How can I dismiss the on screen keyboard?

For Flutter version 2 or latest :

Since Flutter 2 with null safety this is the best way:

FocusManager.instance.primaryFocus?.unfocus();

Note: using old ways leads to some problems like keep rebuild states;

For Flutter version < 2 :

As of Flutter v1.7.8+hotfix.2, the way to go is:

FocusScope.of(context).unfocus();

Comment on PR about that:

Now that #31909 (be75fb3) has landed, you should use
FocusScope.of(context).unfocus() instead of
FocusScope.of(context).requestFocus(FocusNode()), since FocusNodes are
ChangeNotifiers, and should be disposed properly.

-> DO NOT use ̶r̶e̶q̶u̶e̶s̶t̶F̶o̶c̶u̶s̶(̶F̶o̶c̶u̶s̶N̶o̶d̶e̶(̶)̶ anymore.

 F̶o̶c̶u̶s̶S̶c̶o̶p̶e̶.̶o̶f̶(̶c̶o̶n̶t̶e̶x̶t̶)̶.̶r̶e̶q̶u̶e̶s̶t̶F̶o̶c̶u̶s̶(̶F̶o̶c̶u̶s̶N̶o̶d̶e̶(̶)̶)̶;̶

Read more about the FocusScope class in the flutter docs.