Add UI template, koa mount

ui/src/features/counter/counterSlice.ts

@@ -0,0 +1,50 @@
+import { createSlice, PayloadAction } from '@reduxjs/toolkit';
+import { AppThunk, RootState } from '../../app/store';
+
+interface CounterState {
+  value: number;
+}
+
+const initialState: CounterState = {
+  value: 0,
+};
+
+export const counterSlice = createSlice({
+  name: 'counter',
+  initialState,
+  reducers: {
+    increment: state => {
+      // Redux Toolkit allows us to write "mutating" logic in reducers. It
+      // doesn't actually mutate the state because it uses the Immer library,
+      // which detects changes to a "draft state" and produces a brand new
+      // immutable state based off those changes
+      state.value += 1;
+    },
+    decrement: state => {
+      state.value -= 1;
+    },
+    // Use the PayloadAction type to declare the contents of `action.payload`
+    incrementByAmount: (state, action: PayloadAction<number>) => {
+      state.value += action.payload;
+    },
+  },
+});
+
+export const { increment, decrement, incrementByAmount } = counterSlice.actions;
+
+// The function below is called a thunk and allows us to perform async logic. It
+// can be dispatched like a regular action: `dispatch(incrementAsync(10))`. This
+// will call the thunk with the `dispatch` function as the first argument. Async
+// code can then be executed and other actions can be dispatched
+export const incrementAsync = (amount: number): AppThunk => dispatch => {
+  setTimeout(() => {
+    dispatch(incrementByAmount(amount));
+  }, 1000);
+};
+
+// The function below is called a selector and allows us to select a value from
+// the state. Selectors can also be defined inline where they're used instead of
+// in the slice file. For example: `useSelector((state: RootState) => state.counter.value)`
+export const selectCount = (state: RootState) => state.counter.value;
+
+export default counterSlice.reducer;