refactor(app): Making impossible states impossible

[kyu08]

Motivation

• Resolves [Refactor] Making impossible states impossible #262

What's changed

Made impossible states impossible.

At first, I show main change point.

Before

pub struct Model<'a> {
    pub app_state: AppState,
    pub current_pane: CurrentPane,
    pub makefile: Makefile,
    pub search_text_area: TextArea_<'a>,
    pub targets_list_state: ListState,
    pub histories: Option<Histories>, // When home dir could not be found, this is None.
    pub histories_list_state: ListState,
}

pub enum AppState {
    SelectingTarget,
    ExecuteTarget(Option<String>),
    ShouldQuit,
}

After

pub struct Model<'a> {
    pub app_state: AppState<'a>,
}

pub enum AppState<'a> {
    SelectTarget(SelectTargetState<'a>),
    ExecuteTarget(Option<String>),
    ShouldQuit,
}

pub struct SelectTargetState<'a> {
    pub current_pane: CurrentPane,
    pub makefile: Makefile,
    pub search_text_area: TextArea_<'a>,
    pub targets_list_state: ListState,
    pub histories: Option<Histories>,
    pub histories_list_state: ListState,
}

By this change, we can no longer express impossible states like below.

Model {
    app_state: AppState::ShouldQuit,
    current_pane: CurrentPane::Main,
    makefile: Makefile::new_for_test(),
    targets_list_state: ListState::with_selected(ListState::default(), Some(0)),
    search_text_area: TextArea_(TextArea::default()),
    histories: init_histories(vec!["history0".to_string(), "history1".to_string()]),
    histories_list_state: ListState::with_selected(ListState::default(), Some(0)),
}

When app_state is AppState::ShouldQuit, the app need not have any data.

The above code become like this:

Model {
    app_state: AppState::ShouldQuit,
}

Good effect of this change

This makes some good effect:

1. Improve readability

It made readability improved because the data structure describes the application state more precisely.

This may good especially for the person who are new to this project too.

2. Reduce the chance of potential bugs because invalid data accesses can't happen.

As the title says.

3. Remove unnecessary fields.

This is not huge advantage for now, but the effort for testing will not increase even though the data structure grows.

// Before
Case {
    title: "Quit",
    model: init_model(),
    message: Some(Message::Quit),
    expect_model: Model {
        app_state: AppState::ShouldQuit,
        ..init_model() // this is unnecessary field
    },
},

// After
Case {
    title: "Quit",
    model: Model {
        app_state: AppState::SelectTarget(SelectTargetState {
            ..SelectTargetState::new_for_test()
        }),
    },
    message: Some(Message::Quit),
    expect_model: Model {
        app_state: AppState::ShouldQuit,
    },
},

Also, this makes refactoring easier because only necessary fields are defined.

Minor changes

• Improve cohesiveness by extract logics to method.
• Improve ordering of type definition and thier methods in app.rs.