Conditional function that lets you know if your site is being viewed through the Customizer

[nathanrice]

I can provide a use case if necessary, but this should be relatively simple.

If I need to do something special with my theme when my site is being viewed through the customizer, there's no easy way determine if it is.

A simple is_customizer() (or similar) function would be incredibly useful in this scenario.

I'd write up a patch, but I don't know where to put the new function.

This seems to work for me:

global $wp_customize;
if ( isset( $wp_customize ) ) {
	// ...
}

Submitted a patch that creates a function is_customizer() that checks to make sure $wp_customize is of type WP_Customize_Manager and returns the value of $wp_customize->is_preview();