[API] Add a mechanism for Dear ImGui assertion callbacks (#100)

* Syncing wip to github

* Assertion callback works

* Restore example that does not crash

* Add comment in build script. Merge with upstream.

* Fix assert always triggering

* Resolve jvm env at runtime to prevent looking up bad stack address

* Fixed EXCEPTION_ACCESS_VIOLATION within native code after callback was complete.

* Fix linting errors

* Change ImGui to Dear ImGui

Co-authored-by: SpaiR <klimbetoo@gmail.com>

* Move jvm native layer to its own header/source

Co-authored-by: SpaiR <klimbetoo@gmail.com>

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

@@ -1,5 +1,6 @@
 package imgui;
 
+import imgui.assertion.ImAssertCallback;
 import imgui.flag.ImGuiCond;
 import imgui.flag.ImGuiDragDropFlags;
 import imgui.flag.ImGuiInputTextFlags;
@@ -76,6 +77,14 @@ public class ImGui {
         ImFontAtlas.nInit();
         ImGuiPlatformIO.init();
         nInitInputTextData();
+        setAssertCallback(new ImAssertCallback() {
+            @Override
+            public void imAssertCallback(String assertion, int line, String file) {
+                System.err.println("Dear ImGui Assertion Failed: " + assertion);
+                System.err.println("Assertion Located At: " + file + ":" + line);
+                Thread.dumpStack();
+            }
+        });
     }
 
     private static String resolveFullLibName() {
@@ -131,7 +140,9 @@ public class ImGui {
      */
 
     private static native void nInitJni(); /*
+        Jni::InitJvm(env);
         Jni::InitCommon(env);
+        Jni::InitAssertion(env);
         Jni::InitCallbacks(env);
         Jni::InitBindingStruct(env);
     */
@@ -190,6 +201,17 @@ public class ImGui {
         ImGui::SetCurrentContext((ImGuiContext*) ptr);
     */
 
+    /**
+     * Set a custom assertion callback for ImGui assertions.
+     * Take note: Any custom assertion callback SHOULD NOT throw any exception.
+     * After any callback the application will be terminated, any attempt to bypass this behavior
+     * will result in a EXCEPTION_ACCESS_VIOLATION from within the native layer.
+     * @param callback The custom ImGui assertion callback
+     */
+    public static native void setAssertCallback(ImAssertCallback callback); /*
+        Jni::SetAssertionCallback(callback);
+    */
+
     // Main
 
     /**

imgui-binding/src/main/java/imgui/assertion/ImAssertCallback.java

@@ -0,0 +1,45 @@
+
+package imgui.assertion;
+
+/**
+ * Callback for native IM_ASSERT calls.
+ */
+public abstract class ImAssertCallback {
+    /**
+     * Called from native code to bring the callback into java side.
+     * Will force the application to terminate with exit code 1 after forwarding the callback, this is required, otherwise
+     * if execution is returned to the native layer, a EXCEPTION_ACCESS_VIOLATION will be thrown.
+     * If the callback throws an exception, it will be caught and printed along with a warning to prevent execution
+     * from returning to the native layer.
+     *
+     * @param assertion The assertion string
+     * @param line      The line number of the assertion in the source file
+     * @param file      The source file where the assertion occurred
+     */
+    public void imAssert(final String assertion, final int line, final String file) {
+        try {
+            imAssertCallback(assertion, line, file);
+        } catch (Exception ex) {
+            System.err.println("WARNING: Exception thrown in Dear ImGui Assertion Callback!");
+            System.err.println("Dear ImGui Assertion Failed: " + assertion);
+            System.err.println("Assertion Located At: " + file + ":" + line);
+            ex.printStackTrace();
+        }
+        System.exit(1);
+    }
+
+    /**
+     * The assertion callback from ImGui.
+     * Do not throw an exception within this callback, to prevent
+     * execution from returned to the native layer and cause a EXCEPTION_ACCESS_VIOLATION,
+     * the callback will catch any exceptions and print a warning.
+     * <p>
+     * There is no way to catch the assertion and continue execution.
+     * You may however call <code>System.exit(code)</code> with your own exit code.
+     *
+     * @param assertion The assertion string
+     * @param line      The line number of the assertion in the source file
+     * @param file      The source file where the assertion occurred
+     */
+    public abstract void imAssertCallback(String assertion, int line, String file);
+}