Refactor View to fix litany of issues in v1

[tig]

THIS IS A WORK IN PROGRESS. Nothing here is definitive. I will update this Issue as we design/debate/discuss...

V2 Spec for View refactor

IMPORTANT: I am critical of the existing codebase below. Do not take any of this personally. It is about the code, not the amazing people who wrote the code.

ALSO IMPORTANT: I've written this to encourage and drive DEBATE. My style is to "Have strong opinions, weakly held." If you read something here you don't understand or don't agree with, SAY SO. Tell me why. Take a stand.

This covers my thinking on how we will refactor View and the classes in the View heirarchy(inclidng Responder). It does not cover

• Text formatting which will be covered in another spec.
• TrueColor support which will be covered separately.
• ConsoleDriver refactor.

What's wrong with the View and the View-class Heirarchy in v1?

• Frame, Bounds, and ClipRect are confusing and not consistently applied...

  • Bounds is Rect but is used to describe a Size (e.g. Bounds.Size is the size of the View's content area). It literaly is implemented as a property that returns new Rect(0, 0, Width, Height). Throughtout the codebase bounds is used for things that have non-zero Size (and actually descibe either the cliprect or the Frame).
  • The restrictive nature of how Bounds is defined led to the hacky FrameView and Window classes with an embedded ContentView in order to draw a border around the content.
    • The only reason FrameView exists is because the original architecture didn't support offsetting View.Bounds such that a border could be drawn and the interior content would clip correctly. Thus Miguel (or someone) built
      FrameView with nested ContentView that was at new Rect(+1, +1, -2, -2).
    • Border was added later, but couldn't be retrofitted into View such that if View.Border ~= null just worked like FrameView.
    • Thus devs are forced to use the clunky FrameView instead of just setting View.Border.
  • Border has a bunch of confusing concepts that don't match other systems (esp the Web/HTML)
    • Margin on the web means the space between elements - Border doesn't have a margin property, but does has the confusing DrawMarginFrame property.
    • Border on the web means the space where a border is drawn. The current implementaiton confuses the term Frame and Border. BorderThickness is provided.
    • Padding on the web means the padding inside of an element between the Border and Content. In the current implementation Padding is actually OUTSIDE of the Border. This means it's not possible for a view to offset internally by simply changing Bounds.
    • Content on the web means the area inside of the Margin + Border + Padding. View does not currently have a concept of this (but FrameView and Window do via the embeded ContentViews.
    • Border has a Title property. So does Window and FrameView. This is unneeded duplicate code.
    • It is not possilble for a class drived from View to orverride the drawing of the "Border" (frame, title, padding, etc...). Multiple devs have asked to be able to have the border frame to be drawn with a different color than View.ColorScheme. The API should explicitly enable devs to override the drawing of Border independently of the View.Draw method. See how WM_NCDRAW works in wWindows (Draw non-client). It should be easy to do this from within a View sub-class (e.g. override OnDrawBorder) and externally (e.g. DrawBorder += () => ....

• AutoSize mostly works, but only because of heroic special-casing logic all over the place by @BDisp. This should be massively simplified.

• FrameView is superlufous and should be removed from the heirarchy (instead devs should just be able to manipulate View.Border (or similar) to achieve what FrameView provides). The internal FrameView.ContentView is a bug-farm and un-needed if View.Border worked correctly.

• TopLevel is currently built around several concepts that are muddled:

  • Views that host a Menu and StatusBar. It is not clear why this is and if it's needed as a concept.
  • Views that can be run via Application.Run<TopLevel> (need a separate RunState). It is not clear why ANY VIEW can't be run this way, but it seems to be a limitation of the current implementation.
  • Views that can be used as a pop-up (modal) (e.g. Dialog). As proven by Wizard, it is possible to build a View that works well both ways. But it's way too hard to do this today.
  • Views that can be moved by the user must inherit from Window today. It should be possilbe to enable moving of any View (e.g. View.CanMove = true).

• The MdiContainer stuff is complex, perhaps overly so, and is not actually used by anyone outside of the project. It's also mis-named because Terminal.Gui doesn't actually support "documents" nor does it have a full "MDI" system like Windows (did). It seems to represent features useful in overlapping Views, but it is super confusing on how this works, and the naming doesn't help. This all can be refactored to support specific scenarios and thus be simplified.

• There is no facility for users' resizing of Views. @tznind's awesome work on LineCanvas and TileView combined with @tig's experiments show it could be done in a great way for both modal (overlapping) and tiled Views.

• DrawFrame and DrawTitle are implemented in ConsoleDriver and can be replaced by a combination of LineCanvas and Border.

• Colors -

  • As noted above each of Margin, Border, Padding, and Content should support independent colors.
  • Many View sub-classes bastardize the exiting ColorSchemes to get look/feel that works (e.g. TextView and Wizard). Separately we should revamp ColorSchemes to enable more scenarios.
  • TrueColor support is needed and should be the default.

• Responder is supposed to be where all common, non-visual-related, code goes. We should ensure this is the case.

• View should have default support for scroll bars. e.g. assume in the new world View.ContentBounds is the clip area (defined by VIew.Frame minus Margin + Border + Padding) then if any view is added with View.Add that has Frame coordinates outside of ContentBounds the appropriate scroll bars show up automatgically (optioally of course). Without any code, scrolling just works.

• We have many requests to support non-full-screen apps. We need to ensure the View class heirachy suppports this in a simple, understandable way. In a world with non-full-screen (where screen is defined as the visible terminal view) apps, the idea that Frame is "screen relative" is broken. Although we COULD just define "screen" as "the area that bounds the Terminal.GUI app.".

Terminal.Gui v2 View-related Lexicon & Taxonomy

• View - The most basic visual element in Terminal.Gui. Implemented in the View base-class.
• SubView - A View that is contained in antoher view and will be rendered as part of the containing view's ContentArea. SubViews are added to another view via the View.Add method. A View may only be a SubView of a single View.
• SuperView - The View that a SubView was added to.
• Child View - A view that is held by another view in a parent/child relationshiop, but is NOT a SubView. Examples of this are sub-menus of MenuBar.
• Parent View - A view that holds a reference to another view in a parent/child relationship, but is NOT a SuperView of the child.
• Thickness - Describes how thick a rectangle is on each of the rectangle's four sides. Valid thickness values are >= 0.
• Margin - Means the Thickness that separtes a View from other SubViews of the same SUperView.
• Title - Means text that is displayed for the View that describes the View to users. For most Views the Title is displayed at the top-left, overlaying the Border.
• Border - Means the Thickness where a visual border (drawn using line-drawing glyphs) and the Title are drawn. The Border expands inward; in other words if Border.Thickness.Top == 2 the border & title will take up the first row and the second row will be filled with spaces.
• Adornments - The Thickness between the Margin and Padding. The Adornments property of View is a View-subclass that hosts SubViews that are not part of the View's content and are rendered within the Adornment Thickness. Examples of Adornments:
  • A TitleBar renders the View's Title and a horizontal line defining the top of the View. Adds thickness to the top of Adornments.
  • One or more LineViews that render the View's border (NOTE: The magic of LineCanvas lets us automatically have the right joins for these and TitleBar!).
  • A Vertical Scrollbar adds thickness to Adornments.Right (or .Left when right-to-left language support is added).
  • A Horizontal Scrollbar adds thickness to Adornments.Bottom when enabled.
  • A MenuBar adds thickness to Adornments.Top (NOTE: This is a change from v1 where subview.Y = 1 is required).
  • A StatusBar adds thickness ot Adornments.Bottom and is rendered at the bottom of Padding.
  • NOTE: The use of View.Add in v1 to add adornments to Views is the cause of much code complexity. Changing the API such that View.Add is ONLY for subviews and adding a View.Adornments.Add API for menu, statusbar, scroll bar... will enable us to signficantly simplify the codebase.
• Padding - Means the Thickness inside of an element that offsets the Content from the Border. (NOTE: in v1 Padding is OUTSIDE of the Border). Padding is {0, 0, 0, 0} by default.
• Frame - Means the Rect that defines the location and size of the View including all of the margin, border, padding, and content area. The coordinates are relative to the SuperView of the View (or, in the case of Application.Top, ConsoleDriver.Row == 0; ConsoleDriver.Col == 0). The Frame's location and size are controlled by either Absolute or Computed positioning via the .X, .Y, .Height, and .Width properties of the View.
• VisibleArea - Means the area inside of the Margin + Border (Title) + Padding. VisibleArea.Location is always {0, 0}. VisibleArea.Size is the View.Frame.Size shrunk by Margin + Border + Padding.
• ContentArea - The Rect that describes the location and size of the View's content, relative to VisibleRect. If ContentArea.Location is negative, anything drawn there will be clipped and any subview positioned in the negative area will cause (optional) scrollbars to appear (making the Thickness of Padding thicker on the appropriate sides). If ContentArea.Size is changed such that the dimensions fall outside of Frame.Size shrunk by Margin + Border + Padding, drawning will be clipped and (optional) scrollbars will appear.
• Bounds - Synomous with VisibleArea. (Debate: Do we rename Bounds to VisbleArea in v2?)
• ClipArea - Means the currently vislble portion of the Content. This is defined as aRect in coordinates relative to ContentArea (NOT VisibleArea) (e.g. ClipArea {X = 0, Y = 0} == ContentArea {X = 0, Y = 0}). This Rect is passed to View.Redraw (and should be named "clipArea" not "bounds"). It defines the clip-region the caller desires the Redraw implementation to clip itself to (see notes on clipping below).
• Modal - The term used when describing a View that was created using the Application.Run(view) or Application.Run<T> APIs. When a View is running as a modal, user input is restricted to just that View until Application.Run exits.
• TopLevel - The term used to describe a view that is both Modal and can have a MenuBar and/or StatusBar.
• Window - A View that is

Thoughts on Built-in Views

• LineView can be replaced by LineCanvas?
• Button and Label can be merged.
• StatusBar and Menu could be combined. If not, then at least made more consistent (e.g. in how hotkeys are specified).

Design

• Responder("Responder base class implemented by objects that want to participate on keyboard and mouse input.") remains mostly unchanged, with minor changes:
  • Methods that take View parametsrs (e.g. OnEnter) change to take Responder (bad OO design).
  • Nuke IsOverriden (bad OO design)
  • Move View.Data to Responder (primitive)
  • Move Command and KeyBinding stuff from View.
  • Move generic mouse and keyboard stuff from View (e.g. WantMousePositionReports)

Example of creating Adornments

In the current PR, I didn't actually implement this fully yet. Instead of using LineView for the border, I used LineCanvas directly.

// ends up looking just like the v1 default Window with a menu & status bar
// and a vertical scrollbar. In v2 the Window class would do all of this automatically.
var top = new TitleBar() {
    X = 0, Y = 0,
    Width = Dim.Fill(),
    Height = 1
    LineStyle = LineStyle.Single
};
var left = new LineView() {
    X = 0, Y = 0,
    Width = 1,
    Height = Dim.Fill(),
    LineStyle = LineStyle.Single
};
var right = new LineView() {
    X = Pos.AnchorEnd(), Y = 0,
    Width = 1,
    Height = Dim.Fill(),
    LineStyle = LineStyle.Single
};
var bottom = new LineView() {
    X = 0, Y = Pos.AnchorEnd(),
    Width = Dim.Fill(),
    Height = 1,
    LineStyle = LineStyle.Single
};

var menu = new MenuBar() { 
    X = Pos.Right(left), Y = Pos.Bottom(top)
};
var status = new StatusBar () {
    X = Pos.Right(left), Y = Pos.Top(bottom)
};
var vscroll = new ScrollBarView () {
    X = Pos.Left(right),
    Y = Dim.Fill(2) // for menu & status bar
};

Adornments.Add(titleBar);
Adornments.Add(left);
Adornments.Add(right);
Adornments.Add(bottom);
Adornments.Add(vscroll);

var treeView = new TreeView () {
    X = 0, Y = 0, Width = Dim.Fill(), Height = Dim.Fill()
};
Add (treeView);

[tig]

Fixed by #2361 (mostly)