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#473 in Data structures




A undo-redo framework.

Generative approach

This section describes a basic concept of this crate. The concept is that "Undoing is regenerating (recomputing) the old state."

For explanation, a sample history of changes is shown as follows,

t: state
c: command
s: snapshot

old <----------------------> new
     c1        c2        c3
t0 -----> t1 -----> t2 -----> t3
|  +-------------->           |
|  |                          |
s0 +--------------------------+
           undo t3 -> t2

Where tx is an application state at time point x, cx is a command to change a state tx-1 to next tx, and sx is a snapshot of a state tx.

The application state have changed in order of t0, t1, t2, and t3. Now, the current state is t3.

Let us consider undoing the current state t3 to its previous state t2. First, the system restores an old state from its snapshot at any point in the history. In this case, We would have to restore the state t0 from s0 because there is only one snapshot s0. Then the system reruns the commands (c1 and c2) in order. Finally, the target state t2 will be obtained.

Sample code

use gur::prelude::*;
use gur::cur::{Cur, CurBuilder};

// Appication state
struct MyState {
    data: String

fn main() {
    // Initialize
    let mut state: Cur<MyState> = CurBuilder::new().build(MyState{ data: "My".to_string() });
    assert_eq!("My", state.data);

    // Change state
    state.edit(|mut state: MyState| { state.data += "State"; state });
    assert_eq!("MyState", state.data);

    // Undo
    assert_eq!("My", state.data);

    // Redo
    assert_eq!("MyState", state.data);

Where Cur<T> is a type providing undo-redo functionality. The MyState is a type of user's application state. MyState implements the Clone trait required by Cur<T>. Then the variable state as type Cur<MyState> is created to get the ability to undo-redo.

The edit takes a closure to change the variable. The closure is a function that consumes a current state as internal type MyState and returns a new state. The undo can restore the previous state. The redo is reverse operation of the undo.

The Cur<T> implements Deref. So the variable can be used as like smart pointers.

No runtime deps