Options

## 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:

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

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 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.

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

`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}}`.

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