Tag: Binding

Implicit ViewModel 2-Way Data Bindings

I’ve come across a rather interesting… let’s call it “feature” of ExtJS ViewModel and data binding system. Turns out it is possible to establish “implicit” two-way data binds between parent & child view model data (within same component hierarchy) by setting the data of “lower” view model to “undefined”, so long as the view models both define that data property with the same name. Picture the following simple hierarchy:

  • Parent Panel

    • Child Panel

Now imagine that both of the panels define their own ViewModels (implicit or not doesn’t matter) that both define the same two properties, like so:

  • Parent Panel

    • undefinedText: “default Parent text”
    • definedText: “default Parent text”
  • Child Panel

    • undefinedText: undefined
    • definedText: “default Child text”

This is what the default rendering of these panels would look like:
Screen

Read on for a walk through of various scenarios and a Sencha Fiddle…
Continue reading Implicit ViewModel 2-Way Data Bindings

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Declarative Listeners vs. Control

Back in ExtJS 5 Sencha rolled out a rather cool concept of declarative listeners, where you could write something like this:

{
    xtype: 'button',
    handler: 'onButtonClick'
}

…note that onButtonClick is a string, rather than a function reference. The idea here is that the actual function would be resolved at runtime and will be located somewhere in the ViewController world. It was described well by Don Griffin in his blog post: https://www.sencha.com/blog/using-viewcontrollers-in-ext-js-5/.

However, what was omitted in this blog post and the guides, is the potentially powerful and risky “up the component tree” resolution of the ViewController. Read on to learn more with a Sencha Fiddle example…

Continue reading Declarative Listeners vs. Control

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ExtJS 6 DataBinding vs. Premium Licensing

I recently ran into some frustration as I was doing some refactoring and my databinding stopped working for some view. Turned out I misspelled “viewmodel” where it should’ve been “viewModel” and it didn’t get declared for one of the views. As a result, a simple HTML bind like this stopped working:

xtype: 'component',
bind:'<div class="stacked-label-group">' +
        '<div class="label-title">{i18n:translate("appointment_info.appt_info.promise_time")}</div>' +
        '<div class="label-value">{promisedDateOrWaiter}</div>' +
     '</div>'

Turned out that I didn’t have a “promisedDateOrWaiter” to bind to (since I didn’t actually have a view model), which caused even the label data binding to fail to render (even though that one did exist at a different higher level). So the danger with these view models is that even if one of many fails in the binding, the whole binding fails (at least in case of html type of bindings).

Unfortunately this isn’t easy to troubleshoot as there are no error messages in the console or exceptions to pause on.

However, Sencha does offer an Inspection tool: https://www.sencha.com/products/inspector/

I evaluated it and one of the features it offers is being able to catch faulty bindings like this one. The ironic part is that you need to purchase a “premium” license to get the tool, which is almost double the cost of the standard license just to get the framework. I really hope Sencha will write some tutorials on how to debug these data bindings effectively without the custom tool.

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