# Some useful building blocks

Apps can be composed of a wide variety of building blocks that you
can use in your functions, including a small number of functions that get
automatically called for you as appropriate.

## Variables you can read

* `App.screen`
  * `width` and `height` -- integer dimensions for the app window in pixels.
  * `flags` -- some properties of the app window. See [`flags` in `love.graphics.getMode`](https://love2d.org/wiki/love.window.getMode)
    for details.

* `Version` -- the running version of LÖVE as a string, e.g. '11.4'.
* `Major_version` -- just the part before the period as an int, e.g. 11.

## Functions that get automatically called

* `App.initialize_globals()` -- called before running each test and also
  before the app starts up. As the name suggests, use this to initialize all
  your global variables to something consistent. I also find it useful to be
  able to see all my global variables in one place, and avoid defining
  top-level variables anywhere else (unless they're constants and never going
  to be modified).

* `App.initialize(arg)` -- called when app starts up after
  `App.initialize_globals`. Provides in `arg` an array of words typed in if
  you ran it from a terminal window.
  (Based on [LÖVE](https://love2d.org/wiki/love.load).)

* `App.quit()` -- called before the app shuts down.
  (Based on [LÖVE](https://love2d.org/wiki/love.quit).)

* `App.focus(start?)` -- called when the app starts or stops receiving
  keypresses. `start?` will be `true` when app starts receiving keypresses and
  `false` when keypresses move to another window.
  (Based on [LÖVE](https://love2d.org/wiki/love.focus).)

* `App.resize(w,h)` -- called when you resize the app window. Provides new
  window dimensions in `w` and `h`. Don't bother updating `App.screen.width`
  and `App.screen.height`, that will happen automatically before calling
  `App.resize`.
  (Based on [LÖVE](https://love2d.org/wiki/love.resize))

* `App.filedropped(file)` -- called when a file icon is dragged and dropped on
  the app window. Provides in `file` an object representing the file that was
  dropped, that will respond to the following messages:

    * `file:getFilename()` returning a string name
    * `file:read()` returning the entire file contents in a single string

  (Based on [LÖVE](https://love2d.org/wiki/love.filedropped).)

* `App.draw()` -- called to draw on the window, around 30 times a second.
  (Based on [LÖVE](https://love2d.org/wiki/love.draw).)

* `App.update(dt)` -- called after every call to `App.draw`. Make changes to
  your app's variables here rather than in `App.draw`. Provides in `dt` the
  time since the previous call to `App.update`, which can be useful for things
  like smooth animations.
  (Based on [LÖVE](https://love2d.org/wiki/love.update).)

* `App.mousepressed(x,y, mouse_button)` -- called when you press down on a
  mouse button. Provides in `x` and `y` the point on the screen at which the
  click occurred, and in `mouse_button` an integer id of the mouse button
  pressed.
  `1` is the primary mouse button (the left button on a right-handed mouse),
  `2` is the secondary button (the right button on a right-handed mouse),
  and `3` is the middle button. Further buttons are mouse-dependent.
  (Based on [LÖVE](https://love2d.org/wiki/love.mousepressed).)

* `App.mousereleased(x,y, mouse_button)` -- called when you release a mouse
  button. Provides the same arguments as `App.mousepressed()` above.
  (Based on [LÖVE](https://love2d.org/wiki/love.mousereleased).)

* `App.mousemoved(x,y, dx,dy, is_touch)` -- called any time the mouse moves.
  (Based on [LÖVE](https://love2d.org/wiki/love.mousemoved).)

* `App.wheelmoved(dx,dy)` -- called when you use the scroll wheel on a mouse
  that has it. Provides in `dx` and `dy` an indication of how fast the wheel
  is being scrolled. Positive values for `dx` indicate movement to the right.
  Positive values for `dy` indicate upward movement.
  (Based on [LÖVE](https://love2d.org/wiki/love.wheelmoved).)

* `App.mousefocus(in_focus)` -- called when the mouse pointer moves on or off
  the app window.
  (Based on [LÖVE](https://love2d.org/wiki/love.mousefocus).)

* `App.keychord_press(chord, key)` -- called when you press a key-combination.
  Provides in `key` a string name for the key most recently pressed ([valid
  values](https://love2d.org/wiki/KeyConstant)). Provides in `chord` a
  string representation of the current key combination, consisting of the key
  with the following prefixes:
    * `C-` if one of the `ctrl` keys is pressed,
    * `M-` if one of the `alt` keys is pressed,
    * `S-` if one of the `shift` keys is pressed, and
    * `s-` if the `windows`/`cmd`/`super` key is pressed.

* `App.textinput(t)` -- called when you press a key combination that yields
  (roughly) a printable character. For example, `shift` and `a` pressed
  together will call `App.textinput` with `A`.
  (Based on [LÖVE](https://love2d.org/wiki/love.textinput).)

* `App.keyreleased(key)` -- called when you press a key on the keyboard.
  Provides in `key` a string name for the key ([valid values](https://love2d.org/wiki/KeyConstant)).
  (Based on [LÖVE](https://love2d.org/wiki/love.keyreleased), including other
  variants.)

## Functions you can call

Everything in the [LÖVE](https://love2d.org/wiki/Main_Page) and
[Lua](https://www.lua.org/manual/5.1/manual.html) guides is available to you,
but here's a brief summary of the most useful primitives. Some primitives have
new, preferred names under the `App` namespace, often because these variants
are more testable. If you run them within a test you'll be able to make
assertions on their side-effects.

### regarding the app window

* `width, height, flags = App.screen.size()` -- returns the dimensions and
  some properties of the app window.
  (Based on [LÖVE](https://love2d.org/wiki/love.window.getMode).)

* `App.screen.resize(width, height, flags)` -- modify the size and properties
  of the app window. The OS may or may not act on the request.
  (Based on [LÖVE](https://love2d.org/wiki/love.window.setMode).)

* `x, y, displayindex = App.screen.position()` -- returns the coordinates and
  monitor index (if you have more than one monitor) for the top-left corner of
  the app window.
  (Based on [LÖVE](https://love2d.org/wiki/love.window.getPosition).)

* `App.screen.move(x, y, displayindex)` -- moves the app window so its
  top-left corner is at the specified coordinates of the specified monitor.
  The OS may or may not act on the request.
  (Based on [LÖVE](https://love2d.org/wiki/love.window.setPosition).)

### drawing to the app window

* `App.screen.print(text, x,y)` -- print the given `text` in the current font
  using the current color so its top-left corner is at the specified
  coordinates of the app window.
  (Based on [LÖVE](https://love2d.org/wiki/love.graphics.print).)

* `love.graphics.getFont()` -- returns a representation of the current font.
  (From [LÖVE](https://love2d.org/wiki/love.graphics.getFont).)

* `love.graphics.setFont(font)` -- switches the current font to `font`.
  (From [LÖVE](https://love2d.org/wiki/love.graphics.setFont).)

* `love.graphics.newFont(filename)` -- creates a font from the given font
  file.
  (From [LÖVE](https://love2d.org/wiki/love.graphics.newFont), including other
  variants.)

* `App.width(text)` returns the width of `text` in pixels when rendered using
  the current font.
  (Based on [LÖVE](https://love2d.org/wiki/Font:getWidth).)

* `App.color(color)` -- sets the current color based on the fields `r`, `g`,
  `b` and `a` (for opacity) of the table `color`.
  (Based on [LÖVE](https://love2d.org/wiki/love.graphics.setColor).)

* `love.graphics.line(x1,y1, x2,y2)` -- draws a line from (`x1`,`y1`) to
  (`x2`, `y2`) in the app window using the current color, clipping data for
  negative coordinates and coordinates outside (`App.screen.width`,
  `App.screen.height`)
  (From [LÖVE](https://love2d.org/wiki/love.graphics.line), including other
  variants.)

* `love.graphics.rectangle(mode, x, y, w, h)` -- draws a rectangle using the
  current color, with a top-left corner at (`x`, `y`), with dimensions `width`
  along the x axis and `height` along the y axis
  (though check out https://love2d.org/wiki/love.graphics for ways to scale
  and rotate shapes).
  `mode` is a string, either `'line'` (to draw just the outline) and `'fill'`.
  (From [LÖVE](https://love2d.org/wiki/love.graphics.circle), including other
  variants.)

* `love.graphics.circle(mode, x, y, r)` -- draws a circle using the current
  color, centered at (`x`, `y`) and with radius `r`.
  `mode` is a string, either `'line'` and `'fill'`.
  (From [LÖVE](https://love2d.org/wiki/love.graphics.circle), including other
  variants.)

* `love.graphics.arc(mode, x, y, r, angle1, angle2)` -- draws an arc of a
  circle using the current color, centered at (`x`, `y`) and with radius `r`.
  `mode` is a string, either `'line'` and `'fill'`.
  `angle1` and `angle2` are in [radians](https://en.wikipedia.org/wiki/Radian).
  (From [LÖVE](https://love2d.org/wiki/love.graphics.circle), including other
  variants.)

There's much more I could include here; check out [the LÖVE manual](https://love2d.org/wiki/love.graphics).

### text editor primitives

The text-editor widget includes extremely thorough automated tests to give you
early warning if you break something.

* `state = edit.initialize_state(top, left, right, font, font_height, line_height)`
  -- returns an object that can be used to render an interactive editor widget
  for text and line drawings starting at `y=top` on the app window, between
  `x=left` and `x=right`. Wraps long lines at word boundaries where possible,
  or in the middle of words (no hyphenation yet) when it must.

* `edit.draw(state)` -- call this from `App.draw` to display the current
  editor state on the app window as requested in the call to
  `edit.initialize_state` that created `state`.

* `edit.mouse_press(state, x,y, mouse_button)` and `edit.mouse_release(x,y,
  mouse_button)` -- call these to position the cursor or select some text.

* `edit.mouse_wheel_move(state, dx,dy)` -- call this to scroll the editor in
  response to a mouse wheel.

* `edit.keychord_press(state, chord, key)` and `edit.key_release(state, key)`
  -- call these to perform some standard shortcuts: insert new lines,
  backspace/delete, zoom in/out font size, cut/copy/paste to and from the
  clipboard, undo/redo.

* `edit.text_input(state, t)` -- call this to insert keystrokes into the
  buffer.

* `Text.redraw_all(state)` -- call this to clear and recompute any cached
  state as the cursor moves and the buffer scrolls.

* `edit.update(state, dt)` -- call this from `App.update` to periodically
  auto-save editor contents to disk.

* `edit.quit(state)` -- call this from `App.quit` to ensure any final edits
  get saved before quitting.

If you need more precise control, look at the comment at the top of
`edit.initialize_state` in edit.lua. In brief, the widget contains an array of
`lines`. Positions in the buffer are described in _schema-1_ locations
consisting of a `line` index and a code-point `pos`. We may also convert them
at times to _schema-2_ locations consisting of a `line`, `screen_line` and
`pos` that better indicates how long lines wrap. Schema-2 locations are never
persisted, just generated as needed from schema-1. Important schema-1
locations in the widget are `cursor1` describing where text is inserted or
deleted and `screen_top1` which specifies how far down the lines is currently
visible on screen.

Some constants that affect editor behavior:
* `Margin_top`, `Margin_left`, `Margin_right` are integers in pixel units that
  affect where the editor is drawn on window (it always extends to bottom of
  window as needed)
* `Drawing_padding_top` and `Drawing_padding_bottom` affect spacing around
  drawings.

* Various color constants are represented as tables with r/g/b keys:
  * `Text_color`, `Cursor_color`, `Highlight_color` for drawing text.
  * `Stroke_color`, `Current_stroke_color` for line drawings.
  * `Icon_color` affects the color of the little mode icon on the top right of
    a drawing.
  * `Current_name_background_color` manages the color when naming points using
    `ctrl+n`.
  * `Focus_stroke_color` affects the color of a point or line when you hover
    over it.
  * `Help_color` and `Help_background_color` affect the color of online help
    within line drawings.

### clickable buttons

There's a facility for rendering buttons and responding to events when they're
clicked. It requires setting up 3 things:
  - a `state` table housing all buttons. Can be the same `state` variable the
    text-editor widget uses, but doesn't have to be.
  - specifying buttons to create in `state`. This must happen either directly
    or indirectly within `App.draw`.
  - responding to clicks on buttons in `state`. This must happen either
    directly or indirectly within `App.mousepressed`.

The following facilities help set these things up:

* Clear `state` at the start of each frame:

    ```
    state.button_handlers = {}
    ```

  Don't forget to do this, or your app will get slower over time.

* `button` creates a single button. The syntax is:

    ```
    button(state, name, {x=..., y=..., w=..., h=..., bg={r,g,b},
      icon = function({x=..., y=..., w=..., h=...}) ... end,
      onpress1 = ...
    })
    ```

  Call this either directly or indirectly from `App.draw`. It will assign a
  rectangle with the given dimensions and trigger the provided (zero-arg)
  `onpress1` callback when the primary mouse button is clicked within.
  It will also optionally paint the rectangle with the specified background
  color `bg` and a foreground described by the `icon` callback (which will
  receive the same dimensions).

  This way you can see everything about a button in one place. Create as many
  buttons as you like within a single shared `state`.

* `mouse_press_consumed_by_any_button(state, x,y, mouse_button)`

  Call this either directly or indirectly from `App.mousepressed`. It will
  pass on a click to any button registered in `state`. It's also helpful to
  ensure clicks on a button don't have other effects, so I prefer the
  following boilerplate early in `mousepressed`:

    ```
    if mouse_press_consumed_by_any_button(state, x,y, mouse_button) then
      return
    end
    ```

### mouse primitives

* `App.mouse_move(x, y)` -- sets the current position of the mouse to (`x`,
  `y`).
  (Based on [LÖVE](https://love2d.org/wiki/love.mouse.setPosition).)

* `App.mouse_down(mouse_button)` -- returns `true` if the button
  `mouse_button` is pressed. See `App.mousepressed` for `mouse_button` codes.
  (Based on [LÖVE](https://love2d.org/wiki/love.mouse.isDown).)

* `App.mouse_x()` -- returns the x coordinate of the current position of the
  mouse.
  (Based on [LÖVE](https://love2d.org/wiki/love.mouse.getX).)

* `App.mouse_y()` -- returns the x coordinate of the current position of the
  mouse.
  (Based on [LÖVE](https://love2d.org/wiki/love.mouse.getY).)

### keyboard primitives

* `App.is_cursor_movement(key)` -- return `true` if `key` is a cursor movement
  key (arrow keys, page-up/down, home/end)

* `App.cmd_down()`, `App.ctrl_down`, `App.alt_down()`, `App.shift_down()` --
  predicates for different modifier keys.

* `App.any_modifier_down()` -- returns `true` if any of the modifier keys is
  currently pressed.

* `App.key_down(key)` -- returns `true` if the given key is currently pressed.
  (Based on [LÖVE](https://love2d.org/wiki/love.keyboard.isDown).)

### interacting with files

* `App.open_for_reading(filename)` -- returns a file handle that you can
  [`read()`](https://www.lua.org/manual/5.1/manual.html#pdf-file:read) from.
  Make sure `filename` is an absolute path so that your app can work reliably
  by double-clicking on it.
  (Based on [Lua](https://www.lua.org/manual/5.1/manual.html#pdf-io.open).)

* `App.open_for_writing(filename)` -- returns a file handle that you can
  [`write()`](https://www.lua.org/manual/5.1/manual.html#pdf-file:write) to.
  Make sure `filename` is an absolute path so that your app can work reliably
  by double-clicking on it.
  (Based on [Lua](https://www.lua.org/manual/5.1/manual.html#pdf-io.open).)

* `json.encode(obj)` -- returns a JSON string for an object `obj` that will
  recreate `obj` when passed to `json.decode`. `obj` can be of most types but
  has some exceptions.
  (From [json.lua](https://github.com/rxi/json.lua).)

* `json.decode(obj)` -- turns a JSON string into a Lua object.
  (From [json.lua](https://github.com/rxi/json.lua).)

* `App.files(dir)` -- returns an unsorted array of the files and directories
  available under `dir`.
  (From [LÖVE](https://love2d.org/wiki/love.filesystem.getDirectoryItems).]

* `App.file_info(filename)` -- returns some information about
  `filename`, particularly whether it exists (non-`nil` return value) or not.
  (From [LÖVE](https://love2d.org/wiki/love.filesystem.getInfo).]

* `App.mkdir(path)` -- creates a directory. Make sure `path` is absolute.
  (From [LÖVE](https://love2d.org/wiki/love.filesystem.remove).]

* `App.remove(filename)` -- removes a file or empty directory. Definitely make
  sure `filename` is an absolute path.
  (From [LÖVE](https://love2d.org/wiki/love.filesystem.remove).]

There's much more I could include here; check out [the LÖVE manual](https://love2d.org/wiki/love.filesystem)
and [the Lua manual](https://www.lua.org/manual/5.1/manual.html#5.7).

### desiderata

* `App.get_time()` -- returns the number of seconds elapsed since some
  unspecified start time.
  (Based on [LÖVE](https://love2d.org/wiki/love.timer.getTime).)

* `App.get_clipboard()` -- returns a string with the current clipboard
  contents.
  (Based on [LÖVE](https://love2d.org/wiki/love.system.getClipboardText).)

* `App.set_clipboard(text)` -- stores the string `text` in the clipboard.
  (Based on [LÖVE](https://love2d.org/wiki/love.system.setClipboardText).)

* `array.find(arr, elem)` -- scan table `arr` for `elem` assuming it's
  organized as an array (just numeric indices).

* `array.any(arr, f)` -- scan table `arr` for any elements satisfying
  predicate `f`. Return first such element or `false` if none.

There's much more I could include here; check out [the LÖVE manual](https://love2d.org/wiki)
and [the Lua manual](https://www.lua.org/manual/5.1/manual.html).

### writing tests

* `App.screen.init{width=.., height=..}` -- creates a fake screen for a test

* `App.screen.check(y, expected_contents, msg)` -- verifies text written to
  the fake screen at `y`. This isn't very realistic; `y` must exactly match
  what was displayed, and the expected contents show everything printed to
  that `y` in chronological order, regardless of `x` coordinate. In spite of
  these limitations, you can write lots of useful tests with this.

* `App.run_after_textinput(t)` -- mimics keystrokes resulting in `t` and then
  draws one frame.

* `App.run_after_keychord(chord, key)` -- mimics the final `key` press
  resulting in `chord` and then draws one frame.

* `App.run_after_mouse_press(x,y, mouse_button)` -- mimics a mouse press down
  followed by drawing a frame.

* `App.run_after_mouse_release(x,y, mouse_button)` -- mimics a mouse release
  up followed by drawing a frame.

* `App.run_after_mouse_click(x,y, mouse_button)` -- mimics a mouse press down
  and mouse release up followed by drawing a frame.

* `App.wait_fake_time(t)` -- simulates the passage of time for `App.getTime()`.