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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 . The following example shows one way to create a red button with the text "Press Me" on it:
The evaluation order of options is not defined. This implies that the grouping of options shown in the following example is not recommended:
After the operation, the rectangle can be at position or . The following example produces a deterministic behaviour:
The value of each option can be read individually with the function as shown in the following example:
The next example shows how to read the text and the width options from a button:
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 or the back-end provides the default value. For example, the option can be used as follows:
Most options are read/write key-value tuples such as and , 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
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 . |
Read-Only Options
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.
- An Erlang atom such as .
Bool.
- or
Color.
- , or a the predefined name ,, , , , , or . For example is black and is white.
Cursor.
- A mouse cursor, or any of the following: , , , , , , , or . has a special meaning, namely that this object will have the same cursor as its .
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: or , where is , , or a combination of those in a list. is an arbitrary integer. is a typeface of type , , , , , or (which is a suitable screen font).
Int.
- Any integer number, for example 42.
Label.
- A label can either be a plain text label , or an image where should point out a bitmap.
String.
- An Erlang list of ASCII bytes. For example,
Term.
- Any Erlang term.
In cases where the type is self-explanatory, the name of the parameter is used. For example, .