Friday, 12 June 2015

How do I conditionally apply CSS styles in AngularJS?

Angular provides a number of built-in directives for manipulating CSS styling conditionally/dynamically:
  • ng-class - use when the set of CSS styles is static/known ahead of time
  • ng-style - use when you can't define a CSS class because the style values may change dynamically. Think programmable control of the style values.
  • ng-show and ng-hide - use if you only need to show or hide something (modifies CSS)
  • ng-if - new in version 1.1.5, use instead of the more verbose ng-switch if you only need to check for a single condition (modifies DOM)
  • ng-switch - use instead of using several mutually exclusive ng-shows (modifies DOM)
  • ng-disabled and ng-readonly - use to restrict form element behavior
  • ng-animate - new in version 1.1.4, use to add CSS3 transitions/animations
The normal "Angular way" involves tying a model/scope property to a UI element that will accept user input/manipulation (i.e., use ng-model), and then associating that model property to one of the built-in directives mentioned above.
When the user changes the UI, Angular will automatically update the associated elements on the page.

Q1 sounds like a good case for ng-class -- the CSS styling can be captured in a class. ng-class accepts an "expression" that must evaluate to one of the following:
  1. a string of space-delimited class names
  2. an array of class names
  3. a map/object of class names to boolean values
Assuming your items are displayed using ng-repeat over some array model, and that when the checkbox for an item is checked you want to apply the pending-delete class:
<div ng-repeat="item in items" ng-class="{'pending-delete': item.checked}">
   ... HTML to display the item ...
   <input type="checkbox" ng-model="item.checked">
Above, we used ng-class expression type #3 - a map/object of class names to boolean values.

Q2 sounds like a good case for ng-style -- the CSS styling is dynamic, so we can't define a class for this. ng-style accepts an "expression" that must evaluate to:
  1. an map/object of CSS style names to CSS values
For a contrived example, suppose the user can type in a color name into a texbox for the background color (a jQuery color picker would be much nicer):
<div class="main-body" ng-style="{color: myColor}">
   <input type="text" ng-model="myColor" placeholder="enter a color name">

Fiddle for both of the above. The fiddle also contains an example of ng-show and ng-hide. If a checkbox is checked, in addition to the background-color turning pink, some text is shown. If 'red' is entered in the textbox, a div becomes hidden.
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