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/*!
    \contentspage list-of-language-items.html
    \previouspage propertyoptions-item.html
    \page rule-item.html
    \nextpage scanner-item.html
    \ingroup list-of-language-items
    \ingroup list-of-items

    \title Rule Item
    \brief Creates transformers for input tags.

    In \QBS, rules create \e transformers that produce output files from input files.
    The term \e transformer refers to a list of \l{Command and JavaScriptCommand}{commands}.
    These commands are created in a rule's \e {prepare script}. They do the actual work, either
    directly or by executing external commands.

    \section1 A Simple Example

    The following rule takes text files and replaces Windows-style line endings with their
    Unix-style counterparts. We will look at it one piece at a time.

    \code
    Rule {
        multiplex: false
    \endcode
    A \e {multiplex rule} creates one transformer that takes all input artifacts with the
    matching input file tag and creates one or more output artifacts. We are setting the
    respective property to \c false here, indicating that we want to create one transformer
    per input file.
    \note This is actually the default, so the above assignment is not required.
    \code
        inputs: ["txt_input"]
    \endcode
    Here we are specifying that our rule is interested in input files that have the tag
    "txt_input". Such files could be source files, in which case you would tag them
    using a \l{Group Item}{Group}. Or they could in turn get generated by a different rule,
    in which case that rule would assign the file tag.
    The files matching the tag will be available in the prepare script under the name
    \c inputs (see \l{inputs and outputs}{The inputs and outputs Variables}).
    \code
        Artifact {
            filePath: input.fileName + ".out"
            fileTags: ["txt_output"]
        }
    \endcode
    Here we are specifying that for every input file, we want to create one output file
    whose name is the same as the input file, but with an additional extension. Because we are
    giving a relative path, \QBS will prepend that path by the product's build directory.

    In addition, we tell \QBS that the output files should get the file tag "txt_output". This
    enables other rules to use these files as inputs. You must always assign suitable file tags
    to your output artifacts, or the rule will not be run.
    See \l{Rules and Product Types} for details.

    If you want to create more than one output file per input file, you simply provide multiple
    \c Artifact items. The set of output artifacts will be available in the prepare script
    under the name \c outputs (see \l{inputs and outputs}{The inputs and outputs Variables}).

    \code
        prepare: {
            var cmd = new JavaScriptCommand();
            cmd.description = input.fileName + "->" + output.fileName;
            cmd.highlight = "codegen";
            cmd.sourceCode = function() {
                var file = new TextFile(input.filePath);
                var content = file.readAll();
                file.close()
                content = content.replace(/\r\n/g, "\n");
                file = new TextFile(output.filePath, TextFile.WriteOnly);
                file.write(content);
                file.close();
            }
            return [cmd];
        }
    }
    \endcode
    The prepare script shown above puts everything together by creating the command that does
    the actual transformation of the file contents, employing the help of the
    \l{TextFile Service}{TextFile} class.

    As you can see, the return value is an array, meaning you can provide several commands to
    implement the rule's functionality. For instance, if we had provided two \c Artifact items,
    we might have also provided two commands, each of them creating one output file.

    For the \c input and \c output variables used in the code, see the next section.

    \target inputs and outputs
    \section1 The \c inputs and \c outputs Variables

    We already mentioned that the input and output artifacts are available in the prepare script
    via the variables \c inputs and \c outputs, respectively. These variables are JavaScript
    objects whose property keys are file tags and whose property values are lists of objects
    representing the artifacts matching these tags. In our example, the \c inputs variable
    has a single property \c txt_input, whose value is a list with one element. Similarly, the
    \c outputs variable also has one single property \c txt_output, again with a list containing
    one element.

    The actual artifact objects have the following properties:
    \table
    \header
        \li Property
        \li Description
    \row
        \li \c baseName
        \li The file name without any extension.
    \row
        \li \c completeBaseName
        \li The file name without the last extension.
    \row
        \li \c fileName
        \li The name of the file (that is, \c filePath without any directory components).
    \row
        \li \c filePath
        \li The full file path.
    \row
        \li \c fileTags
        \li The list of the artifact's file tags.
    \endtable

    The artifact object contains a property for every module that is used in the product. That can
    be used to access the module's properties. For instance, for an artifact in a C++ product,
    \c{artifact.cpp.defines} is the list of defines that will be passed when compiling the
    respective file.

    But what about the variables \c input and \c output that appeared in our example? These
    are simply convenience variables which are available in the case that the \c inputs
    and \c outputs variables contain only one artifact, respectively. So in our example, instead
    of \c input we also could have written \c {inputs.txt_input[0]}, which is considerably
    more verbose.

    \section1 Rules and Product Types

    It is important to know that when figuring out which rules to execute, \QBS starts at the
    product type and then looks for a way to produce artifacts with matching file tags from
    source files, using a chain of rules that are connected by their respective input and output
    tags. For instance, consider this simple C++ project:
    \code
    Product {
        type: ["application"]
        Depends { name: "cpp" }
        files: ["main.cpp"]
    }
    \endcode
    Here's how this product is built:
    \list 1
        \li \QBS looks for a rule that can produce artifacts with the file tag
            \c{"application"}. Such a rule is found in the \c cpp module (namely, the rule that
            invokes the linker).
        \li Since the rule found in the previous step takes inputs of type \c{"obj"}, \QBS now
            looks for a rule that produces artifacts of that type. Again, such a rule is found in
            the \c cpp module (the rule that runs the compiler).
        \li The rule found in the previous step takes inputs of type \c{"cpp"}. No rule is found
            that creates such artifacts, but we do have a source file with a matching type (because
            the \c cpp module contains a \l{FileTagger item}{FileTagger} which attached that type
            to \c{"main.cpp"} due to its file extension).
        \li Now that there is a chain of rules leading from a source file tag to the product type,
            the commands of these rules are executed one after the other until we end up with
            our executable.
    \endlist

    \section1 Rule Properties

    \table
    \header
        \li Property
        \li Type
        \li Default
        \li Description
    \row
        \li multiplex
        \li bool
        \li false
        \li Determines whether this is a multiplex rule.
    \row
        \li inputs
        \li string list
        \li undefined
        \li File tags the input artifacts must match.
           All output artifacts will depend on all artifacts in the product with
           the given input file tags. Also these artifacts are available in the
           inputs variable of the prepare script.
    \row
        \li auxiliaryInputs
        \li string list
        \li undefined
        \li A list of file tags. This rule will be dependent on every other rule
            that produces artifacts that are compatible with \a{auxiliaryInputs}.
            Unlike \a{inputs}, the property \a{auxiliaryInputs} has no effect on the content of the
            \a{inputs} variable in the \a{prepare} script.
    \row
        \li excludedAuxiliaryInputs
        \li string list
        \li undefined
        \li A list of file tags. Connections to rules that produce these file tags are prevented.
            This property has no effect on the content of the \a{inputs} variable in the \a{prepare}
            script.
    \row
        \li inputsFromDependencies
        \li string list
        \li undefined
        \li File tags the artifacts of product dependencies must match.
           For example, the product \a foo might appear as follows in the current product:
           \code
            Depends {
                name: "foo"
            }
           \endcode
           All artifacts of \a foo that match the given
           file tags will appear in the \a inputs variable of the prepare
           script.
    \row
        \li outputArtifacts
        \li array of objects
        \li undefined
        \li An array of output artifacts, specified as JavaScript objects.
            Example:
            \code
            outputArtifacts: [{
                filePath: "myfile.cpp",
                fileTags: ["cpp"],
                cpp: { cxxLanguageVersion: "c++11" }
            }]
            \endcode
            For a description of the possible properties, see the documentation of the
            \l{Artifact item}.
            Output artifacts can be specified either by \c{Rule.outputArtifacts} or by \c{Artifact}
            items. Use \c{Rule.outputArtifacts} if the set of outputs is not fixed but dependent on
            the input's content. If no file tags are provided, \QBS will apply all
            \l{FileTagger Item}{file taggers} known in the current context to the output file name.
            The user may set the property \c{explicitlyDependsOn} on artifact objects, which is
            similar to \c{Rule.explicitlyDependsOn}.
    \row
        \li outputFileTags
        \li string list
        \li undefined
        \li If output artifacts are specified by \c{Rule.outputArtifacts}, then
            \c{Rule.outputFileTags} must be a list of file tags the rule potentially produces.
    \row
        \li condition
        \li bool
        \li true
        \li If true, the rule is enabled, otherwise it does nothing.
    \row
        \li explicitlyDependsOn
        \li string list
        \li undefined
        \li Each artifact that matches the file tags in \a explicitlyDependsOn
           is added to the dependencies of each output node.
    \row
        \li prepare
        \li script
        \li undefined
        \li Script that prepares the commands to transform the inputs to outputs.
            The code in this script is treated as a function with the signature
            \c{function(project, product, inputs, outputs, input, output)}.
            The argument \c{input} is \c{undefined} if there's more than one input artifact for this
            rule. Similarly, \c{output} is only defined if there's exactly one output artifact.
    \row
        \li alwaysRun
        \li bool
        \li false
        \li If true, the rule's commands are always executed, even if all output artifacts
            are up to date.
    \endtable

*/
