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/*!
    \previouspage PropertyOptions
    \nextpage Scanner
    \qmltype Rule
    \inqmlmodule QbsLanguageItems
    \ingroup list-of-items
    \keyword QML.Rule

    \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 \l{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.

    \quotefromfile ../examples/rule/rule.qbs

    \skipto Rule
    \printuntil multiplex: false

    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.

    \printuntil inputs: ["txt_input"]

    Here we are specifying that our rule is interested in input files that have the tag
    \c "txt_input". Such files could be source files, in which case you would tag them
    using a \l{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 \l{prepare} script under the name
    \c inputs (see \l{inputs and outputs}{The inputs and outputs Variables}).

    \printuntil }

    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 \c "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
    \l 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}).

    \printuntil /^\s{4}\}/

    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 \l{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} 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 A Complete Example

    The following code snippet shows a single \l{Rule} within a \l{Product} and summarizes the
    previous sections.

    \note The product's type is set up to the \c "txt_output" file tag to tell \QBS that the
    product depends on output artifacts produced by the custom rule. Otherwise the rule would not
    be executed.

    \quotefile ../examples/rule/rule.qbs
*/

/*!
    \qmlproperty bool Rule::multiplex

    Determines whether this is a multiplex rule.

    \defaultvalue \c false
*/

/*!
    \qmlproperty stringList Rule::inputs

    A list of 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
    \c inputs variable of the \l{prepare} script.

    \nodefaultvalue
*/

/*!
    \qmlproperty stringList Rule::auxiliaryInputs

    A list of file tags. This rule will be dependent on (that is: applied after)
    every other rule in the same product that (potentially) produces artifacts
    that are compatible with the value of this property.

    Unlike \l{inputs}, this property has no effect on the content of the
    \c inputs variable of the \l{prepare} script.

    \nodefaultvalue
*/

/*!
    \qmlproperty stringList Rule::auxiliaryInputsFromDependencies

    Like \l auxiliaryInputs, except that instead of rules from the current product,
    those from product dependencies are considered.

    \nodefaultvalue
    \since 2.6
*/

/*!
    \qmlproperty stringList Rule::excludedInputs

    A list of file tags. Connections to rules that produce these file tags are
    prevented.

    \nodefaultvalue
    \since Qbs 1.12
*/

/*!
    \qmlproperty stringList Rule::inputsFromDependencies

    A list of file tags that 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
    \c inputs variable of the \l{prepare} script.

    \nodefaultvalue
*/

/*!
    \qmlproperty var Rule::outputArtifacts

    An array of output artifacts, specified as JavaScript objects.

    For 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 this property or by \l{Artifact}
    items. Use this property if the set of outputs is not fixed but depends the
    input's content. If no file tags are provided, \QBS will apply all
    \l{FileTagger}{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 \l{Rule::explicitlyDependsOn}{Rule.explicitlyDependsOn}.

    \nodefaultvalue
*/

/*!
    \qmlproperty stringList Rule::outputFileTags

    If output artifacts are specified by \l{outputArtifacts}, this property must
    specify a list of file tags that the rule potentially produces.

     \nodefaultvalue
*/

/*!
    \qmlproperty bool Rule::condition

    If \c true, the rule is enabled, otherwise it does nothing.

    \defaultvalue \c true
*/

/*!
    \qmlproperty stringList Rule::explicitlyDependsOn

    A list of file tags. Each artifact that matches the file tags is added to
    the dependencies of each output node. All artifacts in the current product
    are considered.

    The main difference between this property and \l inputs is that all matching artifacts
    are dependencies for all output artifacts, regardless of whether \l multiplex
    is true or not.

    \nodefaultvalue
*/

/*!
    \qmlproperty stringList Rule::explicitlyDependsOnFromDependencies

    Like \l explicitlyDependsOn, except that instead of artifacts from the current product,
    those from product dependencies are considered.

    \nodefaultvalue
    \since Qbs 1.12
*/

/*!
    \qmlproperty script Rule::prepare

    A 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, explicitlyDependsOn)}.

    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 is
    exactly one output artifact.

   \nodefaultvalue

*/

/*!
    \qmlproperty bool Rule::requiresInputs

    Specifies whether a rule's commands should be created even if no inputs are
    available.

    Enabling this property can be useful if you are not sure whether input
    files exist, and you want \QBS to create an output file even if they do not.

    Set to \c true if the rule declares any inputs, \c false otherwise.
*/

/*!
    \qmlproperty bool Rule::alwaysRun

    If \c true, the rule's commands are always executed, even if all output
    artifacts are up to date.

    \defaultvalue \c false
*/
