Monday, 24 June 2013

Knockout - the "visible" binding

The visible binding causes the associated DOM element to become hidden or visible according to the value you pass to the binding.
 
Example:
<div data-bind="visible: shouldShowMessage">
    You will see this message only when "shouldShowMessage" holds a true value.
</div>

<script type="text/javascript">
    var viewModel = {
        shouldShowMessage: ko.observable(true) // Message initially visible
    };
    viewModel.shouldShowMessage(false); // ... now it's hidden
    viewModel.shouldShowMessage(true); // ... now it's visible again
</script>

Parameters
  • Main parameter
    • When the parameter resolves to a false-like value (e.g., the boolean value false, or the numeric value 0, or null, or undefined), the binding sets yourElement.style.display to none, causing it to be hidden. This takes priority over any display style you’ve defined using CSS.
    • When the parameter resolves to a true-like value (e.g., the boolean value true, or a non-null object or array), the binding removes the yourElement.style.display value, causing it to become visible. 
    • Note that any display style you’ve configured using CSS will then apply (so CSS rules like display:table-row work fine in conjunction with this binding).
  • If this parameter is an observable value, the binding will update the element’s visibility whenever the value changes. If the parameter isn’t observable, it will only set the element’s visibility once and will not update it again later.

Using functions and expressions to control element visibility
You can also use a JavaScript function or arbitrary JavaScript expression as the parameter value. If you do, KO will run your function/evaluate your expression, and use the result to determine whether to hide the element.
Example:
<div data-bind="visible: myValues().length > 0">
    You will see this message only when 'myValues' has at least one member.
</div>

<script type="text/javascript">
    var viewModel = {
        myValues: ko.observableArray([]) // Initially empty, so message hidden
    };
    viewModel.myValues.push("some value"); // Now visible
</script>



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