Previous Value within connect callback.

[loeiks]

Can we get the previous value of a key when working with connect? If not it would be great to have a feature like that to compare values to perform an update or not. Currently I'm handling it manually.

Like in @ changed

store.on("@changed", (state, change) => {
    // we can get the changes
})

I would expect something like this for connect:

connect("key", (previousState, state) => {
   // we can compare states
})

or

connect("key", (previousValueOfKey, state) => {
   // we can compare values only not all state
})

I think both would work.

[shoonia]

Hello @loeiks

The connect is a method to render your data. You shouldn't check your state in connect callback, only the render logic.

connect('name', (state) => {
  $w('#name').value = state.name;
});

The idea of split your app: 1) state manage layer and 2) render layer. The connect will run only if the state is changed. The changing/check/update state you should manage in the modules.

store.on('add/item', (state, newItem) => {
  if (state.items.length > 100) {
    return; // the state won't be update, if you return nothing
  }
  
  return {
    items: [...state.items, newItem],
  };
});

In the module you have the current state and it should return partial of state that you want to update or undefined if you don't want to update state.

state.on('your/event', (state, data) => {
  const previousState = { ...state };
  const nextState = { ...state, ...data };
  
  if (businessLogic(previousState, nextState)) {
    return nextState;
  }
});