Mfcttrf

I tried the following MWE:

documentclassarticle
usepackageetoolbox

AtBeginEnvironmenttableAtBeginEnvironmentpar
AtEndEnvironmenttableAtEndEnvironmentpar
BeforeBeginEnvironmenttableBeforeBeginEnvironmentpar
AfterEndEnvironmenttableAfterEndEnvironmentpar

begindocument
 Text beforepar
begintable [t]%
 Table textpar
endtable
 Text afterpar
enddocument

and got the following result:

Table text
AtEndEnvironment
Text before
BeforeBeginEnvironment
AtBeginEnvironment
AfterEndEnvironment
Text after

I was expecting AtBeginEnvironment to travel to the top of the page together with the float.
Here is what the manual has to say on the matter:

AtBeginEnvironment<environment><code>
Appends arbitrary <code> to a hook executed by the begin command
at the beginning of a given <environment>, immediately before
<environment>, inside the group opened by begin.

I do not know what happens inside an environment definition; I guess it declares some special commands called <environment> (e.g. table) and end<environment> (e.g. endtable).

But in this case, shouldn't it be immediately after <environment>, in order to be a complement to AtEndEnvironment just as BeforeBeginEnvironment is a complement to AfterEndEnvironment?

Anyway, how can I add code to the beginning of an environment? I'm trying to set a certain font style for all tables, and AtBeginEnvironmenttablesmall does not work. Also I guess this behavior makes this answer incorrect, because addvspace would stay at the point of declaration of the figure instead of moving together with it.

The etoolbox package redefines begin so that

1. 
   @beforebegin@<envname>@hook is executed


2. the usual bookkeeping is performed (setting @currenvir, for instance)


3. 
   @begin@<envname>@hook is executed
   1 a group is started


4. the macro <envname> is executed

Thus what you get is

@beforebegin@table@hook
<bookkeeping>
@begin@table@hook
begingroup
table

The macro table is responsible for starting the creation of a float and expands to @floattable.

Thus the tokens AtBeginEnvironmentpar and BeforeBeginEnvironmentpar are seen and the paragraphs are typeset before the float is started.

To the contrary, end is redefined to execute the AtEndEnvironment tokens before end<envname> is executed.

This should explain the output you get.

There is a hook for doing things when the float has already begun and is @floatboxreset.

I would be wary of using the hooks for adding text: they're meant for additional setup.