Correctly respond to size class changes in Download Manager.

Old code used IsCompactWidth() function inside
viewWillTransitionToSize: callback to check the current size
class. This is incorrect and would return previous size class
instead of new size class.

New code uses willTransitionToTraitCollection: instead, which
provides correct new size class.

Also download bar width for regular size class is now anchored to
superview width with 0.6 multiplier. This is because anchoring to
constant (what old code tried to do) would require more complex
logic and is not really much better than anchoring to multiplier.

Bug: 823698,791806
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ide994e3d76b773d20d462d782f370a0491009916
Reviewed-on: https://chromium-review.googlesource.com/973464Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544914}

ios/chrome/browser/ui/download/download_manager_view_controller.mm

@@ -10,7 +10,6 @@
 #include "ios/chrome/browser/ui/download/download_manager_animation_constants.h"
 #import "ios/chrome/browser/ui/download/download_manager_state_view.h"
 #import "ios/chrome/browser/ui/download/radial_progress_view.h"
-#import "ios/chrome/browser/ui/uikit_ui_util.h"
 #import "ios/chrome/browser/ui/util/named_guide.h"
 #include "ios/chrome/grit/ios_strings.h"
 #include "ios/public/provider/chrome/browser/chrome_browser_provider.h"
@@ -31,10 +30,6 @@ GuideName* const kActionButtonGuide = @"kDownloadManagerActionButtonGuide";
 // button). Defines the minimal distance between elements.
 const CGFloat kElementMargin = 16;
 
-// Ideal download UI width if user interface has regular width. For compact
-// user interface width download UI will take full width of the superview.
-const CGFloat kPreferredWidthForRegularUIWidth = 500;
-
 // Returns formatted size string.
 NSString* GetSizeString(long long size_in_bytes) {
   return [NSByteCountFormatter
@@ -86,9 +81,9 @@ NSString* GetSizeString(long long size_in_bytes) {
 // deactivating the old constraint.
 @property(nonatomic) NSLayoutConstraint* bottomConstraint;
 
-// Represents constraint for self.view.widthAnchor, which can either be
-// constrained to superview's width or to kPreferredWidthForRegularUIWidth
-// constant. Stored in a property to allow deactivating the old constraint.
+// Represents constraint for self.view.widthAnchor, which is anchored to
+// superview with different multipliers depending on size class. Stored in a
+// property to allow deactivating the old constraint.
 @property(nonatomic) NSLayoutConstraint* widthConstraint;
 
 @end
@@ -135,7 +130,7 @@ NSString* GetSizeString(long long size_in_bytes) {
   // self.view constraints.
   UIView* view = self.view;
   [self updateBottomConstraints];
-  [self updateWidthConstraints];
+  [self updateWidthConstraintsForTraitCollection:self.traitCollection];
 
   // shadow constraints.
   UIImageView* shadow = self.shadow;
@@ -275,10 +270,14 @@ NSString* GetSizeString(long long size_in_bytes) {
   ]];
 }
 
-- (void)viewWillTransitionToSize:(CGSize)size
-       withTransitionCoordinator:
-           (id<UIViewControllerTransitionCoordinator>)coordinator {
-  [self updateWidthConstraints];
+- (void)willTransitionToTraitCollection:(UITraitCollection*)newCollection
+              withTransitionCoordinator:
+                  (id<UIViewControllerTransitionCoordinator>)coordinator {
+  void (^alongsideBlock)(id<UIViewControllerTransitionCoordinatorContext>) =
+      ^(id<UIViewControllerTransitionCoordinatorContext> context) {
+        [self updateWidthConstraintsForTraitCollection:newCollection];
+      };
+  [coordinator animateAlongsideTransition:alongsideBlock completion:nil];
 }
 
 #pragma mark - Public
@@ -565,29 +564,23 @@ NSString* GetSizeString(long long size_in_bytes) {
   self.bottomConstraint.active = YES;
 }
 
-// Updates and activates self.widthConstraint. self.widthConstraint
-// constraints self.view.widthAnchor to superview's width if user interface has
-// compact width or there is no room to use kPreferredWidthForRegularUIWidth.
-// Otherwise self.widthConstraint constraints self.view.widthAnchor to
-// kPreferredWidthForRegularUIWidth constant.
-- (void)updateWidthConstraints {
+// Updates and activates self.widthConstraint anchored to superview width. Uses
+// smaller multiplier for regular ui size class, because otherwise the bar will
+// be too wide.
+- (void)updateWidthConstraintsForTraitCollection:
+    (UITraitCollection*)traitCollection {
   self.widthConstraint.active = NO;
 
-  NSLayoutDimension* firstAnchor = self.view.widthAnchor;
-  CGFloat superviewWidth = self.view.superview.frame.size.width;
-  if (superviewWidth < kPreferredWidthForRegularUIWidth || IsCompactWidth()) {
-    // Superview is too narrow to accomodate kPreferredWidthForRegularUIWidth or
-    // user interface has compant width. In this case download view should
-    // occupy all superview width.
-    NSLayoutDimension* secondAnchor = self.view.superview.widthAnchor;
-    self.widthConstraint = [firstAnchor constraintEqualToAnchor:secondAnchor];
-  } else {
-    // Superview is wide enough to accomodate kPreferredWidthForRegularUIWidth
-    // or user interface has regular width. In this case download view width
-    // should be constant. Otherwise the UI will jhave too much blank space.
-    self.widthConstraint = [firstAnchor
-        constraintEqualToConstant:kPreferredWidthForRegularUIWidth];
-  }
+  // With regular horizontal size class, UI is too wide to take the full width,
+  // because there will be a lot of blank space.
+  CGFloat multiplier =
+      traitCollection.horizontalSizeClass == UIUserInterfaceSizeClassRegular
+          ? 0.6
+          : 1.0;
+
+  self.widthConstraint = [self.view.widthAnchor
+      constraintEqualToAnchor:self.view.superview.widthAnchor
+                   multiplier:multiplier];
 
   self.widthConstraint.active = YES;
 }