Handle backspace on Android by using the engine's ICU grapheme breaker

fml/platform/android/jni_util.cc

@@ -165,5 +165,21 @@ std::string GetJavaExceptionInfo(JNIEnv* env, jthrowable java_throwable) {
   return JavaStringToString(env, exception_string.obj());
 }
 
+void ThrowException(JNIEnv* env, const char* class_name, const char* message) {
+  jclass clazz = env->FindClass(class_name);
+  FML_DCHECK(clazz);
+  env->ThrowNew(clazz, message);
+}
+
+ScopedJavaStringChars::ScopedJavaStringChars(JNIEnv* env, jstring str)
+    : env_(env), str_(str) {
+  chars_ = env_->GetStringChars(str_, nullptr);
+  FML_DCHECK(chars_);
+}
+
+ScopedJavaStringChars::~ScopedJavaStringChars() {
+  env_->ReleaseStringChars(str_, chars_);
+}
+
 }  // namespace jni
 }  // namespace fml

fml/platform/android/jni_util.h

@@ -38,6 +38,21 @@ bool ClearException(JNIEnv* env);
 
 std::string GetJavaExceptionInfo(JNIEnv* env, jthrowable java_throwable);
 
+void ThrowException(JNIEnv* env, const char* class_name, const char* message);
+
+class ScopedJavaStringChars {
+ public:
+  ScopedJavaStringChars(JNIEnv* env, jstring str);
+  ~ScopedJavaStringChars();
+
+  const jchar* chars() { return chars_; }
+
+ private:
+  JNIEnv* env_;
+  jstring str_;
+  const jchar* chars_;
+};
+
 }  // namespace jni
 }  // namespace fml
 

shell/platform/android/io/flutter/embedding/engine/FlutterJNI.java

@@ -146,6 +146,15 @@ public static native void nativeOnVsync(
   @NonNull
   public static native FlutterCallbackInformation nativeLookupCallbackInformation(long handle);
 
+  /**
+   * Return the index of the first grapheme cluster preceding {@code offset} in {@code text}.
+   *
+   * <p>This is similar to {@link android.text.TextUtils#getOffsetBefore(CharSequence, int, int)}
+   * but it uses the engine's ICU library which is up to date and acts consistently on all versions
+   * of Android.
+   */
+  public static native int nativeGetTextOffsetBefore(String text, int offset);
+
   @Nullable private Long nativePlatformViewId;
   @Nullable private AccessibilityDelegate accessibilityDelegate;
   @Nullable private PlatformMessageHandler platformMessageHandler;

shell/platform/android/io/flutter/plugin/editing/InputConnectionAdaptor.java

@@ -13,15 +13,16 @@
 import android.text.Layout;
 import android.text.Selection;
 import android.text.TextPaint;
-import android.text.method.TextKeyListener;
 import android.view.KeyEvent;
 import android.view.View;
 import android.view.inputmethod.BaseInputConnection;
 import android.view.inputmethod.EditorInfo;
 import android.view.inputmethod.ExtractedText;
 import android.view.inputmethod.ExtractedTextRequest;
 import android.view.inputmethod.InputMethodManager;
+import androidx.annotation.VisibleForTesting;
 import io.flutter.Log;
+import io.flutter.embedding.engine.FlutterJNI;
 import io.flutter.embedding.engine.systemchannels.TextInputChannel;
 
 class InputConnectionAdaptor extends BaseInputConnection {
@@ -111,6 +112,23 @@ public InputConnectionAdaptor(
     mImm = (InputMethodManager) view.getContext().getSystemService(Context.INPUT_METHOD_SERVICE);
   }
 
+  interface GetOffsetBefore {
+    int getOffsetBefore(String text, int offset);
+  }
+
+  private GetOffsetBefore getOffsetBefore =
+      new GetOffsetBefore() {
+        @Override
+        public int getOffsetBefore(String text, int offset) {
+          return FlutterJNI.nativeGetTextOffsetBefore(text, offset);
+        }
+      };
+
+  @VisibleForTesting
+  void setGetOffsetBefore(GetOffsetBefore value) {
+    getOffsetBefore = value;
+  }
+
   // Send the current state of the editable to Flutter.
   private void updateEditingState() {
     // If the IME is in the middle of a batch edit, then wait until it completes.
@@ -270,18 +288,16 @@ public boolean sendKeyEvent(KeyEvent event) {
       if (event.getKeyCode() == KeyEvent.KEYCODE_DEL) {
         int selStart = clampIndexToEditable(Selection.getSelectionStart(mEditable), mEditable);
         int selEnd = clampIndexToEditable(Selection.getSelectionEnd(mEditable), mEditable);
+        if (selStart == selEnd && selStart > 0) {
+          // Extend selection to the left of the last character.
+          selStart = getOffsetBefore.getOffsetBefore(mEditable.toString(), selStart);
+        }
         if (selEnd > selStart) {
           // Delete the selection.
           Selection.setSelection(mEditable, selStart);
           mEditable.delete(selStart, selEnd);
           updateEditingState();
           return true;
-        } else if (selStart > 0) {
-          if (TextKeyListener.getInstance().onKeyDown(null, mEditable, event.getKeyCode(), event)) {
-            updateEditingState();
-            return true;
-          }
-          return false;
         }
       } else if (event.getKeyCode() == KeyEvent.KEYCODE_DPAD_LEFT) {
         int selStart = Selection.getSelectionStart(mEditable);

shell/platform/android/platform_view_android_jni.cc

@@ -7,6 +7,7 @@
 #include <android/native_window_jni.h>
 
 #include <utility>
+#include "unicode/brkiter.h"
 
 #include "flutter/assets/directory_asset_bundle.h"
 #include "flutter/common/settings.h"
@@ -484,6 +485,28 @@ static void InvokePlatformMessageEmptyResponseCallback(JNIEnv* env,
       );
 }
 
+// Return the index of the first grapheme cluster that precedes the given
+// offset in the text.
+static jint GetTextOffsetBefore(JNIEnv* env,
+                                jclass jcaller,
+                                jstring text,
+                                jint offset) {
+  std::unique_ptr<icu::BreakIterator> breaker;
+  UErrorCode status = U_ZERO_ERROR;
+  breaker.reset(
+      icu::BreakIterator::createCharacterInstance(icu::Locale(), status));
+  if (!U_SUCCESS(status)) {
+    fml::jni::ThrowException(env, "java/lang/IllegalStateException",
+                             "unable to create character iterator");
+    return -1;
+  }
+
+  fml::jni::ScopedJavaStringChars chars(env, text);
+  breaker->setText(
+      icu::UnicodeString(false, chars.chars(), env->GetStringLength(text)));
+  return breaker->preceding(offset);
+}
+
 bool RegisterApi(JNIEnv* env) {
   static const JNINativeMethod flutter_jni_methods[] = {
       // Start of methods from FlutterJNI
@@ -599,6 +622,12 @@ bool RegisterApi(JNIEnv* env) {
           .signature = "(J)Lio/flutter/view/FlutterCallbackInformation;",
           .fnPtr = reinterpret_cast<void*>(&LookupCallbackInformation),
       },
+
+      {
+          .name = "nativeGetTextOffsetBefore",
+          .signature = "(Ljava/lang/String;I)I",
+          .fnPtr = reinterpret_cast<void*>(&GetTextOffsetBefore),
+      },
   };
 
   if (env->RegisterNatives(g_flutter_jni_class->obj(), flutter_jni_methods,

shell/platform/android/test/io/flutter/plugin/editing/InputConnectionAdaptorTest.java

@@ -16,6 +16,7 @@
 import android.text.InputType;
 import android.text.Selection;
 import android.text.SpannableStringBuilder;
+import android.text.TextUtils;
 import android.view.KeyEvent;
 import android.view.View;
 import android.view.inputmethod.EditorInfo;
@@ -318,6 +319,13 @@ public void testSendKeyEvent_delKeyDeletesBackward() {
     int selStart = 29;
     Editable editable = sampleRtlEditable(selStart, selStart);
     InputConnectionAdaptor adaptor = sampleInputConnectionAdaptor(editable);
+    adaptor.setGetOffsetBefore(
+        new InputConnectionAdaptor.GetOffsetBefore() {
+          @Override
+          public int getOffsetBefore(String text, int offset) {
+            return TextUtils.getOffsetBefore(text, offset);
+          }
+        });
 
     KeyEvent downKeyDown = new KeyEvent(KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_DEL);