[MDCAlertControllerView] Avoid cutting off the top of Thai characters…

… with diacritics.

In iOS platform, text font are not strictly required to fit within the UIFont.lineHeight. Some Thai characters with diacritics are rendered outside the UILabel/UITextView's bounds.

One solution is to increase the top margin of the UILabel/UITextView so we leave extra padding between the parent view's top edge and the top edge of the UILabel/UITextView (Like the _titleLabel in MDCAlertControllerView).

closes #10161

PiperOrigin-RevId: 363491472

components/Dialogs/src/private/MDCAlertControllerView+Private.m

@@ -36,6 +36,35 @@
 
 static const CGFloat MDCDialogMessageOpacity = 0.54f;
 
+/** Calculates the minimum text height for a single line text using device metrics. */
+static CGFloat SingleLineTextViewHeight(NSString *_Nullable title, UIFont *_Nullable font) {
+  if (title.length == 0) {
+    return font.lineHeight;
+  }
+
+  NSDictionary<NSAttributedStringKey, id> *attributes =
+      font == nil ? nil : @{NSFontAttributeName : font};
+  CGRect boundingRect = [title boundingRectWithSize:CGSizeMake(CGFLOAT_MAX, CGFLOAT_MAX)
+                                            options:NSStringDrawingUsesDeviceMetrics
+                                         attributes:attributes
+                                            context:nil];
+  // Some text height may exceed the UIFont.lineHeight.
+  // https://developer.apple.com/documentation/uikit/uifont?language=objc
+  // --------------------
+  //  |               |
+  //  | <= ascender   |
+  //  |               | <= lineHeight
+  // -----------------|
+  //  | <= descender  |
+  // --------------------
+  // UIFont.lineHeight consist of UIFont.ascender and UIFont.descender. UIFont.descender is the
+  // bottom offset from the baseline. The boundingRect.origin.y is the amount of space the text will
+  // occupy in UIFont.descender. So the minimum text view line height is text height + the leftover
+  // space in the UIFont.descender not occupied by the rendered text.
+  CGFloat bottomPadding = boundingRect.origin.y - font.descender;
+  return boundingRect.size.height + bottomPadding;
+}
+
 @interface MDCNonselectableTextView : UITextView
 @end
 
@@ -1059,6 +1088,41 @@ - (void)updateFonts {
 
 @implementation MDCNonselectableTextView
 
+#pragma mark - UITextView
+
+- (void)setAttributedText:(NSAttributedString *)attributedText {
+  if ([self.attributedText isEqual:attributedText]) {
+    return;
+  }
+
+  [super setAttributedText:attributedText];
+  [self updateTopInsetAndTextContainerInset:self.textContainerInset];
+}
+
+- (void)setFont:(UIFont *)font {
+  if ([self.font isEqual:font]) {
+    return;
+  }
+
+  [super setFont:font];
+  [self updateTopInsetAndTextContainerInset:self.textContainerInset];
+}
+
+- (void)setText:(NSString *)text {
+  if ([self.text isEqual:text]) {
+    return;
+  }
+
+  [super setText:text];
+  [self updateTopInsetAndTextContainerInset:self.textContainerInset];
+}
+
+- (void)setTextContainerInset:(UIEdgeInsets)textContainerInset {
+  [self updateTopInsetAndTextContainerInset:textContainerInset];
+}
+
+#pragma mark - UIView
+
 // Disabling text selection when selectable is YES, while allowing gestures for inlined links.
 - (BOOL)pointInside:(CGPoint)point withEvent:(UIEvent *)event {
   if (UIAccessibilityIsVoiceOverRunning()) {
@@ -1086,4 +1150,28 @@ - (BOOL)pointInside:(CGPoint)point withEvent:(UIEvent *)event {
   return link != nil;
 }
 
+#pragma mark - Private
+
+/**
+ * Updates the top text inset of the UITextView to avoid rendering text outside the view
+ * boundary. We need to calculate the text margin when we change the text or the text font.
+ *
+ * @note Our title UILabel already has a top margin, so its text won't exceed the parent view's
+ * boundary.
+ *
+ * @param textContainerInset The target text insets to display. It will update the following text
+ * margin: left, right and bottom.
+ */
+- (void)updateTopInsetAndTextContainerInset:(UIEdgeInsets)textContainerInset {
+  // To avoid changing the top margin when the view width changes, we only compare the text height
+  // for a single line text.
+  UIFont *font = self.font;
+  CGFloat singleLineTextViewHeight = SingleLineTextViewHeight(self.text, font);
+  textContainerInset.top = MAX(0.0f, singleLineTextViewHeight - font.lineHeight);
+  if (!UIEdgeInsetsEqualToEdgeInsets(super.textContainerInset, textContainerInset)) {
+    // Note that |self setTextContainerInset:| will call this method.
+    super.textContainerInset = textContainerInset;
+  }
+}
+
 @end

components/Dialogs/tests/snapshot/MDCAlertControllerLocalizationTests.m

@@ -21,6 +21,7 @@
 #import "MDCAlertController+Testing.h"
 #import "MaterialSnapshot.h"
 
+static NSString *const kThaiTextWithDiacritics = @"นี้ Thai text นี้";
 static NSString *const kTitleUrdu = @"عنوان";
 static NSString *const kMessageUrdu =
     @"براہ کرم اپنا نیٹ ورک کنکشن چیک کریں اور دوبارہ کوشش کریں۔براہ کرم اپنا نیٹ ورک کنکشن چیک "
@@ -86,6 +87,35 @@ - (void)changeToRTL:(MDCAlertController *)alertController {
 
 #pragma mark - Tests
 
+/** Verifies the top of Thai attributed message with diacritics won't be cut off. */
+- (void)testPreferredContentSizeWithThaiAttributedMessage {
+  // When
+  self.alertController.attributedMessage =
+      [[NSAttributedString alloc] initWithString:kThaiTextWithDiacritics];
+
+  // Then
+  [self generateSizedSnapshotAndVerifyForAlert:self.alertController];
+}
+
+/** Verifies the top of Thai message with diacritics won't be cut off. */
+- (void)testPreferredContentSizeWithThaiMessage {
+  // When
+  self.alertController.message = kThaiTextWithDiacritics;
+
+  // Then
+  [self generateSizedSnapshotAndVerifyForAlert:self.alertController];
+}
+
+/** Verifies the top of Thai message and title with diacritics won't be cut off. */
+- (void)testPreferredContentSizeWithThaiMessageAndTitle {
+  // When
+  self.alertController.title = kThaiTextWithDiacritics;
+  self.alertController.message = kThaiTextWithDiacritics;
+
+  // Then
+  [self generateSizedSnapshotAndVerifyForAlert:self.alertController];
+}
+
 - (void)testPreferredContentSizeWithNotoNastaliqUrdu {
   // When
   self.alertController.title = kTitleUrdu;