Seed `view` optimization by tracking data changes in `view` functions arguments

view_optimization.rs

#![allow(dead_code)]

use std::collections::BTreeMap;
use std::collections::hash_map::DefaultHasher;
use std::hash::{Hash, Hasher};

struct Model {
    todos: BTreeMap<String, Todo>,
    new_todo_title: String,
    selected_todo: Option<SelectedTodo>,
    filter: Filter,
}

struct Todo {
    title: String,
    completed: bool,
}

struct SelectedTodo {
    title: String,
}

enum Filter {
   All,
   Active,
}

fn main() {
    let mut view_arg_1_hash: Option<u64> = None;
    let mut view_new_todo_title_arg_1_hash: Option<u64> = None;

    let mut model = Model {
        todos: BTreeMap::new(),
        new_todo_title: "my todo".to_owned(),
        selected_todo: None,
        filter: Filter::All,
    };
    
    println!("--- 1. view call ---");
    view(&model, &mut view_arg_1_hash, &mut view_new_todo_title_arg_1_hash);
    
    println!("--- 2. view call --- ");
    view(&model, &mut view_arg_1_hash, &mut view_new_todo_title_arg_1_hash);
    
    println!("--- 3. view call --- ");
    // change `Model` a bit to trigger at least root `view` logic
    model.filter = Filter::Active;
    view(&model, &mut view_arg_1_hash, &mut view_new_todo_title_arg_1_hash);
    
    println!("--- 4. view call --- ");
    // change `new_todo_title` to trigger root `view` logic and also `view_new_todo_title` logic
    model.new_todo_title = "change my todo title".to_owned();
    view(&model, &mut view_arg_1_hash, &mut view_new_todo_title_arg_1_hash);
}

// Arguments `view_arg_1_hash` and `view_new_todo_title_arg_1_hash` + hashing + match
// should be refactored and then hidden by the attribute `#[view]`
fn view(model: &Model, view_arg_1_hash: &mut Option<u64>, view_new_todo_title_arg_1_hash: &mut Option<u64>) {
    let hash = as_bytes_and_calculate_hash(model);
    let old_hash = view_arg_1_hash.replace(hash);
    match old_hash {
        Some(old_hash) if hash == old_hash => return,
        _ => ()
    }
    
    println!("view called");
    view_new_todo_title(&model.new_todo_title, view_new_todo_title_arg_1_hash);
}

fn view_new_todo_title(title: &str, view_new_todo_title_arg_1_hash: &mut Option<u64>) {
    // Note: `to_owned` is needed because of `::std::mem::size_of::<T>(),` in `any_as_u8_slice`
    // but there is probably a way how to get bytes from reference to unsized types.
    let hash = as_bytes_and_calculate_hash(&title.to_owned());
    let old_hash = view_new_todo_title_arg_1_hash.replace(hash);
    match old_hash {
        Some(old_hash) if hash == old_hash => return,
        _ => ()
    }
    
    println!("view new todo title called");
}

fn calculate_hash<T: Hash + ?Sized>(t: &T) -> u64 {
    let mut s = DefaultHasher::new();
    t.hash(&mut s);
    s.finish()
}

fn as_bytes_and_calculate_hash<T>(t: &T) -> u64 {
    let bytes: &[u8] = unsafe { any_as_u8_slice(t) };
    calculate_hash(bytes)
}

unsafe fn any_as_u8_slice<T: Sized>(p: &T) -> &[u8] {
    ::std::slice::from_raw_parts(
        (p as *const T) as *const u8,
        ::std::mem::size_of::<T>(),
    )
}

It can be run in https://play.rust-lang.org/