You can slightly decouple things by using the widget registry - instead of assigning the self.my_input = toga.TextInput(...), you give the TextInput an id, and then in the button handler, you look up the widget ID with either app.widgets[id] or window.widgets[id] to retrieve the widget by ID. Fundamentally, this isn't really any different - you're still using the app (or window) as a point from which you can obtain a reference to other widgets.

Your comments about "shared data source" hint in the direction of the longer term solution - but that solution doesn't currently exist. Toga does contain a ValueSource, which is the "single value" equivalent of TreeSource/ListSource - but, at present, you can't use a ValueSource for anything. Ideally, you would be able to use a ValueSource as the data behind a TextInput - so then, whenever the ValueSource is changed, the TextInput would change, and vice versa.

However, that won't change the underlying issue that made the "pragmatic" solution possible. You need a reference to something to represent the data- and that will mean either assigning something to the app, or using some other "registry" (be that the app/window widget registry, or something else).

If you want an example of this - Toga Tutorial 1 (The Fahrenheit to Celsius converter) in the examples folder of the Toga folder demonstrates this - you press a button, and the value of one text input is used to update another text input.

[tfcroft4]

Thanks again for your insight. Very Helpful. But a few more questions.
Are app and window global and accessible from within a class? I have created separate classes for each Tab display.

The programmatic change to a text value would seem to have a solution. I will test the proposed methods to identify the widgets.
Are there any constraints on widget ids? Any sensible Python string.

My aim is to have a settings summary in a table with a data source then separate Input widgets that can update row items in the table source. (and trigger a display update unless that would be auto magical)
{Not visible in the screen shot but the data changes on the Query Tab and the table is on the Settings Tab.}
Some widgets may be text entry but one may be a File Dialog returned file name. i.e. a programmatic update rather than a text input as such.
If I understand this correctly a ListSource is a separate entity.
The data shown by a table can be changed by setting or resetting the data property of the table.
Change Data in Text Input => Change Table Data Source = > Table updated to reflect changes
(input of programmatic) => lost focus or on change event to update List Source? = > trigger table update or is this automatic?

Cheers
Ted

[freakboy3742]

App is a global singleton, because you can only have one app object - you can access it via toga.App.app.

There's no "window" global because your app can have many windows. But the list of windows is accessible via windows attribute on the App.

Or - you pass the window as context to the thing that needs to know about the window. If you need f() to know about x... you pass x as an argument to f... f(x). Or, you reverse the dependency, and pass a reference of the handler that does know about the window to f(), and then install that handler in the body of f().

As for the rest of your description of your problem - a ListSource is representation of data that is decoupled from the Table widget. If you update an entry in a ListSource, the display in the Table widget will be updated. How you choose to update the ListSource is up to you - but one option would be a handler on another widget reacting to a change in value, a button press, etc.

[tfcroft4]

I am getting my head around this.
This button callback updates a textbox with id = 'weblog' with the id of the Button that was pressed. So using widget id resolved the cross app communication

    def upload_callback(self,button):
        # handle event                
        toga.App.app.widgets["weblog"].value = button.id

Now looking at ListSource updates so making progress.
Thanks again