AngularJS ng-condition

Mark Rajcok’s explanation of How do I conditionally apply CSS styles in AngularJS? I copy here for quick retrieving:

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.

(A) ng-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:

 ng-repeat="item in items" ng-class="{'pending-delete': item.checked}">
   ... HTML to display the item ...
    type="checkbox" ng-model="item.checked">

Above, we used ng-class expression type #3 - a map/object of class names to boolean values.

(B) ng-style

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.

(C) ng-if, ng-show/ng-hide

1. ng-if will remove elements from DOM. This means that all your handlers or anything else attached to those elements will be lost. For example, if you bound a click handler to one of child elements, when ng-if evaluates to false, that element will be removed from DOM and your click handler will not work any more, even after ng-if later evaluates to true and displays the element. You will need to reattach the handler.
2. ng-show/ng-hide does not remove the elements from DOM. It uses CSS styles to hide/show elements (note: you might need to add your own classes). This way your handlers that were attached to children will not be lost.
3. ng-if creates a child scope while ng-show/ng-hide does not.