How to execute an Ant task only when source files

问题:

There must be an easy way to do this. I build a Flex app using ant that depends on a SWC library, which works fine except that it rebuilds the library whether it needs to or not. How do I tell ant to only run the task if any of the sources files of the library (*.as, *.mxml) are newer than the SWC?

I've looked at <dependset> but it only seems to delete files, not determine whether a task should be run or not. <depend> seems to expect a one-to-one relationship between the source and target files rather than a one-to-many relationship -- I have many input files and one output file, but no intermediate object files.

Thanks a lot, Alex

回答1:

You may use the Ant uptodate task to create a property, and execute your other target only if that property is set.

I don't know much about flex, but you probably want something like this:

<?xml version="1.0" encoding="UTF-8"?>
<project name="test" default="compile">

   <target name="checkforchanges">
      <uptodate property="nochanges">
         <srcfiles dir="." includes="**/*.as"/>
         <srcfiles dir="." includes="**/*.mxml"/>
         <mapper to="applicaton.flex"/>
      </uptodate>
   </target>

   <target name="compile" depends="checkforchanges" unless="nochanges">
      ...
   </target>

</project>

回答2:

The OutOfDate task from the ant contrib library is IMO much cleaner than the Ant uptodate option. The reason is that you have to define additional targets just to set the property.

The solution with Ant contrib (from their example page):

<outofdate>
    <sourcefiles>
        <pathelement path="build.xml"/>
        <fileset dir="${lib.dir}"/>
    </sourcefiles>
    <targetfiles path="${jrun.file}"/>
    <sequential>
        <mkdir dir="${build.bin.dir}"/>
        <echo file="${jrun.file}" message="java -cp ${jrun.path} $*"/>
        <chmod file="${jrun.file}" perm="ugo+rx"/>
    </sequential>
</outofdate> 

Everything is kept nicely inside one single target.