Add basic support of Java<->Native callbacks

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

imgui-binding/src/main/java/imgui/ImGui.java

@@ -18,7 +18,7 @@ public final class ImGui {
 
     static {
         System.loadLibrary(System.getProperty(LIB_NAME_PROP, LIB_NAME_DEFAULT));
-        initJniCommon();
+        initJni();
         ImDrawList.nInit();
         ImDrawData.nInit();
         nInitInputTextData();
@@ -31,10 +31,12 @@ public final class ImGui {
         #include <stdint.h>
         #include <imgui.h>
         #include "jni_common.h"
+        #include "jni_callbacks.h"
      */
 
-    private static native void initJniCommon(); /*
+    private static native void initJni(); /*
         Jni::InitCommon(env);
+        Jni::InitCallbacks(env);
     */
 
     // Context creation and access

imgui-binding/src/main/java/imgui/ImGuiIO.java

@@ -1,5 +1,8 @@
 package imgui;
 
+import imgui.callbacks.ImStrConsumer;
+import imgui.callbacks.ImStrSupplier;
+
 /**
  * Communicate most settings and inputs/outputs to Dear ImGui using this structure.
  * Access via ImGui::GetIO(). Read 'Programmer guide' section in .cpp file for general usage.
@@ -11,6 +14,7 @@ public final class ImGuiIO {
     /*JNI
         #include <imgui.h>
         #include "jni_common.h"
+        #include "jni_callbacks.h"
      */
 
     //------------------------------------------------------------------
@@ -253,7 +257,41 @@ public final class ImGuiIO {
         ImGui::GetIO().BackendRendererName = obj_backendRendererName == NULL ? NULL : (char*)env->GetStringUTFChars(obj_backendRendererName, JNI_FALSE);
     */
 
-    // TODO clipboard callback
+    // Optional: Access OS clipboard
+    // (default to use native Win32 clipboard on Windows, otherwise uses a private clipboard. Override to access OS clipboard on other architectures)
+
+    /*JNI
+        jobject _setClipboardTextCallback = nullptr;
+        jobject _getClipboardTextCallback = nullptr;
+
+        void setClipboardTextStub(void* userData, const char* text) {
+            Jni::CallImStrConsumer(Jni::GetEnv(), _setClipboardTextCallback, text);
+        }
+
+        const char* getClipboardTextStub(void* user_data) {
+            JNIEnv* env = Jni::GetEnv();
+            jstring jstr = Jni::CallImStrSupplier(env, _getClipboardTextCallback);
+            return env->GetStringUTFChars(jstr, 0);
+        }
+     */
+
+    public native void setSetClipboardTextFn(ImStrConsumer setClipboardTextCallback); /*
+        if (_setClipboardTextCallback != nullptr) {
+            env->DeleteGlobalRef(_setClipboardTextCallback);
+        }
+
+        _setClipboardTextCallback = env->NewGlobalRef(setClipboardTextCallback);
+        ImGui::GetIO().SetClipboardTextFn = setClipboardTextStub;
+    */
+
+    public native void setGetClipboardTextFn(ImStrSupplier getClipboardTextCallback); /*
+        if (_getClipboardTextCallback != nullptr) {
+            env->DeleteGlobalRef(_getClipboardTextCallback);
+        }
+
+        _getClipboardTextCallback = env->NewGlobalRef(getClipboardTextCallback);
+        ImGui::GetIO().GetClipboardTextFn = getClipboardTextStub;
+    */
 
     //------------------------------------------------------------------
     // Input - Fill before calling NewFrame()

imgui-binding/src/main/java/imgui/callbacks/ImStrConsumer.java

@@ -0,0 +1,11 @@
+package imgui.callbacks;
+
+import java.util.function.Consumer;
+
+/**
+ * Callback to supply Java string form native code.
+ */
+public abstract class ImStrConsumer implements Consumer<String> {
+    @Override
+    public abstract void accept(String str);
+}

imgui-binding/src/main/java/imgui/callbacks/ImStrSupplier.java

@@ -0,0 +1,11 @@
+package imgui.callbacks;
+
+import java.util.function.Supplier;
+
+/**
+ * Callback to get Java string from native code.
+ */
+public abstract class ImStrSupplier implements Supplier<String> {
+    @Override
+    public abstract String get();
+}