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 200 matching lines @@
 
   // 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"]]);
+  // -[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.
+  NSAttributedString* as = [[[NSAttributedString alloc]

[tapted, 2017/05/01 06:48:05]

since this is a bit heavyweight, and barely anything calls
GetExpectedTextWidth(..), can we calculate this inside GetExpectedTextWidth?

the data members should have initializers in the .h file (some code above
becomes redundant), then GetExpectedTextWidth can check whether average_width_
has been changed from 0.

Also we like to avoid sending objects to the autorelease pool if it can be
avoided. We can use scoped_nsobject rather than autorelease to do that.

[Elly Fong-Jones, 2017/05/02 14:41:23]

Done.

+      initWithString:@"abcdefghijklmnopqrstuvwxyz"
+          attributes:@{NSFontAttributeName : font}] autorelease];
+  average_width_ = [as size].width / [as length];

[tapted, 2017/05/01 06:48:05]

rounding is occurring here, and GetExpectedTextWidth() takes a "length" argument
that will magnify the rounding. I think we should just make average_width_ a
float, and explicitly do ceil(average_width_ * length) in GetExpectedTextWidth()

[Elly Fong-Jones, 2017/05/02 14:41:23]

Done.

 
   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