morristech / buildtimegradlecodegen

The most trivial (and hacky) code generation with Gradle for Android

The most trivial (and hacky) code generation with Gradle for Android

For example given an interface in root package of an application (let's assume it's com.sample)

package com.sample;

interface Configuration {
    boolean hasFeature(int flag);
}

There is a need to have a specific implementation depending on buildType or productFlavor or (which is more applicable in this example) a specifc property during a build time. For example, if build is triggered with property: supportedFeatures=1,3,5,10 we want all builds to reflect that. Here is what we can do:

gradlew assemble -PsupportedFeatures=1,3,5,10

android {

    compileSdkVersion 25
    buildToolsVersion '25.0.2'
    
    defaultConfig {
        applicationId 'com.sample'
        minSdkVersion 16
        targetSdkVersion 25
        versionCode 1
        versionName '1.0.0'
        
        final def conf = project.hasProperty('supportedFeatures') \
                ? 'flags == ' + project.get('supportedFeatures').split(',').join(' || flags == ') \
                : 'false'

        final def configurationImpl = """
                public static class ConfigurationImpl implements Configuration {
                    @Override
                    public boolean hasFeature(int flags) {
                        return $conf;
                    }
                }
        """
        
        buildConfigField 'byte', '$$__$$', '0;\n' +
                configurationImpl
    }
}

So, first we check if our project has supportedFeatures property, if it does we construct the return statement with applied logic, if not we just return false.

Then we write a basic Configuration implementation and insert into it our return statement.

Then, we declare a mock buildConfigField with byte type (the smallest possible type in Java), give it unreadable name (to avoid its usage by random caller), place a semicolon ; and a new line \n and concat with our configuration implementation source code. When the project will be build we will see this in com.sample.BuildConfig:

package com.sample;

public final class BuildConfig {
    public static final boolean DEBUG = Boolean.parseBoolean("true");
    public static final String APPLICATION_ID = "com.sample";
    public static final String BUILD_TYPE = "debug";
    public static final String FLAVOR = "";
    public static final int VERSION_CODE = 1;
    public static final String VERSION_NAME = "1.0.0";
    
    // Fields from default config.
    public static final byte $$__$$ = 0;

    public static class ConfigurationImpl implements Configuration {
        @Override
        public boolean hasFeature(int flags) {
            return flags == 1 || flags == 3 || flags == 5 || flags == 10;
        }
    }
    ;
}

Then it can be used as usual:

final Configuration configuration = new BuildConfig.ConfigurationImpl();

Please note that I'm not advocating to abuse Android Gradle plugin like this. Most likely there is a better way to achieve the same result, but if you need a quick solution to test something or you are in a hacky mood this can help a bit.