PlatformFontMac: better guess for average width

ui/gfx/platform_font_mac.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/platform_font_mac.h"
 
 #include <cmath>
 
 #include <Cocoa/Cocoa.h>
 
@@ skipping 115 matching lines @@
 
 int PlatformFontMac::GetBaseline() {
   return ascent_;
 }
 
 int PlatformFontMac::GetCapHeight() {
   return cap_height_;
 }
 
 int PlatformFontMac::GetExpectedTextWidth(int length) {
-  return length * average_width_;
+  if (!average_width_) {

[tapted, 2017/05/03 00:43:35]

I think we need a `&& native_font_` in this condition -- there are weird cases
where it's null for invalid font names and I'm not sure whether
NSAttributedString will behave in that case 

[Elly Fong-Jones, 2017/05/03 18:06:07]

Done.

+    // -[NSFont boundingRectForGlyph:] seems to always return the largest
+    // bounding rect that could be needed, which produces very wide expected
+    // widths for strings. Instead, compute the actual width of a string
+    // containing all the lowercase characters to find a reasonable guess at the
+    // average.
+    base::scoped_nsobject<NSAttributedString> as([[NSAttributedString alloc]
+        initWithString:@"abcdefghijklmnopqrstuvwxyz"
+            attributes:@{NSFontAttributeName : native_font_.get()}]);
+    average_width_ = [as size].width / [as length];

[tapted, 2017/05/03 00:43:35]

nit: DCHECK(!!average_width_) ?

[Elly Fong-Jones, 2017/05/03 18:06:07]

Done.

+  }
+  return ceil(length * average_width_);
 }
 
 int PlatformFontMac::GetStyle() const {
   return font_style_;
 }
 
 Font::Weight PlatformFontMac::GetWeight() const {
   return font_weight_;
 }
 
@@ skipping 48 matching lines @@
 }
 
 void PlatformFontMac::CalculateMetricsAndInitRenderParams() {
   NSFont* font = native_font_.get();
   if (!font) {
     // This object was constructed from a font name that doesn't correspond to
     // an actual font. Don't waste time working out metrics.
     height_ = 0;
     ascent_ = 0;
     cap_height_ = 0;
-    average_width_ = 0;
     return;
   }
 
   ascent_ = ceil([font ascender]);
   cap_height_ = ceil([font capHeight]);
 
   // PlatformFontMac once used -[NSLayoutManager defaultLineHeightForFont:] to
   // initialize |height_|. However, it has a silly rounding bug. Essentially, it
   // gives round(ascent) + round(descent). E.g. Helvetica Neue at size 16 gives
   // ascent=15.4634, descent=3.38208 -> 15 + 3 = 18. When the height should be
   // at least 19. According to the OpenType specification, these values should
   // simply be added, so do that. Note this uses the already-rounded |ascent_|
   // to ensure GetBaseline() + descender fits within GetHeight() during layout.
   height_ = ceil(ascent_ + std::abs([font descender]) + [font leading]);
 
-  average_width_ =
-      NSWidth([font boundingRectForGlyph:[font glyphWithName:@"x"]]);
-
   FontRenderParamsQuery query;
   query.families.push_back(font_name_);
   query.pixel_size = font_size_;
   query.style = font_style_;
   query.weight = font_weight_;
   render_params_ = gfx::GetFontRenderParams(query, NULL);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // PlatformFont, public:
 
 // static
 PlatformFont* PlatformFont::CreateDefault() {
   return new PlatformFontMac;
 }
 
 // static
 PlatformFont* PlatformFont::CreateFromNativeFont(NativeFont native_font) {
   return new PlatformFontMac(native_font);
 }
 
 // static
 PlatformFont* PlatformFont::CreateFromNameAndSize(const std::string& font_name,
                                                   int font_size) {
   return new PlatformFontMac(font_name, font_size);
 }
 
 }  // namespace gfx