streams - Directing output of PrintTemporary to an additional OutputStream

I want to log messages that I send to the screen, even temporary ones.

By setting $Output appropriately, it is possible to log Print messages, but not PrintTemporary messages:

oldout = $Output;
    logfile = FileNameJoin[{$TemporaryDirectory, "example.log"}];
    log = OpenWrite[logfile];

    $Output = Append[oldout, log];
Print["Log message printed on screen."];
PrintTemporary["Log message printed temporarily on screen."]; Pause[1];
$Output = oldout;
Close[log];
Print["Contents of file:"];
FilePrint[logfile];

gives

Log message printed on screen.

(Log message printed temporarily on screen.)

Contents of file:

"Log message printed on screen."

Perhaps this is not surprising, but if you start Mathematica in non-gui mode (i.e., "MathKernel" on MacOsX), the PrintTemporary output stays permanently on the screen.

How can one direct the output of PrintTemporary permanently to an additional output stream?

Answer

Since PrintTemporary does not use output streams, but rather is akin to CellPrint, I believe you will need to add this functionality manually. For example:

Unprotect[PrintTemporary];


$log = OpenAppend["logfile.txt"];

PrintTemporary[expr_] /; ! TrueQ[ptLog] :=
 Block[{ptLog = True},
  WriteString[$log, expr];
  PrintTemporary @ expr
 ]

Protect[PrintTemporary];