#### 16.5.15 Defining project attributes

The project files are required by GPS, and are used to store various pieces of information related to the current set of source files. This includes how to find the source files, how the files should be compiled, or manipulated through various tools, ....

However, the default set of attributes that are usable in a project file is limited to the attributes needed by the tool packaged with GPS or GNAT.

If you are delivering your own tools, you might want to store similar information in the project files themselves, since these are a very convenient place to associate some specific settings with a given set of source files.

GPS lets manipulate the contents of projects through XML customization files and script commands. You can therefore add you own typed attributes into the projects, so that they are saved automatically when the user saves the project, and reloaded automatically the next time GPS is started.

##### 16.5.15.1 Declaring the new attributes

New project attributes can be declared in two ways: either using the advanced XML tags below, or using the `<tool>` tag (see Defining tool switches).

The customization files support the `<project_attribute>` tag, which is used to declare all the new attributes that GPS should expect in a project. Attributes that have not been declared explictly will not be accessible through the GPS scripting languagues, and will generate warnings in the Messages window.

Project attributes are typed: they can either have a single value, or have a set of such values (a list). The values can in turn be a free-form string, a file name, a directory name, or a value extracted from a list of preset values.

Attributes that have been declared in these customization files will also be graphically editable through the project properties dialog, or the project wizard. Therefore, you should specify when an attribute is defined how it should be presented to the GPS user.

The `<project_attribute>` tag accepts the following attributes:

- `package` (a string, default value: "")

  This is the package in the project file in which the attribute is stored. Common practice suggests that one such package should be used for each tool. These packages provide namespaces, so that attributes with the same name, but for different tools, do not conflict with each other.

- `name` (a string, mandatory)

  This is the name of the attribute. This should be a string with no space, and that represents a valid Ada identifier (typically, it should start with a letter and be followed by a set of letters, digits or underscore characters). This is an internal name that is used when saving the attribute in a project file.

- `editor_page` (a string, default value: "General")

  This is the name of the page in the Project Properties editor dialog in which the attribute is presented. If no such page already exists, a new one will be created as needed. If the page already exists, the attribute will be appended at its bottom.

- `editor_section` (a string, default value: "")

  This is the name of the section, inside editor page, in which the attribute is displayed. These sections are surrounded by frames, the title of which is given by the `editor_section` attribute. If this attribute is not specified, the attribute is put in an untitled section.

- `label` (a string, default value: the name of the attribute)

  If this attribute is set to a value other than the empty string `""`, a textual label is displayed to the left of the attribute in the graphical editor. This should be used to identify the attribute. However, it can be left to the empty string if the attribute is in a named section of its own, since the title of the section might be a good enough indication.

- `description` (a string, default value: "")

  This is the help message that describes the role of the attribute. It is displayed in a tooltip if the user leaves the mouse on top of the attribute for a while.

- `list` (a boolean, default value: "false")

  If this is set to `"true"`, the project attribute will in fact contains a list of values, as opposed to a single value. This is used for instance for the list of source directories in standard projects.

- `ordered` (a boolean, default value: "false")

  This is only relevant if the project attribute contains a list of values. This indicates whether the order of the values is relevant. In most cases, it will not matter. However, for instance, the order of source directories matters, since this also indicates where the source files will be searched, stopping at the first match.

- `omit_if_default` (a boolean, default value: "true")

  This indicates whether the project attribute should be set explicitly in the project if the user has left it to its default value. This can be used to keep the project files a simple as possible, if all the tools that will use this project attribute know about the default value. If this isn't the case, set `omit_if_default` to "false" to force the generation of the project attribute.

- `base_name_only` (a boolean, default value: "false")

  If the attribute contains a file name or a directory name, this indicates whether the full path should be stored, or only the base name. In most cases, the full path should be used. However, since GPS automatically looks for source files in the list of directories, for instance, the list of source files should only contain base names. This also increases the portability of project files.

- `case_sensitive_index` (a boolean, default value: "false")

  This XML attribute is only relevant for project attributes that are indexed on another one (see below for more information on indexed attributes). It indicates whether two indexes that differ only by their casing should be considered the same. For instance, if the index is the name of one of the languages supported by GPS, the index is case insensitive since "Ada" is the same as "C". However, if the index is the name of a file on Windows, the index is case-insensitive.

- `hide_in` (a string, default value: "")

  This XML attribute defines the various context in which this attribute should not be editable graphically. Currently, GPS provides three such contexts ("wizard", "library_wizard" and "properties", corresponding to the project creation wizards and the project properties editor). If any of those context is specified in hide_in, then the widget to edit this attribute will not be shown. The goal is to keep the graphical interface simple.

- `disable_if_not_set` (a boolean, default value: "false")

  If this attribute is set to "true", the editor for this attribute will be greyed out if the attribute is not explicitly set in the project. In most cases, this is not needed, since the default value of the attribute can be used to leave the editor active at all time. However, when the value of the attribute is automatically computed depending on other attributes, the default value cannot be easily specified in the XML file, and in this case it might be easier to grey out the editor. An extra check box is displayed next to the attribute so that the user can choose to activate the editor and add the attribute to the project.

- `disable` (a space-separated list of attribute names, default: "")

  This is a list of attribute whose editor should be greyed out if the current attribute is specified. This only works if both the current attribute and the referenced attributes have their `disable_if_not_set` attribute set to "true". This can be used to have mutually exclusive attributes present in the editor

##### 16.5.15.2 Declaring the type of the new attributes

The type of the project attribute is specified through one or several child tags of `<project_attribute>`. The following tags are recognized.

- `<string>`

  This tag indicates that the attribute is made of one (or more if it is a list) strings. This tag accepts the following XML attributes:

  - `default` (a string, default value: "")

    This gives the default value to be used for the string (and therefore the project attribute), in case the user hasn't overridden it.

    If the attribute's type is a file or a directory, the default value will be normalized (ie an absolute path will be generated from it, based on the project's location, where `"."` will represent the project's directory). As a special case, if default is surrounded by parenthesis, no normalization takes place, so that you can later on test whether the user is still using the default value or not).

    A special case if when `default` is set to "project source files". In this case, this is automatically replaced by the known list of source files for the project. This doesn't work from the project wizard, since the list of source files hasn't been computed at that stage.

  - `type` (one of "", "file", "directory", default "")

    This indicates what the string represents. In the first case, any value can be used. In the second case, it should represent a file name, although no check is done to make sure the file actually exists on the disk. But GPS will be able to do some special marshalling with the file name. The third case indicates that GPS should expect a directory.

  - `filter` (one of "none", "project", "extending_project")

    This attribute is ignored for all types except `"file"`. In this case, it further specifies what kind of files can be used in this attribute. If the filter is `"none"`, then any file anywhere on the system is valid. If the filter is `"project"`, then only files from the selected project can be specified. If the filter is `"extended_project"`, then only the files from the project extended by the current project can be specified. The attribute will not be shown if the current project is not an extending project.

  - `allow_empty` (one of "True" or "False, default "True")

    This attribute indicates whether the value for this attribute can be an empty string. If not, the user must specify a value or an error message will be displayed in the project properties editor and project wizard.

- `<choice>`

  This tag can be repeated several times. It indicates one of the valid values for the attribute, and can be used to provide a static list of such values. If it is combined with a `<string>` tag, this indicates that the attribute can be any string, although a set of possible values is provided to the user for ease of use. This tag accepts one optional attribute, `"default"`, which is a boolean. It indicates whether this value is the default to use for the project attribute.

  If several `<choice>` tags are used, it is possible that several of them are part of the default value if the project attribute is a list, as opposed to a single value.

- `<shell>`

  This tag is a GPS scripting command to execute to get a list of valid values for the attribute. The command should return a list. As for the `<choice>` tag, the `<shell>` tag can be combined with a `<string>` tag to indicate that the list of values returned by the scripting command is only a set of possible values, but that the project attribute can in fact take any value.

  The `<shell>` tag accepts two attributes:

  - `lang` (a string, default value: "shell")

    The scripting language in which the command is written. Currently, the only other possible value is "python".

  - `default` (a string, default value: "")

    The default value that the project attribute takes if the user hasn't overridden it.

In some cases, the type of the project attribute, or at least its default value, depends on what the attribute applies to. The project file support this in the form of indexed project attribute. This is for instance used to specify what should be the name of the executable generated when compiling each of the main files in the project (ie the executable name for `gps.adb` should be `gps.exe`, the one for `main.c` should be `myapp.exe`, and so on).

Such attributes can also be declared through XML files. In such cases, the `<project_attribute>` tag should have one `<index>` child, and zero or more `<specialized_index>` children. Each of these two tags in turn take one of the already mentioned `<string>`, `<choice>` or `<shell>` tag.

The `<index>` tag indicates what other project attribute is used to index the current one. In the example given above for the executable names, the index is the attribute that contains the list of main files for the project.

It accepts the following XML attributes:

- `attribute` (a string, mandatory)

  The name of the other attribute. This other attribute must be declared elsewhere in the customization files, and must be a list of values, not a single value.

- `package` (a string, default value: "")

  The package in which the index project attribute is defined. This is used to uniquely identify homonym attributes.

The `<specialized_index>` is used to override the default type of the attribute for specific values of the index. For instance, the project files contains an attribute that specify what the name of the compiler is for each language. It is indexed on the project attribute that list the languages used for the source files of the project. Its default value depends on the language ("gnatmake" for Ada, "gcc" for C, and so on). This attribute accepts requires one XML attribute:

- `value` (a string, mandatory)

  This is the value of the attribute for which the type is overriden.

Note that almost all the standard project attributes are defined through an XML file, `projects.xml`, which is part of the GPS installation. Check this file to get advanced examples on how to declare project attributes.

##### 16.5.15.3 Examples

The following example declares three attributes, with a single string as their value. This string represents a file or a directory in the last two cases. You can simply copy this into a `.xml` file in your `$HOME/.gps/plug-ins` directory, as usual.

         <?xml version="1.0"?>
         <custom>
           <project_attribute
               name="Single1"
               package="Test"
               editor_page="Tests single"
               editor_section="Single"
               description="Any string">

              <string default="Default value" />
           </project_attribute>

           <project_attribute
               name="File1"
               package="Test"
               editor_page="Tests single"
               editor_section="Single"
               description="Any file" >

               <string type="file" default="/my/file" />
           </project_attribute>

           <project_attribute
               name="Directory1"
               package="Test"
               editor_page="Tests single"
               editor_section="Single"
               description="Any directory" >

               <string type="directory" default="/my/directory/" />
           </project_attribute>
         </custom>

The following example declares an attribute whose value is a string. However, a list of predefined possible values is also provided, as an help for interactive edition for the user. If the `<string>` tag wasn't given, the attribute's value would have two be one of the three possible choices.

         <?xml version="1.0" ?>
         <custom>
           <project_attribute
               name="Static2"
               package="Test"
               editor_page="Tests single"
               editor_section="Single"
               description="Choice from static list (or any string)" >

               <choice>Choice1</choice>
               <choice default="true" >Choice2</choice>
               <choice>Choice3</choice>
               <string />
           </project_attribute>
         </custom>

The following example declares an attribute whose value is one of the languages currently supported by GPS. Since this list of languages is only know when GPS is executed, a script command is used to query this list.

         <?xml version="1.0" ?>
         <custom>
          <project_attribute
               name="Dynamic1"
               package="Test"
               editor_page="Tests single"
               editor_section="Single"
               description="Choice from dynamic list" >

               <shell default="C" >supported_languages</shell>
           </project_attribute>
         </custom>

The following example declares an attribute whose value is a set of file names. The order of files in this list matters to the tools that are using this project attribute.

         <?xml version="1.0" ?>
         <custom>
          <project_attribute
               name="File_List1"
               package="Test"
               editor_page="Tests list"
               editor_section="Lists"
               list="true"
               ordered="true"
               description="List of any file" >

               <string type="file" default="Default file" />
           </project_attribute>
         </custom>

The following example declares an attribute whose value is a set of predefined possible values. By default, two such values are selected, unless the user overrides this default setting.

         <?xml version="1.0" ?>
         <custom>
           <project_attribute
               name="Static_List1"
               package="Test"
               editor_page="Tests list"
               editor_section="Lists"
               list="true"
               description="Any set of values from a static list" >

               <choice>Choice1</choice>
               <choice default="true">Choice2</choice>
               <choice default="true">Choice3</choice>
           </project_attribute>
         </custom>

The following example declares an attribute whose value is a string. However, the value is specific to each language (this could for instance be used for the name of the compiler to use for a given language). This is an indexed project attribute. It has two default values, one for Ada, one for C. All other languages have no default value.

         <?xml version="1.0" ?>
         <custom>
           <project_attribute
               name="Compiler_Name"
               package="Test"
               editor_page="Tests indexed"
               editor_section="Single"
               <index attribute="languages" package="">
                  <string default="" />
               </index>
               <specialized_index value="Ada" >
                  <string default="gnatmake" />
               </specialized_index>
               <specialized_index value="C" >
                  <string default="gcc" />
               </specialized_index>
           </project_attribute>
         </custom>

##### 16.5.15.4 Accessing the project attributes

The new attributes that were defined are accessible from the GPS scripting languages, like all the standard attributes, see Querying project switches.

You can for instance access the Compiler_Name attribute we created above with a python command similar to:

         GPS.Project.root().get_attribute_as_string ("Compiler_Name", "Test", "Ada")

You can also access the list of main files for the project, for instance, by calling

         GPS.Project.root().get_attribute_as_list ("main")