3 Options

3.1 The Option Concept

Each object has a set of options. The options are key-value tuples and the key is an atom. Depending on the option, the value can be any Erlang term. Typical options are: x, y, width, height, text, and color. A list of options should be supplied when an object is created . It is also possible to reconfigure an object with the function gs:config/2. The following example shows one way to create a red button with the text "Press Me" on it:

Butt = gs:create(button,Win, [{x,10},{y,10}]),
gs:config(Butt, [{width,50},{height,50},{bg,red}]),
gs:config(Butt, [{label, {text,"Press Me"}},{y,20}]),

The evaluation order of options is not defined. This implies that the grouping of options shown in the following example is not recommended:

Rect = gs:create(rectangle,Can, [{coords,[{10,10},{20,20}]},
                                 {move,{5,5}}]),

After the operation, the rectangle can be at position [{10,10},{20,20}] or [{15,15},{25,25}]. The following example produces a deterministic behaviour:

Rect = gs:create(rectangle,Can,[{coords,[{10,10},{20,20}]},
gs:config(Rect,[{move,{5,5}}]),

The value of each option can be read individually with the read/2 function as shown in the following example:

Value = gs:read(ObjectId,Option)

The next example shows how to read the text and the width options from a button:

Text = gs:read(Butt, text),
Width = gs:read(Butt, width),

3.2 The Option Tables

Each object is described in terms of its options. The options are listed in a table as is shown in the following example:

{Option,Value}	Default	Description
{fg, Color}	<unspec>	Foreground color of the object
{map, Bool}	false	Visibility on the screen
...	...	...

Table 3.1: Options

The <unspec> default value means that either gs or the back-end provides the default value. For example, the fg option can be used as follows:

Rect = gs:create(rectangle, Window, [{fg, red}]),
Color = gs:read(Rect, fg),

3.3 Config-Only Options

Most options are read/write key-value tuples such as {select,true|false} and {map,true|false, but some options are by nature write-only, or read-only. For example, buttons can flash for a short time and canvas objects can be moved dx, dy. The following table exemplifies some config-only options:

Config-Only	Description
flash	Causes the object to flash for 2 seconds.
raise	Raises the object on top of other overlapping objects.
{move, {Dx, Dy}}	Moves the object relative to its current position.

Table 3.2: Config-Only Options

gs:config(Button,[flash]), causes the button to flash.

3.4 Read-Only Options

The opposite of config-only options are read-only options. The following table exemplifies some read-only options:

Read-Only	Return	Description
size	Int	The number of items (entries).
{get, Index}	String	The entry at index Index.

Table 3.3: Read-Only Options

EntryString = gs:read(Listbox,{get, Index}), is an example.

3.5 Data Types

As previously stated, each object is described in terms of its options. This section defines the data types for options.

Anchor|Align.: n|w|s|e|nw|se|ne|sw|center
Atom.: An Erlang atom such as myWay.
Bool.: true or false
Color.: {R,G,B}, or a the predefined name red,green, blue, white, black, grey, or yellow. For example {0,0,0} is black and {255,255,255} is white.
Cursor.: A mouse cursor, or any of the following: arrow, busy, cross, hand, help, resize, text, or parent. parent has a special meaning, namely that this object will have the same cursor as its parent.
FileName.: FileName is a string. The file name may include a directory path and should point out a file of a suitable type. The path can be either absolute or relative to the directory from where Erlang was started.
Float.: Any float, for example 3.1415.
Font.: A Font is represented as a two or three tuple:{Family,Size} or {Family,Style,Size}, where Style is bold, italic, or a combination of those in a list. Size is an arbitrary integer. Family is a typeface of type times, courier, helvetica, symbol, new_century_schoolbook, or screen (which is a suitable screen font).
Int.: Any integer number, for example 42.
Label.: A label can either be a plain text label {text, String}, or an image {image, FileName} where FileName should point out a bitmap.
String.: An Erlang list of ASCII bytes. For example, "Hi there"=[72,105,32,116,104,101,114,101]
Term.: Any Erlang term.

In cases where the type is self-explanatory, the name of the parameter is used. For example, {move, {Dx,Dy}}.