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Options

gs_chapter3.xml

## 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),`

## 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
... ... ...
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),`

## 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.
Config-Only Options

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

## 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.
Read-Only Options

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

## 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}}.