How to transfer the localvalue of one interface to another interface?

You should consider changing the local variable to an interface variable. Local variables value can't be passed.

it would be best if you give a example

I never see interface value before only ri!value and local!value

i meant ri!value as interface value. My bad!

but how to let ri!value get retrieve from another interface? I tried lots of times

how to let ri!value be retrieved from previous interface then passed to second interface?

for example the first interface fill a!save(target:ri!username,value:"SAAAAM"), then the second interface can retrieve like rule!interface1().username, is this ok?

You will likely need to map the rule inputs together from the child interface to the parent. For example, suppose you have a rule input for ri!case on the child interface. You will need to create a second rule input to match that on the parent interface and map them together like this:

rule!ChildInterface(case: ri!case)

Then, any changes you make in the child interface will also be saved to the parent.

But if i create ri!case at second interface how can i make sure it is not null, any interface create ri!value is null automatically right?

/*parent Interface*/

rule!childInterface(
username:ri!username
)

/*child interface - Create a rule input with name username*/
a!buttonWidget(
/*your code*/
saveInto:a!save(target:ri!username,value:"SAAAAM")
)