Polpetta

It's "another" MVI library but with a funny name. With this library you will be able to mutate your State by using a Action defined by using DSL.

Setup

repositories {
  mavenCentral()
}
dependencies {
  implementation 'dev.mcatta:polpetta:0.0.8'
}

The full reference documentation is available here.

How it works

You application's State must extend State and your action the Action class. Every Action can prompt a Reducer which basically manipulate your State.

sealed interface CounterState : State {
    data class Count(val counter: Int) : TestState
    data class Result(val message: String) : TestState
}

sealed interface CounterAction : Action {
    object Increase : CounterAction
    object Decrease : CounterAction
    data class Set(val n: Int) : CounterAction
    object DoNothing : CounterAction
    object ToString : CounterAction
}

interface MySideEffect : SideEffect

This is you State and Actions definition, now you need to write a StateStore which basically will persist your state and your Action/Reducer definition.

class CounterStore(scope: CoroutineScope) : StateStore<CounterAction, CounterState, MySideEffect>(
    coroutineScope = scope,
    initialState = CounterState.Count(0),
    reducerFactory =  {
        on<CounterAction.Decrease, CounterState.Count> { action, state ->
            state.mutate { copy(counter = counter - 1) }
        }
        on<CounterAction.Increase, CounterState.Count> { action, state ->
            state.mutate { copy(counter = counter + 1) }
        }
        on<CounterAction.Set, CounterState.Count> { action, state ->
            state.mutate { copy(counter = action.n) }
        }
        on<CounterAction.DoNothing> { action, state ->
            state.nothing()
        }
        on<CounterAction.ToString, CounterState.Count> { action, state ->
            state.transform<CounterState.Result> { CounterState.Result(counter.toString()) }
        }
        // ...
    }
)

You can add the debugMode = true to enable the logging.

StateStore<CounterAction, CounterState, MySideEffect>(
    coroutineScope = scope,
    debugMode = true,
    initialState = CounterState.Count(0),
    reducerFactory =  {}
)

Supported StateModifiers

The reducer supports three types of operations:

{ action, state -> state.nothing() }

which basically doesn't change the state

{ action, state -> state.mutate { copy(counter = counter + 1) } }

which mutate the properties of the current state (Note: your state must be data class in order to copy it)

{ action, state -> state.transform { CounterState.Result(counter.toString()) } }

which allows to change the current state into a new one of different type

Side Effects

Polpetta supports also side effects. In order to support that we need to specify which SideEffect class we want to use:

class CounterStore(scope: CoroutineScope) : StateStore<CounterAction, CounterState, MySideEffect>

otherwise we can say Nothing

class CounterStore(scope: CoroutineScope) : StateStore<CounterAction, CounterState, Nothing>

Then given a specific SideEffect event we can prompt it inside the Reducer's scope, like this:

// Inside the StateStore
on<TestAction.Increase, TestState.Count> { _, stateModifier ->
    sideEffect(TestSideEffect.Toast("Show message"))
    
    stateModifier.mutate<TestState.Count> { copy(counter + 1) }
}
// On the View
testStore.sideEffectFlow.collect {}

License

Copyright 2025 Marco Cattaneo  
 
Licensed under the Apache License, Version 2.0 (the "License");  
you may not use this file except in compliance with the License.  
You may obtain a copy of the License at  
 
     https://www.apache.org/licenses/LICENSE-2.0  
 
Unless required by applicable law or agreed to in writing, software  
distributed under the License is distributed on an "AS IS" BASIS,  
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  
See the License for the specific language governing permissions and  
limitations under the License.