Upstream Chrome on iOS source code [4/11].

ios/chrome/browser/ui/contextual_search/contextual_search_mask_view.mm

+// Copyright 2015 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 "ios/chrome/browser/ui/contextual_search/contextual_search_mask_view.h"
+
+#include "base/logging.h"
+#import "ios/chrome/browser/ui/contextual_search/contextual_search_panel_view.h"
+#import "ios/chrome/browser/ui/uikit_ui_util.h"
+
+// Linearly map |delta| in the range [0, 1] to a value in [min, max].
+#define LERP(min, max, delta) (min * (1 - delta) + max * delta)
+
+namespace {
+const CGFloat kPhoneMaskLimit = 1.0;
+const CGFloat kPadMaskLimit = 0.8;
+}
+
+@implementation ContextualSearchMaskView {
+  CGFloat _maskLimit;
+}
+
+- (instancetype)init {
+  if ((self = [super initWithFrame:CGRectZero])) {
+    self.userInteractionEnabled = NO;
+    self.translatesAutoresizingMaskIntoConstraints = NO;
+    self.accessibilityIdentifier = @"contextualSearchMask";
+    self.alpha = 0.0;
+    self.backgroundColor = [UIColor blackColor];
+    _maskLimit = IsIPadIdiom() ? kPadMaskLimit : kPhoneMaskLimit;
+  }
+  return self;
+}
+
+- (instancetype)initWithCoder:(NSCoder*)aDecoder NS_UNAVAILABLE {
+  NOTREACHED();
+  return nil;
+}
+
+- (instancetype)initWithFrame:(CGRect)frame NS_UNAVAILABLE {
+  NOTREACHED();
+  return nil;
+}
+
+#pragma mark - UIView methods.
+
+- (void)updateConstraints {
+  DCHECK(self.superview);
+  NSArray* constraints = @[ @"V:|[mask]|", @"H:|[mask]|" ];
+  ApplyVisualConstraints(constraints, @{ @"mask" : self }, self.superview);
+  [super updateConstraints];
+}
+
+#pragma mark - ContextualSearchPanelMotionObserver methods
+
+- (void)panel:(ContextualSearchPanelView*)panel
+    didMoveWithMotion:(ContextualSearch::PanelMotion)motion {
+  CGFloat ratio;
+  if (motion.state < ContextualSearch::PEEKING) {
+    ratio = 0;
+  } else if (motion.state == ContextualSearch::COVERING) {
+    ratio = 1;
+  } else {
+    ratio = [panel.configuration gradationToState:ContextualSearch::COVERING
+                                        fromState:ContextualSearch::PEEKING
+                                       atPosition:motion.position];
+  }
+
+  ratio = LERP(0, _maskLimit, ratio);
+
+  self.alpha = ratio * ratio;
+}
+
+- (void)panelWillPromote:(ContextualSearchPanelView*)panel {
+  [panel removeMotionObserver:self];
+}
+
+@end