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/*!
    \contentspage list-of-items.html
    \previouspage qtapplication-item.html
    \page rule-item.html
    \nextpage staticlibrary-item.html
    \ingroup list-of-items

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

    A \e {multiplex rule} creates one \e transformer that takes all
    input artifacts with the matching input file tag and creates
    one or more artifacts (e.g. C++ linker).
    A \e {simplex rule} creates one transformer per matching input file
    (e.g. C++ compiler).
    As a real-world example of a simplex rule, here is a simplified version
    of \QBS' rule for transforming C++ sources into object files using gcc:
    \code
    import qbs.ModUtils
    import qbs.Utilities
    Rule {
        id: compiler
        inputs: ['cpp']
        auxiliaryInputs: ['hpp']

        Artifact {
            fileTags: ['obj']
            filePath: '.obj/' + Utilities.getHash(input.baseDir) + '/' + input.fileName + '.o'
        }

        prepare: {
            var args = [];
            if (product.moduleProperty('cpp', 'debugInformation'))
                args.push('-g');
            var warnings = product.moduleProperty('cpp', 'warningLevel')
            if (warnings === 'none')
                args.push('-w');
            if (warnings === 'all') {
                args.push('-Wall');
                args.push('-Wextra');
            }
            if (product.moduleProperty('cpp', 'treatWarningsAsErrors'))
                args.push('-Werror');
            var includePaths = product.moduleProperties('cpp', 'includePaths');
            for (i in includePaths)
                args.push('-I' + includePaths[i]);
            var defines = product.moduleProperties('cpp', 'defines');
            for (i in defines)
                args.push('-D' + defines[i]);
            args.push('-c');
            args.push(input.filePath);
            args.push('-o');
            args.push(output.filePath);
            var compilerPath = ModUtils.moduleProperty(product, 'compilerPath');
            var cmd = new Command(compilerPath, args);
            cmd.description = 'compiling ' + input.fileName;
            cmd.highlight = 'compiler';
            return cmd;
        }
    }
    \endcode

    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 and
            transformer 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. Also, each output artifact of this rule will be dependent on
           those artifacts.
    \row
        \li outputArtifacts
        \li array of objects
        \li undefined
        \li An array of output artifacts, specified as JavaScript objects.
            Example:
            \code
            outputArtifacts: [{filePath: "myfile.txt", fileTags: ["foo", "bar"]}]
            \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

*/
