Vim’s event model

Vim’s editing functions behave as if they are event-driven. For performance reasons, the actual implementation is more complex than that, with much of the event handling optimized away or handled several layers below the event loop itself, but you can still think of the editor as a simple while loop responding to a series of editing events.

Whenever you start a Vim session, open a file, edit a buffer, change your editing mode, switch windows, or interact with the surrounding filesystem, you are effectively queuing an event that Vim immediately receives and handles.

For example, if you start Vim, edit a file named demo.txt, swap into Insert mode, type in some text, save the file, and then exit, your Vim session receives a series of events like what is shown in Listing 1.

> vim
    BufWinEnter     (create a default window)
    BufEnter        (create a default buffer)
    VimEnter        (start the Vim session)
:edit example.txt
    BufNew          (create a new buffer to contain demo.txt)
    BufAdd          (add that new buffer to the session’s buffer list)
    BufLeave        (exit the default buffer)
    BufWinLeave     (exit the default window)
    BufUnload       (remove the default buffer from the buffer list)
    BufDelete       (deallocate the default buffer)
    BufReadCmd      (read the contexts of demo.txt into the new buffer)
    BufEnter        (activate the new buffer)
    BufWinEnter     (activate the new buffer's window)
i
    InsertEnter     (swap into Insert mode)
Hello
    CursorMovedI    (insert a character)
    CursorMovedI    (insert a character)
    CursorMovedI    (insert a character)
    CursorMovedI    (insert a character)
    CursorMovedI    (insert a character)
<ESC>
    InsertLeave     (swap back to Normal mode)
:wq
    BufWriteCmd     (save the buffer contents back to disk)
    BufWinLeave     (exit the buffer's window)
    BufUnload       (remove the buffer from the buffer list)
    VimLeavePre     (get ready to quit Vim)
    VimLeave        (quit Vim)

More interestingly, Vim provides "hooks" that allow you to intercept any of these editing events. So you can cause a particular Vimscript command or function to be executed every time a specific event occurs: every time Vim starts, every time a file is loaded, every time you leave Insert mode … or even every time you move the cursor. This makes it possible to add automatic behaviors almost anywhere throughout the editor.

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