# 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.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.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.quit()` -- calling this ensures any final edits are flushed to disk
before the app exits.
* `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()`.