EditText: maintain cursor position when text changes

Summary:
Similar to D29786190 (b0e39b2) on iOS, keeps cursor position constant to the end of the text whenever text changes without selection updates.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D29879663

fbshipit-source-id: da1b50a99ae3b9ef796423146ba49e4172e286df

Author: Andrei Shikov

ReactAndroid/src/main/java/com/facebook/react/views/textinput/ReactEditText.java

@@ -331,10 +331,20 @@ public void maybeSetSelection(int eventCounter, int start, int end) {
     }
 
     if (start != UNSET && end != UNSET) {
+      // clamp selection values for safety
+      start = clampToTextLength(start);
+      end = clampToTextLength(end);
+
       setSelection(start, end);
     }
   }
 
+  private int clampToTextLength(int value) {
+    int textLength = getText() == null ? 0 : getText().length();
+
+    return Math.max(0, Math.min(value, textLength));
+  }
+
   @Override
   public void setSelection(int start, int end) {
     if (DEBUG_MODE) {

ReactAndroid/src/main/java/com/facebook/react/views/textinput/ReactTextInputManager.java

@@ -328,9 +328,22 @@ public void updateExtraData(ReactEditText view, Object extraData) {
         Spannable spannable = update.getText();
         TextInlineImageSpan.possiblyUpdateInlineImageSpans(spannable, view);
       }
+
+      // Ensure that selection is handled correctly on text update
+      boolean isCurrentSelectionEmpty = view.getSelectionStart() == view.getSelectionEnd();
+      int selectionStart = update.getSelectionStart();
+      int selectionEnd = update.getSelectionEnd();
+      if ((selectionStart == UNSET || selectionEnd == UNSET) && isCurrentSelectionEmpty) {
+        // if selection is not set by state, shift current selection to ensure constant gap to
+        // text end
+        int textLength = view.getText() == null ? 0 : view.getText().length();
+        int selectionOffset = textLength - view.getSelectionStart();
+        selectionStart = update.getText().length() - selectionOffset;
+        selectionEnd = selectionStart;
+      }
+
       view.maybeSetTextFromState(update);
-      view.maybeSetSelection(
-          update.getJsEventCounter(), update.getSelectionStart(), update.getSelectionEnd());
+      view.maybeSetSelection(update.getJsEventCounter(), selectionStart, selectionEnd);
     }
   }