You might first want to read the tutorial for a few examples of using variants.

Here is an example how build variants can be specified. This will be used to explain how it works.

     :variant Opt
         some
             OPTIMIZE = 2
         much
             OPTIMIZE = 6        
         *
             OPTIMIZE = 1

"Opt" is the name of a variable. It is used to select one of the variants. Each possible value is listed in the following line and further lines with the same indent. In the example these are "some" and "much". "*" is used to accept any value, it must be the last one. The first value mentioned is the default when the variable isn't set.

You can now start Aap with various arguments to specify the kind of optimizing you want to use:

Note that when "Opt" is not given a value the first entry is used, resulting in OPTIMIZE being set to 2. But when it is set to a value that isn't mentioned the last entry "*" is used.

The BDIR Variable

The $BDIR variable will be adjusted for the variant used. CAREFUL: this means that using $BDIR before :variant commands will use a different value, that might not always be what you want.

Inside the :variant command the value of $BDIR has already been adjusted.

When a target that is being build starts with $BDIR and $BDIR doesn't exist, it is created. (Actually, this happens when an item in the path is "build" or starts with "build-".

$BDIR is relative to the recipe. When using ":child dir/main.aap" the child recipe will use a different build directory dir/$BDIR. Note that when building the same source file twice from recipes that are in different directories, you will get two results. Best is to always build a target from the same recipe (that makes it easier to understand the recipe anyway).

Compile only when needed

This continues the last example of the tutorial.

We happen to know that the main.c file does not depend on the GUI used. With the recipe above it will nevertheless be compiled again for every GUI version. Although this is a small thing in this example, in a bigger project it becomes more important to skip compilation when it is not needed. Here is the modified recipe:

1    Source = main.c version.c gui.c
2
3    :variant Build
4        release
5            OPTIMIZE = 4
6            Target = myprog
7        debug
8            DEBUG = yes
9            Target = myprogd
10
11   :attr {var_DEFINE = $DEFINE} {var_BDIR = $BDIR} main.c
12    
13   Gui ?= motif
14   :variant Gui
15       console
16       motif
17            Source += motif.c
18       gtk
19            Source += gtk.c
20
21   DEFINE += -DGUI=$Gui
22
23   :program $Target : $Source

The only new line is line 11. The "main.c" file is given two extra attributes: var_DEFINE and var_BDIR. What happens is that when "main.c" is being build, Aap will check for attributes of this source file that start with "var_". The values will be used to set variables with the following name to the value of the attribute. Thus DEFINE gets the value of var_DEFINE. This means that the variable is overruled by the attribute while building "main.c".

The var_BDIR attribute is set to "$BDIR" before the second :variant command. It does not yet have the selected GUI appended there. The list of directories used is now:

Building multiple variants at once

If you want to build all the variants that are possible, use a few lines of Python code. Here is an example:

 1   :variant license
 2       trial
 3           DEFINE += -DTRIAL
 4       demo
 5           DEFINE += -DDEMO
 6       full
 7           DEFINE += -DFULL
 8   
 9   :variant language
10       chinese
11           DEFINE += -DCHINESE
12       bulgarian
13           DEFINE += -DBULGARIAN
14       *
15           DEFINE += -DENGLISH
16    
17   build:
18       :print Building with $license license for language $language.
19       :print DEFINE=$DEFINE
10    
21   all:
22       @for a in ['trial', 'demo', 'full']:                    #license
23       @   for c in ['chinese', 'bulgarian', 'english']:       #language
24               :execute main.aap build license=$a language=$c

Invoking Aap without arguments builds the "all" target, which loops over all possible variants and invokes the :execute command with the "build" target. The reason to use the "build" target is that without it the "all" target would be built again and result in an endless loop.

This is the resulting output: