Creating a Macro
Imagine that you need to run a tool as part of your build. For example, you may want to generate or preprocess a source file, or compress a binary. In this tutorial, you are going to create a macro that resizes an image.
Macros are suitable for simple tasks. If you want to do anything more complicated, for example add support for a new programming language, consider creating a rule. Rules give you more control and flexibility.
The easiest way to create a macro that resizes an image is to use a
genrule( name = "logo_miniature", srcs = ["logo.png"], outs = ["small_logo.png"], cmd = "convert $< -resize 100x100 $@", ) cc_binary( name = "my_app", srcs = ["my_app.cc"], data = [":logo_miniature"], )
If you need to resize more images, you may want to reuse the code. To do that,
define a function in a separate
.bzl file, and call the file
def miniature(name, src, size="100x100", **kwargs): """Create a miniature of the src image. The generated file is prefixed with 'small_'. """ native.genrule( name = name, srcs = [src], outs = ["small_" + src], cmd = "convert $< -resize " + size + " $@", **kwargs )
A few remarks:
By convention, macros have a
nameargument, just like rules.
To document the behavior of a macro, use docstring like in Python.
To call a
genrule, or any other native rule, use
**kwargsto forward the extra arguments to the underlying
genrule(it works just like in Python). This is useful, so that a user can use standard attributes like
Now, use the macro from the
load("//path/to:miniature.bzl", "miniature") miniature( name = "logo_miniature", src = "image.png", ) cc_binary( name = "my_app", srcs = ["my_app.cc"], data = [":logo_miniature"], )