How to Call a Function on the Android UI Thread from C++ Using JNI

How can I call a function on the Android UI thread from C++ using JNI?

// Example C++ code to attach JNI to the Android UI thread
JNIEnv* env;
JavaVM* jvm;
// Assume jvm initialization code here
jvm->AttachCurrentThread(&env, NULL);
// Proceed to call Java method here

Invoking a function on the Android UI thread from C++ requires the use of JNI (Java Native Interface). This process involves obtaining a reference to the current activity context and utilizing the UI thread handler to execute the required functions. Here’s a detailed approach to achieve this.

extern "C" JNIEXPORT void JNICALL
Java_com_example_myapp_MainActivity_nativeCallFunc(JNIEnv* env, jobject thiz) {
    jclass cls = env->GetObjectClass(thiz);
    jmethodID mid = env->GetMethodID(cls, "runOnUiThread", "(Ljava/lang/Runnable;)V");
    jobject runnable = env->NewGlobalRef(env->NewObject(env->FindClass("java/lang/Runnable"), env->GetMethodID(env->FindClass("java/lang/Runnable"), "<init>", "()V")));
    env->CallVoidMethod(thiz, mid, runnable);
    env->DeleteGlobalRef(runnable);
}

Causes

• Running non-UI operations on the UI thread can cause application freezes.
• Need to perform UI updates from native code after background processing.

Solutions

• Utilize the Android JNI to get the Java environment (JNIEnv).
• Attach the C++ thread to the Java VM to gain access to the Android UI thread.
• Utilize a Java Runnable to post UI updates to the Android UI thread.

Mistake: Not attaching the current thread to the JVM correctly, resulting in null pointers.

Solution: Always ensure that your thread is attached before making JNI calls.

Mistake: Directly calling UI methods from C++ without posting to the UI thread.

Solution: Use the runOnUiThread method or a similar executor to ensure UI methods execute on the main thread.