# Proposed Design for a modern Cursor system in v2

See end for list of issues this design addresses.

## Tenets for Cursor Support (Unless you know better ones...)

1. **More GUI than Command Line**. The concept of a cursor on the command line of a terminal is intrinsically tied to enabling the user to know where keybaord import is going to impact text editing. TUI apps have many more modalities than text editing where the keyboard is used (e.g. scrolling through a `ColorPicker`). Terminal.Gui's cursor system is biased towards the broader TUI experiences.

2. **Be Consistent With the User's Platform** - Users get to choose the platform they run *Terminal.Gui* apps on and the cursor should behave in a way consistent with the terminal.

## Lexicon & Taxonomy

- Cursor - A visual indicator to the user where keyboard input will have an impact. There is one Cursor per terminal sesssion.
- Cursor Location - The top-left corner of the Cursor. In text entry scenarios, new text will be inserted to the left/top of the Cursor Location. 
- Cursor Size - The width and height of the cursor. Currently the size is limited to 1x1.
- Cursor Style - How the cursor renders. Some terminals support various cursor styles such as Block and Underline.
- Cursor Visibilty - Whether the cursor is visible to the user or not. NOTE: Some ConsoleDrivers overload Cursor Style and Cursor Visibility, making "invisible" a style. Terminal.Gui HIDES this from developers and changing the visibilty of the cursor does NOT change the style.
- Caret - Visual indicator that  where text entry will occur. 
- Selection - A visual indicator to the user that something is selected. It is common for the Selection and Cursor to be the same. It is also common for the Selection and Cursor to be distinct. In a `ListView` the Cursor and Selection (`SelectedItem`) are the same, but the `Cursor` is not visible. In a `TextView` with text selected, the `Cursor` is at either the start or end of the `Selection`. A `TableView' supports mutliple things being selected at once.

## Requirements

- No flickering. The Cursor should blink/pulse at the rate dictated by the terminal. Typing, moving the mouse, view layout, etc... should not caue the cursor to flicker.
- By default, the Cursor should not be visible. A View or View subclass should have to do anything (this is already the case) to keep the Cursor invisible.
- Views that just want to show the cursor at a particular location in the Viewport should only have to:
  - Optionally, declare a desired Cursor Style. Set `Application.CursorStyle`.
  - Indicate the Cursor Locaiton when internal state dictates the location has changed (debatable if this should be in content or viewport-relative coords). Just set `this.CursorPosition`.
  - To hide the cursor, simply set `this.CursorPostion` to `null`.
- The Cursor should only be visible in Views where
  - `Enabled == true`
  - `Visible == true`
  - `CanFocus == true`
  - `this == SuperView.MostFocused`
- If a `ConsoleDriver` supports Cursor Styles other than Default, they should be supported per-application (NOT View). 
- Ensuring the cursor is visible or not should be handled by `Application`, not `View`.
- General V2 Requirement: View sub-class code should NEVER call a `Driver.` API. Only `Application` and the `View` base class should call `ConsoleDriver` APIs; before we ship v2, all `ConsoleDriver` APIs will be made `internal`.

## Design

### `View` Focus Changes

It doesn't make sense the every View instance has it's own notion of `MostFocused`. The current implemention is overly complicated and fragile because the concept of "MostFocused" is handled by `View`. There can be only ONE "most focused" view in an application. `MostFocused` should be a property on `Application`.

* Remove `View.MostFocused`
* Change all references to access `Application.MostFocusedView` (see `Application` below)
* Find all instances of `view._hasFocus = ` and change them to use `SetHasFocus` (today, anyplace that sets `_hasFocus` is a BUG!!).
* Change `SetFocus`/`SetHasFocus` etc... such that if the focus is changed to a different view heirarchy, `Application.MostFocusedView` gets set appropriately. 

**MORE THOUGHT REQUIRED HERE** - There be dragons given how `Toplevel` has `OnEnter/OnLeave` overrrides. The above needs more study, but is directioally correct.

### `View` Cursor Changes
* Add `public Point? CursorPosition`
    - Backed with `private Point? _cursorPosition`
    - If `!HasValue` the cursor is not visible
    - If `HasValue` the cursor is visible at the Point.
    - On set, if `value != _cursorPosition`, call `OnCursorPositionChanged()`
* Add `public event EventHandler<LocaitonChangedEventArgs>? CursorPositionChanged`
* Add `internal void OnCursorPositionChanged(LocationChangedEventArgs a)`
  * Not virtual
  * Fires `CursorPositionChanged`

### `ConsoleDriver`s

* Remove `Refresh` and have `UpdateScreen` and `UpdateCursor` be called separately. The fact that `Refresh` in all drivers currently calls both is a source of flicker.

* Remove the `xxxCursorVisibility` APIs and replace with:
  * `internal int CursorStyle {get; internal set; }`
    - Backed with `private int _cursorStyle`
    - On set, calls `OnCursorStyleChanged()`
  * Add `internal abstract void OnCursorStyleChanged()`
    - Called by `base` whenever the cursor style changes, but ONLY if `value != _cursorStyle`.

  * Add `internal virtual (int Id, string StyleName) []  GetCursorStyles()`
    - Returns an array of styles supported by the driver, NOT including Invisible. 
    - The first item in array is always "Default".
    - Base implementation returns `{ 0, "Default" }`
    - `CursesDriver` and `WindowsDriver` will need to implement overrides.

  * Add `internal Point? CursorPosition {get; internal set; }`
    - Backed with `private Point? _cursorPosition`
    - If `!HasValue` the cursor is not visible
    - If `HasValue` the cursor is visible at the Point.
    - On set, calls `OnCursorPositionChanged` ONLY if `value != _cursorPosition`.
  * Add `internal abstract void OnCursorPositionChanged()`
    - Called by `base` whenever the cursor position changes. 
    - Depending on the value of `CursorPosition`:
        - If `!HasValue` the cursor is not visible - does whatever is needed to make the cursor invisible.
        - If `HasValue` the cursor is visible at the `CursorPosition` - does whatever is needed to make the cursor visible (using `CursorStyle`).

  * Make sure the drivers only make the cursor visible (or leave it visible) when `CursorPosition` changes!

### `Application`

* 

* Add `internal static View FocusedView {get; private set;}` 
  - Backed by `private static _focusedView`
  - On set, 
    - if `value != _focusedView` 
        - Unsubscribe from `_focusedView.CursorPositionChanged`
        - Subscribe to `value.CursorPositionChanged += CursorPositionChanged`        
        - `_focusedView = value`
        - Call `UpdateCursor` 

* Add `internal bool CursorPositionChanged (object sender, LocationChangedEventArgs a)`

    Called when:

    - `FocusedView`
        - Has changed to another View (should cover `FocusedView.Visible/Enable` changes)
        - Has changed layout - 
        - Has changeed it's `CursorPosition`
    - `CursorStyle` has changed

    Does:

    - If `FocusedView is {}` and `FocusedView.CursorPosition` is visible (e.g. w/in `FocusedView.SuperView.Viewport`) 
        - Does `Driver.CursorPosition = ToScreen(FocusedView.CursorPosition)`
    - Else
        - Makes driver cursor invisible with `Driver.CursorPosition = null`

* Add `public static int CursorStyle {get; internal set; }`
  - Backed with `private static int _cursorStyle
  - If `value != _cursorStyle`
    - Calls `ConsoleDriver.CursorStyle = _cursorStyle` 
    - Calls `UpdateCursor`

* Add `public (int Id, string StyleName) []  GetCursorStyles()`
  - Calls through to `ConsoleDriver.GetCursorStyles()`



# Issues with Current Design

## `Driver.Row/Pos`, which are changed via `Move` serves two purposes that confuse each other:

a) Where the next `AddRune` will put the next rune
b) The current "Cursor Location"

If most TUI apps acted like a command line where the visible cursor was always visible, this might make sense. But the fact that only a very few View subclasses we've seen actually care to show the cursor illustrates a problem:

Any drawing causes the "Cursor Position" to be changed/lost. This means we have a ton of code that is constantly repositioning the cursor every MainLoop iteration.

## The actual cursor position RARELY changes (relative to `Mainloop.Iteration`).

Derived from abo`ve, the current design means we need to call `View.PositionCursor` every iteration. For some views this is a low-cost operation. For others it involves a lot of math. 

This is just stupid. 

## Flicker

Related to the above, we need constantly Show/Hide the cursor every iteration. This causes ridiculous cursor flicker. 

## `View.PositionCursor` is poorly spec'd and confusing to implement correctly

Should a view call `base.PositionCursor`? If so, before or after doing stuff? 

## Setting cursor visibility in `OnEnter` actually makes no sense

First, leaving it up to views to do this is fragile.

Second, when a View gets focus is but one of many places where cursor visibilty should be updated. 
