Hooks are an important component to extend knitr. A hook is a user-defined R function to fulfill tasks beyond the default capability of knitr. There are two types of hooks: chunk hooks and output hooks. We have already introduced some built-in output hooks in Section 5.3, and how to customize both the chunk and inline R output. In this chapter we focus on chunk hooks.
A chunk hook is a function stored in knit_hooks and triggered by a custom chunk option. All chunk hooks have three arguments: before, options, and envir (explained later).
A chunk hook can be arbitrarily named, as long as it does not clash with existing hooks in knit_hooks. Names of all built-in hooks are:
For example, ...
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