Fix tabs are not rendered properly when restore tabs after crash on iPad

ios/chrome/browser/ui/tabs/tab_strip_controller.mm

 // Copyright 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.
 
 #import "ios/chrome/browser/ui/tabs/tab_strip_controller.h"
 #import "ios/chrome/browser/ui/tabs/tab_strip_controller_private.h"
 
 #include <cmath>
 #include <vector>
 
@@ skipping 1245 matching lines @@
                               isNewTab:(BOOL)isNewTab {
   DCHECK_NE(NSNotFound, static_cast<NSInteger>(tabIndex));
 
   if (experimental_flags::IsTabStripAutoScrollNewTabsEnabled() && isNewTab) {
     // The following code calculates the amount of scroll needed to make
     // |tabIndex| visible in the "virtual" coordinate system, where root is x=0
     // and it contains all the tabs laid out as if the tabstrip was infinitely
     // long. The amount of scroll is calculated as a desired length that it is
     // just large enough to contain all the tabs to the left of |tabIndex|, with
     // the standard overlap.
+    if (tabIndex == [_tabArray count] - 1) {

[rohitrao (ping after 24h), 2017/01/19 01:44:22]

If crash restore adds 100 tabs to the tabstrip, will we shift every tab view 100
times?  If you try with a smaller number of tabs, say 20, is there a noticeable
delay in restoring?

[liaoyuke, 2017/01/20 00:55:51]

Yes, the complexity is n^2, every tab view will be shifted 100 times.

I didn't see any noticeable delay for 20 tabs. While for 100 tabs, there is a
noticeable delay both with or without the change, maybe a little bit longer with
the change.

+      const CGFloat tabStripAvailableSpace =
+          _tabStripView.frame.size.width - _tabStripView.contentInset.right;
+      CGPoint oldOffset = [_tabStripView contentOffset];
+      if (_tabStripView.contentSize.width > tabStripAvailableSpace) {
+        CGFloat scrollToPoint =
+            _tabStripView.contentSize.width - tabStripAvailableSpace;
+        [_tabStripView setContentOffset:CGPointMake(scrollToPoint, 0)];
+      }
+
+      // To handle content offset change without making views appear to jump,
+      // shift all of the subviews by an amount equal to the size change.  This
+      // effectively places the subviews back where they were before the change,
+      // in terms of screen coordinates.
+      CGFloat dx = [_tabStripView contentOffset].x - oldOffset.x;
+      for (UIView* view in [_tabStripView subviews]) {

[rohitrao (ping after 24h), 2017/01/19 01:44:22]

Is there any way to share this code with the other callsite at line 1200 above?

[liaoyuke, 2017/01/20 00:55:51]

Done.

+        CGRect frame = [view frame];
+        frame.origin.x += dx;
+        [view setFrame:frame];
+        _targetFrames.AddFrame(view, frame);
+      }
+      return;

[rohitrao (ping after 24h), 2017/01/19 01:44:22]

This function now has two early returns, which makes me think there's probably a
better way to organize the code.  Should we split this up into three separate
functions, and call one of the three based on which case we are in?

[liaoyuke, 2017/01/20 00:55:51]

From my point of view, I think it would be non-trivial to come up with good
names for those three functions, I tried to think about some names, but I found
them too confusing. Please let me know if you have some good names in mind.

I agree that we should have a better way to organize the code, so I tentatively
moved the new code into a separate function while left the old code unchanged.
Once the old code is deleted, we can move it back. Please let me know how you
think about this.

+    }
+
     NSUInteger numNonClosingTabsToLeft = 0;
     NSUInteger i = 0;
     for (TabView* tab in _tabArray.get()) {
       if ([_closingTabs containsObject:tab])
         ++i;
 
       if (i == tabIndex)
         break;
 
       ++numNonClosingTabsToLeft;
@@ skipping 413 matching lines @@
 
 @implementation TabStripController (Testing)
 
 - (TabView*)existingTabViewForTab:(Tab*)tab {
   NSUInteger tabIndex = [_tabModel indexOfTab:tab];
   NSUInteger tabViewIndex = [self indexForModelIndex:tabIndex];
   return [_tabArray objectAtIndex:tabViewIndex];
 }
 
 @end