{namespace buck.build_rule}

/***/
{template .soyweb}
  {call buck.page}
    {param title: 'Build Rule' /}
    {param content}

A <strong>build rule</strong> is a procedure for producing an output file from
a set of input files.

<p>

Buck comes with a number of built-in build rules, as there are many common
procedures when building Android code. For example, compiling Java code against
the Android SDK is one of the most common operations, so the build rule
{sp}{call buck.android_library /} exists to
facilitate that. Similarly, the primary objective of most Android developers
is to create an APK, so the build rule
{sp}{call buck.android_binary /} can be used
to do that.

<p>

Build rules are defined using built-in Python functions in a
{sp}<a href="build_file.html">build file</a>. (Both the Python function to define
the rule and the corresponding Java object that is ultimately created to
carry out the procedure are referred to as "the build rule,"
even though they are technically different things.) Every build rule takes at
least the following three arguments:

<ul>
  <li><code>name</code> The name of the build rule,
      which must be unique within a build file.
  <li><code>deps</code> The build rule's dependencies,
      expressed as a list of build targets.
  <li><code>visibility</code> The set of build rules that are allowed to
      claim this rule as a dependency, expressed as a list of
      {sp}<a href="build_target_pattern.html">build target patterns</a>.
</ul>

<p>

In Buck, every build rule can produce zero or one output files.
These output files can be used by other rules that declare the rule
responsible for the output file as a dependency. For example,
the output of an {call buck.android_library /}{sp}
is a JAR file, so an
{sp}{call buck.android_binary /}{sp}
that declares the <code>android_library</code> as a dependency can include
the <code>.class</code> files from the <code>android_library</code>'s JAR
file in its resulting APK. Or when one <code>android_library</code>{sp}
depends on another, the JAR of dependent rule will be included on the
classpath when the rule that depends on it compiles its Java code.
Check the documentation for each build rule to see how it uses its <code>deps</code>.
Note that no matter how a rule uses its <code>deps</code>, every rule in a rule's
{sp}<code>deps</code> is guaranteed to be built before the rule attempts to build itself.

<p>

Build rules and their dependencies define a directed graph. Buck requires that this
graph be acyclic. This helps Buck build efficiently, as it makes it easier to
build independent subgraphs in parallel.

<p>

Although Buck tries to provide a rich set of built-in build rules for Android
developers, it will not be able to address all possible needs. As an "escape hatch," Buck
also provides a vanilla build rule called a
{sp}{call buck.genrule /},
which can be used to build an arbitrary file using a Bash script.
<p>

Finally, note that build files are evaluated as Python files.
This means that you can define your own functions within build files that
generate build rules. This should not be something that you need to do often,
but taking advantage of this may help you add something to Buck's build process
without editing its source code.
For more details on this topic, see the article
on <a href="{ROOT}extending/macros.html">macros</a>.

    {/param}
  {/call}
{/template}