added improved scrolling behavior when changing item size in grid vie…

…w demo When changing the item size using the slider, the view controller scrolls to either the selected cell or to what ever cell was in the center of the view. This results in more consistent behavior closer to what IKImageBrowserView does.
jwilling · Dec 31, 2014 · 1a34179 · 1a34179
1 parent 0baf57e
commit 1a34179
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Showing 2 changed files with 65 additions and 3 deletions.
diff --git a/demo/JNWCollectionViewDemo/GridDemoViewController.h b/demo/JNWCollectionViewDemo/GridDemoViewController.h
@@ -9,7 +9,7 @@
 #import <Cocoa/Cocoa.h>
 #import <JNWCollectionView/JNWCollectionView.h>
 
-@interface GridDemoViewController : NSViewController <JNWCollectionViewDataSource, JNWCollectionViewGridLayoutDelegate>
+@interface GridDemoViewController : NSViewController
 
 @property (nonatomic, weak) IBOutlet JNWCollectionView *collectionView;
 

diff --git a/demo/JNWCollectionViewDemo/GridDemoViewController.m b/demo/JNWCollectionViewDemo/GridDemoViewController.m
@@ -9,9 +9,12 @@
 #import "GridDemoViewController.h"
 #import "GridCell.h"
 
-@interface GridDemoViewController()
+@interface GridDemoViewController() <JNWCollectionViewDataSource, JNWCollectionViewDelegate, JNWCollectionViewGridLayoutDelegate>
 @property (nonatomic, strong) NSArray *images;
 @property (nonatomic, strong) IBOutlet NSSlider *sizeSlider;
+@property (nonatomic, strong) NSIndexPath* activeScrollCellIndexPath;
+@property (nonatomic, strong) NSIndexPath* selectedIndexPath;
+@property (nonatomic) BOOL userScroll;
 @end
 
 static NSString * const identifier = @"CELL";
@@ -31,15 +34,63 @@ - (void)awakeFromNib {
 
 	self.collectionView.collectionViewLayout = gridLayout;
 	self.collectionView.dataSource = self;
+    self.collectionView.delegate = self;
 	self.collectionView.animatesSelection = NO; // (this is the default option)
 
 	[self.collectionView registerClass:GridCell.class forCellWithReuseIdentifier:identifier];
 
+    // this is not nice, better would be to have an NSScrollViewDelegate
+    [[NSNotificationCenter defaultCenter] addObserverForName:NSViewBoundsDidChangeNotification
+                                                      object:self.collectionView.clipView
+                                                       queue:[NSOperationQueue mainQueue]
+                                                  usingBlock:^(NSNotification *note) {
+                                                      // coalesced execution for better performance
+                                                      if (!self.userScroll) {
+                                                          [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(_updateScrollAnchor) object:nil];
+                                                          [self performSelector:@selector(_updateScrollAnchor) withObject:nil afterDelay:0.05];
+                                                      }
+    }];
+
 	[self.collectionView reloadData];
 }
 
-- (IBAction)updateSizeSliderValue:(id)sender {
+- (void) _updateScrollAnchor
+{
+    self.activeScrollCellIndexPath = [self _centerCellIndexPath];
+}
+
+- (NSIndexPath*) _centerCellIndexPath
+{
+    NSRect scrollRect = self.collectionView.documentVisibleRect;
+    // don't optimize scrolling when scrolled on top edge
+    if (NSMinY(scrollRect) <= 0) {
+        return nil;
+    }
+
+    NSArray* indexPathes = [self.collectionView.collectionViewLayout indexPathsForItemsInRect:scrollRect];
+    if ([indexPathes count] == 0) {
+        return nil;
+    }
+
+    NSUInteger middleIndex = [indexPathes count]/2;
+    return indexPathes[middleIndex];
+}
+
+- (IBAction)updateSizeSliderValue:(id)sender
+{
+    //NSRect visibleScrollRect = self.collectionView.documentVisibleRect;
 	[self.collectionView.collectionViewLayout invalidateLayout];
+
+    NSIndexPath* scrollIndexPath = (self.selectedIndexPath) ? self.selectedIndexPath : self.activeScrollCellIndexPath;
+    if (scrollIndexPath) {
+        self.userScroll = YES;
+        [self.collectionView scrollToItemAtIndexPath:scrollIndexPath
+                                    atScrollPosition:JNWCollectionViewScrollPositionMiddle
+                                            animated:NO];
+        self.userScroll = NO;
+    }
+
+
 }
 
 #pragma mark Data source
@@ -63,6 +114,17 @@ - (CGSize)sizeForItemInCollectionView:(JNWCollectionView *)collectionView {
 	return CGSizeMake(sizeSliderValue, sizeSliderValue);
 }
 
+- (void)collectionView:(JNWCollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
+{
+    self.selectedIndexPath = indexPath;
+}
+
+- (void)collectionView:(JNWCollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath
+{
+    if ([self.selectedIndexPath isEqualTo:indexPath]) {
+        self.selectedIndexPath = nil;
+    }
+}
 #pragma mark Image creation
 
 // To simulate at least something realistic, this just generates some randomly tinted images so that not every