GRC C++ block parameter value can't be changed

Hello group,

I am very new in writing C++ blocks. I just followed GNU Radio tutorials to write a sync block. I simply used a parameter "double value", so the output = input * value.

The block works. However, if I use a GUI range to change the parameter "value" during running of GRC, the output doesn't change accordingly, the "value" remains the same as the default value in GUI range. Did I miss something in the block?

Please find the source code in the attached file, I appreciate it very much if you could help me figure out the problem.

Regards,

Wei