//! Listen to input method events.
use std::ops::Range;

/// Describes [input method](https://en.wikipedia.org/wiki/Input_method) events.
///
/// This is also called a "composition event".
///
/// Most keypresses using a latin-like keyboard layout simply generate a
/// [`WindowEvent::KeyboardInput`]. However, one couldn't possibly have a key for every single
/// unicode character that the user might want to type
/// - so the solution operating systems employ is to allow the user to type these using _a sequence
///   of keypresses_ instead.
///
/// A prominent example of this is accents - many keyboard layouts allow you to first click the
/// "accent key", and then the character you want to apply the accent to. In this case, some
/// platforms will generate the following event sequence:
///
/// ```ignore
/// // Press "`" key
/// Ime::Preedit("`", Some((0, 0)))
/// // Press "E" key
/// Ime::Preedit("", None) // Synthetic event generated by winit to clear preedit.
/// Ime::Commit("é")
/// ```
///
/// Additionally, certain input devices are configured to display a candidate box that allow the
/// user to select the desired character interactively. (To properly position this box, you must use
/// [`Window::set_ime_cursor_area`].)
///
/// An example of a keyboard layout which uses candidate boxes is pinyin. On a latin keyboard the
/// following event sequence could be obtained:
///
/// ```ignore
/// // Press "A" key
/// Ime::Preedit("a", Some((1, 1)))
/// // Press "B" key
/// Ime::Preedit("a b", Some((3, 3)))
/// // Press left arrow key
/// Ime::Preedit("a b", Some((1, 1)))
/// // Press space key
/// Ime::Preedit("啊b", Some((3, 3)))
/// // Press space key
/// Ime::Preedit("", None) // Synthetic event generated by winit to clear preedit.
/// Ime::Commit("啊不")
/// ```
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub enum Event {
    /// Notifies when the IME was enabled.
    ///
    /// After getting this event you could receive [`Preedit`][Self::Preedit] and
    /// [`Commit`][Self::Commit] events. You should also start performing IME related requests
    /// like [`Window::set_ime_cursor_area`].
    Enabled,

    /// Notifies when a new composing text should be set at the cursor position.
    ///
    /// The value represents a pair of the preedit string and the cursor begin position and end
    /// position. When it's `None`, the cursor should be hidden. When `String` is an empty string
    /// this indicates that preedit was cleared.
    ///
    /// The cursor range is byte-wise indexed.
    Preedit(String, Option<Range<usize>>),

    /// Notifies when text should be inserted into the editor widget.
    ///
    /// Right before this event winit will send empty [`Self::Preedit`] event.
    Commit(String),

    /// Notifies when the IME was disabled.
    ///
    /// After receiving this event you won't get any more [`Preedit`][Self::Preedit] or
    /// [`Commit`][Self::Commit] events until the next [`Enabled`][Self::Enabled] event. You should
    /// also stop issuing IME related requests like [`Window::set_ime_cursor_area`] and clear
    /// pending preedit text.
    Disabled,
}