# 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.
## 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
`on.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 `on.draw`. Make changes to
your app's variables here rather than in `on.draw`. Provides in `dt` the
time since the previous call to `on.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 `on.mouse_press()` 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 `on.textinput` with `A`.
(Based on [LÖVE](https://love2d.org/wiki/love.textinput).)
* `App.keyrelease(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).
### 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 `on.mouse_press` 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.modifier_down(key)` -- returns `true` if the given key (doesn't
actually have to be just a modifier) 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).)
* `love.filesystem.getDirectoryItems(dir)` -- returns an unsorted array of the
files and directories available under `dir`. `dir` must be relative to
[LÖVE's save directory](https://love2d.org/wiki/love.filesystem.getSaveDirectory).
There is no easy, portable way in Lua/LÖVE to list directories outside the
save dir.
(From [LÖVE](https://love2d.org/wiki/love.filesystem.getDirectoryItems).]
* `love.filesystem.getInfo(filename)` -- returns some information about
`filename`, particularly whether it exists (non-`nil` return value) or not.
`filename` must be relative to [LÖVE's save directory](https://love2d.org/wiki/love.filesystem.getSaveDirectory).
(From [LÖVE](https://love2d.org/wiki/love.filesystem.getInfo).]
* `os.remove(filename)` -- removes a file or empty directory. Definitely make
sure `filename` is an absolute path.
(From [Lua](https://www.lua.org/manual/5.1/manual.html#pdf-os.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.getTime()` -- returns the number of seconds elapsed since some
unspecified start time.
(Based on [LÖVE](https://love2d.org/wiki/love.timer.getTime).)
* `App.getClipboardText()` -- returns a string with the current clipboard
contents.
(Based on [LÖVE](https://love2d.org/wiki/love.system.getClipboardText).)
* `App.setClipboardText(text)` -- stores the string `text` in the clipboard.
(Based on [LÖVE](https://love2d.org/wiki/love.system.setClipboardText).)
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).