views: support disabled color for CTA buttons

ui/views/controls/button/md_text_button.cc

Index: ui/views/controls/button/md_text_button.cc
diff --git a/ui/views/controls/button/md_text_button.cc b/ui/views/controls/button/md_text_button.cc
index 44d111c80dff5219baba969b59ba615dc8a2cbe5..8f5e960fcf0ea44b91f125e84fd85be5f9288230 100644
--- a/ui/views/controls/button/md_text_button.cc
+++ b/ui/views/controls/button/md_text_button.cc
@@ -287,16 +287,24 @@ void MdTextButton::UpdateColors() {
   if (!explicitly_set_normal_color())
     LabelButton::SetEnabledTextColors(theme->GetSystemColor(fg_color_id));
 
+  // CTA buttons keep their enabled text color; disabled state is conveyed by
+  // shading the background instead.
+  if (is_cta_)
+    SetTextColor(STATE_DISABLED, theme->GetSystemColor(fg_color_id));
+
   SkColor text_color = label()->enabled_color();
-  SkColor bg_color =
-      bg_color_override_
-          ? *bg_color_override_
-          : is_cta_
-                ? theme->GetSystemColor(
-                      ui::NativeTheme::kColorId_CallToActionColor)
-                : is_default()
-                      ? color_utils::BlendTowardOppositeLuma(text_color, 0xD8)
-                      : SK_ColorTRANSPARENT;
+  SkColor bg_color = SK_ColorTRANSPARENT;
+
+  if (bg_color_override_) {
+    bg_color = *bg_color_override_;
+  } else if (is_cta_) {
+    bg_color = theme->GetSystemColor(
+        ui::NativeTheme::kColorId_CallToActionColor);
+    if (state() == STATE_DISABLED)
+      bg_color = SkColorSetA(bg_color, 0x61);

[Evan Stade, 2016/09/07 17:14:33]

I know it's expressed as an alpha value in the spec, but I'm thinking the blend
towards opposite luma (like below) is somewhat more correct here than setting an
alpha because we probably don't want the color to change based on the
background. For example, if we had a disabled CTA button on an infobar with a
yellow bg, would we want the button to be somewhat yellow? probably not.

[Elly Fong-Jones, 2016/09/07 18:01:45]

Done.

+  } else if (is_default()) {
+    bg_color = color_utils::BlendTowardOppositeLuma(text_color, 0xD8);
+  }
 
   bg_color = PlatformStyle::BackgroundColorForMdButton(bg_color, state());

[Evan Stade, 2016/09/07 17:14:33]

can we make it a todo to get rid of this? Color logic is currently spread across
three places: this file, native theme, and platform style :<

perhaps:

if (pressed()) {
  SkColor shade_color = theme->GetSystemColor(kColorId_ButtonPressedShade);
  bg_color = color_utils::Blend(shade_color, bg_color);
}

(note that AlphaBlend doesn't do quite what we want here so we have to invent a
new utility function. I'm pretty sure I've wanted this function in the past but
worked around its non-existence so this won't be the only time we'd use it.)

[Elly Fong-Jones, 2016/09/07 17:43:50]

Hm - I like this idea, but what would color_utils::Blend() do? As I understand
it there are many different ways to blend two colors together.

Also, I think for this specific use, kColorId_ButtonPressedShade would be
#FFFFFF with an alpha of #08, and we need a function that would:

1) Extract the alpha, and set the shade's alpha to #FF
2) Set the bg color's alpha to #FF
3) Alpha blend the two with the extracted alpha from step 1

Is that the kind of thing you're thinking of? I don't understand color that
well.

[Elly Fong-Jones, 2016/09/07 18:01:45]

I ended up adding a TODO for this.