7. September 2017

Gradle Experimental Plugin Android error: Cannot invoke method dependencies()

Update: Engineers at Google fixed the issue. Thank you.

After initial setup of SDL2 application for Android, I decided to add one activity before launching SDL. So I created the new MainActivity which should launch SDLActivity after a tap on the button.

This time Gradle Experimental Plugin gave me a very fancy error: Cannot invoke method dependencies() on null object.

As always the error message does not provide too many hints how to solve the issue.

Android Studio automatically just appended a dependency into the Gradle file during creation of new MainActivity. So the code in build.gradle looked like this:

tasks.whenTaskAdded { task ->
    if (task.name.contains('compile')) {
        task.dependsOn ':main:distributeLib'
    }
}dependencies {
    compile 'com.android.support.constraint:constraint-layout:+'
}

Little bit weird code for dependencies. The solution was just to add a new line.

tasks.whenTaskAdded { task ->
    if (task.name.contains('compile')) {
        task.dependsOn ':main:distributeLib'
    }
}

dependencies {
    compile 'com.android.support.constraint:constraint-layout:+'
}

The whole project is available at github.com/georgik/sdl2-android-example project.

More articles about SDL2 for Android are available under tag sdl2.

3. September 2017

Difference between libraries and main Android application based on SDL2 built by Gradle 4

Gradle Android Experimental Plugin works without problem with Android Studio and it is the recommended approach use it when starting a new project with NDK.

It’s very easy to get started, but there are some traps. Let’s explore common mistake when writing Gradle build script for Android with NDK.

In case of an application which is based on C libraries, the goal is clear. Build libraries and then link everything into the final project.

The tricky part is the definition of dependencies. The definition for the main and for libraries is different. If you use the same style of definition you may run into error like: Android Studio is not able to import the project, because of missing .so file.

Here is the definition which you can find in build.gradle for the main module:

model {
     repositories {
        libs(PrebuiltLibraries) {
            SDL2 {
                headers.srcDir "../SDL2/include"
                binaries.withType(SharedLibraryBinary) {
                    sharedLibraryFile = file("${gradle.libDistributionRoot}/SDL2/lib/${targetPlatform.getName()}/libSDL2.so")
                }
            }
            SDL2_image {
                headers.srcDir "../SDL2_image/include"
                binaries.withType(SharedLibraryBinary) {
                    sharedLibraryFile = file("${gradle.libDistributionRoot}/SDL2_image/lib/${targetPlatform.getName()}/libSDL2_image.so")
                }
            }

        }
    }

   android {
        compileSdkVersion = gradle.sdkVersion
        buildToolsVersion = '25.0.3'

        defaultConfig {
            minSdkVersion.apiLevel = 13
            versionCode = 1
            versionName = '1.0'
        }
        ndk {
            moduleName = 'main'
            cppFlags.addAll([
                    "-I" + file("../SDL2/include/").absolutePath,
                    "-I" + file("../SDL2_image/include/").absolutePath,
            ])
            CFlags.addAll([
                    "-I" + file("../SDL2/include/").absolutePath,
                    "-I" + file("../SDL2_image/include/").absolutePath,
            ])
            stl "stlport_static"
        }

        sources {
            main {
                jni {
                    dependencies {
                        library 'SDL2' linkage 'shared'
                        library 'SDL2_jpeg' linkage 'shared'
                        library 'SDL2_png' linkage 'shared'
                        library 'SDL2_image' linkage 'shared'
                    }
                    source {
                        srcDirs 'src/'
                    }
                }
            }
        }

    }
}

If you look close enough, you’ll see that the first part of the file contains a definition of PrebuiltLibraries. These libraries are referencing directly the .so file. In the section dependencies, you can find reference to the declared dependencies.

Gradle will just build the app and it will grab all referenced .so files and put them into a final application. That is correct for the main module. When you’re writing build files for libraries, you should take a different approach.

Here is an example of SDL2_image/build.gradle file:

apply plugin: 'com.android.model.native'

model {

    android {
        compileSdkVersion = gradle.sdkVersion
        buildToolsVersion = '25.0.3'

        defaultConfig {
            minSdkVersion.apiLevel = 13
            versionCode = 1
            versionName = '1.0'
        }
        ndk {
            moduleName = 'SDL2_image'
            ldLibs.addAll(["GLESv1_CM", "EGL", "GLESv2", "log", "android", "dl"])
            CFlags.addAll(["-DGL_GLEXT_PROTOTYPES"])
            CFlags.addAll(["-I" + file("include/").absolutePath,
                           "-DGL_GLEXT_PROTOTYPES",
                           "-DLOAD_JPG",
                           "-DLOAD_PNG",
                           "-DLOAD_XPM"
            ])
        }

        sources {
            main {
                jni {
                    dependencies {
                        project ':SDL2' linkage 'shared'
                        project ':SDL2_jpeg' linkage 'shared'
                        project ':SDL2_png' linkage 'shared'
                    }
                    exportedHeaders {
                        srcDir "../SDL2/include"
                        srcDir "../SDL2_jpeg/include"
                        srcDir "../SDL2_png/include"
                    }
                    source {
                        srcDir "src"

                    }
                }
            }
        }

    }
}

Here you can see that there is no definition of PrebuiltLibraries, but the sources contain dependencies on projects of other libraries. In addition to that, there is exportedHeaders declaration which will tell the compiler where to find .h files. That is because you do not need any reference to .so dependencies when building .so library. You need just headers.

The different is very small in the dependencies section. It can simplify your build process if you use it correctly.

Here is an example of an error that occurs when the .so file is missing and library is referencing the file:

project refresh failed Error:Exception thrown while executing model rule: 
NdkComponentModelPlugin.Rules#createNativeBuildModel(NativeBuildConfig, ModelMap<androidbinaryinternal>, ModelMap<ndkabioptions>, NdkHandler) > create...

You can fix either build files or you can build missing libraries one by one.

The correct version of build files is available in sdl2-android-example project.

1. September 2017

SDL2 for Android API level 19

SDL2 example application works without problem with newer Androids API 21+. The problem was with API level 19. It took me some time to figure out reasons for very interesting errors.

Let’s examine error messages and investigate how to fix them.

When I’ve started sdl2-android-example on a device with API 19, it failed with error message: dlopen failed: cannot locate symbol “signal” referenced by libSDL2.so”

The root cause is not that obvious from the error message. It failed because the application has been built for API 21 instead of API 19.

The fixture is easy, just set sdkVersion to 19.

For this purpose, I’ve extracted sdkVersion to settings.gradle which can be found in the root directory of the project.

gradle.ext.sdkVersion = 19

Each module then has reference to this value:

model {
    android {
        compileSdkVersion = gradle.sdkVersion
...

Make complete clean and distributeLib to rebuild for proper API.

gradle clean dL

Hooray! Fixed. Let’s start the application.

Yey. New error: dlopen failed: could not load library “libSDL2_image.so” needed by “libmain.so”; caused by cannot locate symbol “png_set_longjmp_fn” referenced by “libSDL2_image.so”

The root cause of this issue is similar to the problem with JPEG described in the previous article. Paul Asiimwe found out that there is another library the name png and it has a different version so the application crashes.

To fix this issue it was necessary to rename whole png submodule to SDL2_png. Renaming requires clean and new distributeLib:

gradle clean dL

After these small tricks, the sdl2-android-example works even with API level 19.

31. August 2017

SDL2_image for Android with JPEG image format

In the previous article, we were discussing how to add support for PNG image format to SDL2 application for Android. Let’s add another more common format. What about JPEG?

The first step is to enable JPEG in SDL2_image. Just add a proper definition to SDL2_image/build.gradle.

ndk {
...
CFlags.addAll([
    "-DLOAD_JPG"
    "-DLOAD_PNG",
    "-DLOAD_XPM"
])}

Then it is necessary to add C implementation of JPEG library. There is one big catch. Do not call it jpeg. The problem is that the name will collide with another library in Android system and function jpeg_create_decompress(&cinfo) will crash. Thanks to DevMultiTech for a hint about the solution.

Let’s call the jpeg library SDL2_jpeg.

Register modules in settings.gradle:

include ':SDL2_jpeg'

Build all libraries:

gradle distributeLib

Here is a sample code for loading and displaying JPEG image:

SDL_Surface *backgroundSurface = IMG_Load("brno-snow.jpg")
SDL_Texture *backgroundTexture = SDL_CreateTextureFromSurface(renderer, backgroundSurface);
SDL_RenderCopy(renderer, backgroundTexture, NULL, &dstrect);

Result looks like this:

You can find whole source code at https://github.com/georgik/sdl2-android-example.

You can find about SDL2 and libraries more under the topic SDL2.

9. July 2017

SDL2_image for Android with PNG image format

Update of the article #1: If you’d like to support API level 19, the library must have a different name than png. Therefore I changed names in the article to SDL2_png. Read more about SDL2 API level 19 support.

Update of the article #2: Original article contained a dependency on the custom version of zip lib. It is not necessary because it is sufficient to link libz from Android. The library was removed from the example.

In the previous article, we were discussing how to add support for XPM image format to SDL2 application for Android. Let’s add another more common format. What about Portable Network Graphic (a.k.a. PNG)?

The first step is to enable PNG in SDL2_image. Just add a proper definition to SDL2_image/build.gradle.

ndk {
...
CFlags.addAll([
    "-DLOAD_PNG",
    "-DLOAD_XPM"
])}

Unfortunately, it’s not that simple. PNG support has a dependency on PNG library with a patch and PNG library. To get PNG working you need to add these one C library into the project. The approach is the same like when adding SDL2_image. The library is stored as a separate module in the directory with the same name like module name.

Register modules in settings.gradle:

include ':SDL2_image'
include ':SDL2_png'

Add build.gradle: SDL2_png/build.gradle.

You can verify compilation of the whole suite by following commands:

gradle :SDL2_png:distributeLib
gradle :SDL2_image:distributeLib

or simply:

gradle distributeLib

The library is ready. Now it’s necessary to add PNG file to the project. The location for graphic assets is app/src/main/assets.

Here is a sample code for loading and displaying PNG image:

SDL_Surface *loadedSurface = IMG_Load("smiley.png");
SDL_Texture *smileyTexture = SDL_CreateTextureFromSurface(renderer, loadedSurface);
SDL_RenderCopy(renderer, smileyTexture, NULL, NULL);

Result looks like this:

As you can see the PNG version of smiley has support for background transparency.

You can find the whole source code at https://github.com/georgik/sdl2-android-example.

We will discuss how to use further SDL2 libraries in next articles. You can find more under topic SDL2.

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