The foundation of the Customizer is built on the concept of the “setting”, an abstract representation of anything in WordPress: options, theme mods, posts, terms, and so on. Settings are linked to controls which are responsible for manipulating them. When a setting is changed it gets pushed into the preview window so that the modification can be reviewed before being published live. There are two ways (transports) for how a setting can be pushed into the preview: refresh and postMessage. The refresh transport does a full page refresh with the modified settings applied, and so it relies on PHP to apply the setting changes. The refresh transport is designed to be compatible with all themes, but for heavy sites refresh can be excruciating slow and taxing on the server. For this reason, the postMessage transport was introduced for previewing changes. This transport uses JavaScript to apply the changes instantly in the preview without any Ajax requests or PHP calls. This is great, but themes have to opt-in to postMessage transport for settings because they must also include JavaScript code which duplicates the logic which is already being executed on the server by PHP. The postMessage transport
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