Add basic support of Java<->Native callbacks

Implement them for ImGuiIO::GetClipboardTextFn and ImGuiIO::SetClipboardTextFn

imgui-binding/src/main/native/jni_callbacks.cpp

@@ -0,0 +1,23 @@
+#include "jni_callbacks.h"
+
+jmethodID jImStrConsumerAcceptMID;
+jmethodID jImStrSupplierGetMID;
+
+namespace Jni
+{
+void InitCallbacks(JNIEnv* env) {
+    jclass jImStrConsumer = env->FindClass("imgui/callbacks/ImStrConsumer");
+    jImStrConsumerAcceptMID = env->GetMethodID(jImStrConsumer, "accept", "(Ljava/lang/String;)V");
+
+    jclass jImStrSupplier = env->FindClass("imgui/callbacks/ImStrSupplier");
+    jImStrSupplierGetMID = env->GetMethodID(jImStrSupplier, "get", "()Ljava/lang/String;");
+}
+
+void CallImStrConsumer(JNIEnv* env, jobject consumer, const char* str) {
+    env->CallVoidMethod(consumer, jImStrConsumerAcceptMID, env->NewStringUTF(str));
+}
+
+jstring CallImStrSupplier(JNIEnv* env, jobject supplier) {
+    return (jstring)env->CallObjectMethod(supplier, jImStrSupplierGetMID);
+}
+}

imgui-binding/src/main/native/jni_callbacks.h

@@ -0,0 +1,14 @@
+#include <jni.h>
+
+#ifndef JNI_CALLBACK_H
+#define JNI_CALLBACK_H
+
+namespace Jni
+{
+void InitCallbacks(JNIEnv* env);
+
+void CallImStrConsumer(JNIEnv* env, jobject consumer, const char* str);
+jstring CallImStrSupplier(JNIEnv* env, jobject supplier);
+}
+
+#endif

imgui-binding/src/main/native/jni_common.cpp

@@ -8,9 +8,13 @@ jfieldID imVec4YID;
 jfieldID imVec4ZID;
 jfieldID imVec4WID;
 
+static JavaVM* jvm;
+
 namespace Jni
 {
 void InitCommon(JNIEnv* env) {
+    env->GetJavaVM(&jvm);
+
     jclass jImVec2Class = env->FindClass("imgui/ImVec2");
     imVec2XID = env->GetFieldID(jImVec2Class, "x", "F");
     imVec2YID = env->GetFieldID(jImVec2Class, "y", "F");
@@ -22,6 +26,13 @@ void InitCommon(JNIEnv* env) {
     imVec4WID = env->GetFieldID(jImVec4Class, "w", "F");
 }
 
+JNIEnv* GetEnv() {
+    JNIEnv* env;
+    jint res = jvm->GetEnv((void**)(&env), JNI_VERSION_1_8);
+    assert(res == JNI_OK);
+    return env;
+}
+
 void ImVec2Cpy(JNIEnv* env, ImVec2* src, jobject dst) {
     env->SetFloatField(dst, imVec2XID, src->x);
     env->SetFloatField(dst, imVec2YID, src->y);

imgui-binding/src/main/native/jni_common.h

@@ -8,6 +8,8 @@ namespace Jni
 {
 void InitCommon(JNIEnv* env);
 
+JNIEnv* GetEnv();
+
 void ImVec2Cpy(JNIEnv* env, ImVec2* src, jobject dst);
 void ImVec2Cpy(JNIEnv* env, ImVec2 src, jobject dst);