Rule

Creates transformers for input tags. More...

• List of all members, including inherited members
• Rule is part of List of All Items.

Properties

• alwaysRun : bool
• auxiliaryInputs : stringList
• condition : bool
• excludedInputs : stringList
• explicitlyDependsOn : stringList
• explicitlyDependsOnFromDependencies : stringList
• inputs : stringList
• inputsFromDependencies : stringList
• multiplex : bool
• outputArtifacts : var
• outputFileTags : stringList
• prepare : script
• requiresInputs : bool

Detailed Description

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

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.

    Rule {
        multiplex: false

A 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 false here, indicating that we want to create one transformer per input file.

        inputs: ["txt_input"]

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 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 inputs (see The inputs and outputs Variables).

        Artifact {
            filePath: input.fileName + ".out"
            fileTags: ["txt_output"]
        }

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 Rules and Product Types for details.

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

        prepare: {
            var cmd = new JavaScriptCommand();
            cmd.description = "generating" + output.fileName + " from " + input.fileName;
            cmd.highlight = "codegen";
            cmd.sourceCode = function() {
                var file = new TextFile(input.filePath);
                var content = file.readAll();
                file.close()
                content = content.toUpperCase();
                file = new TextFile(output.filePath, TextFile.WriteOnly);
                file.write(content);
                file.close();
            }
            return [cmd];
        }
    }

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 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 Artifact items, we might have also provided two commands, each of them creating one output file.

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

The inputs and outputs Variables

We already mentioned that the input and output artifacts are available in the prepare script via the variables inputs and 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 inputs variable has a single property txt_input, whose value is a list with one element. Similarly, the outputs variable also has one single property txt_output, again with a list containing one element.

The actual artifact objects have the following properties:

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, artifact.cpp.defines is the list of defines that will be passed when compiling the respective file.

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

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:

Product {
    type: ["application"]
    Depends { name: "cpp" }
    files: ["main.cpp"]
}

Here's how this product is built:

1. Qbs looks for a rule that can produce artifacts with the file tag "application". Such a rule is found in the cpp module (namely, the rule that invokes the linker).
2. Since the rule found in the previous step takes inputs of type "obj", Qbs now looks for a rule that produces artifacts of that type. Again, such a rule is found in the cpp module (the rule that runs the compiler).
3. The rule found in the previous step takes inputs of type "cpp". No rule is found that creates such artifacts, but we do have a source file with a matching type (because the cpp module contains a FileTagger which attached that type to "main.cpp" due to its file extension).
4. 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.

A Complete Example

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

import qbs.TextFile

Product {
    type: "txt_output"

    Group {
        name: "lorem_ipsum"
        files: "lorem_ipsum.txt"
        fileTags: "txt_input"
    }

    Rule {
        multiplex: false
        inputs: ["txt_input"]
        Artifact {
            filePath: input.fileName + ".out"
            fileTags: ["txt_output"]
        }
        prepare: {
            var cmd = new JavaScriptCommand();
            cmd.description = "generating" + output.fileName + " from " + input.fileName;
            cmd.highlight = "codegen";
            cmd.sourceCode = function() {
                var file = new TextFile(input.filePath);
                var content = file.readAll();
                file.close()
                content = content.toUpperCase();
                file = new TextFile(output.filePath, TextFile.WriteOnly);
                file.write(content);
                file.close();
            }
            return [cmd];
        }
    }
}

Property Documentation

Depends {
    name: "foo"
}

outputArtifacts: [{
    filePath: "myfile.cpp",
    fileTags: ["cpp"],
    cpp: { cxxLanguageVersion: "c++11" }
}]

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