State

State is what your app remembers. The number in a counter, the todos in a list, the text someone has typed: all state.

state count = 0
state draft = ""
state todos: [Todo] = []

state lines go at the top level of your file, outside any window, function or view. Every part of the app can read and change them.

State drives the screen

The special thing about state is that the UI follows it. When state changes, every part of the screen that reads it updates:

state count = 0

window("Demo") {
  column() {
    label("Count: {count}")         // updates when count changes
    button("+") { count += 1 }
  }
}

You never tell a label to redraw. You change the state, and Nifty does the rest. See How the UI stays up to date for the details.

Changing state

You can change state in two places:

1. In a button’s action (or a function it calls):

   button("Reset") { count = 0 }
   

2. Through a two-way binding, when someone types in a text field or clicks a checkbox:

   text_field($draft)
   

The ways to change a value:

count = 10        // set it
count += 1        // add to it
count -= 1        // take away
count *= 2        // multiply
count /= 2        // divide
todos.push(item)  // add to the end of a list
todos.remove(0)   // remove the first item of a list
todo.done = true  // change one field
todos[0] = item   // replace one list item

Showing state never changes it

Code that builds the screen can read state, but it can’t change it. This is an error:

fn bump() { count += 1 }

window("Oops") {
  label("{bump()}")   // ✗ showing the UI can't change state
}

This rule means drawing the screen is always safe and predictable: looking at your app never changes what it remembers.

Starting values

A state’s starting value can’t use other state, because there’s no order in which state gets created:

state a = 1
state b = a     // ✗ a state's starting value can't use other state

If you need b to always follow a, don’t store it. Compute it with a function instead.

Local variables

Inside functions and button actions, let makes a temporary variable that disappears when the block ends:

fn remaining() -> int {
  let count = 0
  for todo in todos {
    if !todo.done {
      count += 1
    }
  }
  return count
}

Use state for things the app should remember, and let for working things out.