Keeps image views in view hierarchy

Create the image views once per cell, and keep them in the content view even if
there is no image to show (like in the last row of the table).

This gains ~5 FPS on iPad 3 by avoiding expensive calls to -addSubview: and
-removeFromSuperview.

Author: William Green

FastImageCache/FastImageCacheDemo/Classes/FICDPhotosTableViewCell.m

@@ -19,7 +19,7 @@ @interface FICDPhotosTableViewCell () <UIGestureRecognizerDelegate> {
     NSArray *_photos;
     NSString *_imageFormatName;
 
-    NSMutableArray *_imageViews;
+    NSArray *_imageViews;
     UITapGestureRecognizer *_tapGestureRecognizer;
 }
 
@@ -39,42 +39,28 @@ @implementation FICDPhotosTableViewCell
 - (void)setPhotos:(NSArray *)photos {
     if (photos != _photos) {
         _photos = [photos copy];
-        
-        // Either create the image views for this cell or clear them out if they already exist
-        if (_imageViews == nil) {
-            NSInteger photosPerRow = [[self class] photosPerRow];
-            _imageViews = [[NSMutableArray alloc] initWithCapacity:photosPerRow];
-            
-            for (NSInteger i = 0; i < photosPerRow; i++) {
-                UIImageView *imageView = [[UIImageView alloc] init];
-                [imageView setContentMode:UIViewContentModeScaleAspectFill];
-                [_imageViews addObject:imageView];
-            }
-        } else {
-            for (UIImageView *imageView in _imageViews) {
-                [imageView setImage:nil];
-                [imageView removeFromSuperview];
-            }
-        }
-        
-        NSInteger photosCount = [_photos count];
-        for (NSInteger i = 0; i < photosCount; i++) {
-            FICDPhoto *photo = [_photos objectAtIndex:i];
+
+        for (NSInteger i = 0; i < [_imageViews count]; i++) {
             UIImageView *imageView = [_imageViews objectAtIndex:i];
-            
-            if (_usesImageTable) {
-                [[FICImageCache sharedImageCache] retrieveImageForEntity:photo withFormatName:_imageFormatName completionBlock:^(id<FICEntity> entity, NSString *formatName, UIImage *image) {
-                    // This completion block may be called much later. We should check to make sure this cell hasn't been reused for different photos before displaying the image that has loaded.
-                    if (photos == [self photos]) {
-                        [imageView setImage:image];
-                    }
-                }];
+
+            if (i < [_photos count]) {
+                FICDPhoto *photo = [_photos objectAtIndex:i];
+
+                if (_usesImageTable) {
+                    [[FICImageCache sharedImageCache] retrieveImageForEntity:photo withFormatName:_imageFormatName completionBlock:^(id<FICEntity> entity, NSString *formatName, UIImage *image) {
+                        // This completion block may be called much later. We should check to make sure this cell hasn't been reused for different photos before displaying the image that has loaded.
+                        if (photos == [self photos]) {
+                            [imageView setImage:image];
+                        }
+                    }];
+                } else {
+                    [imageView setImage:[photo thumbnailImage]];
+                }
             } else {
-                [imageView setImage:[photo thumbnailImage]];
+                // Last row might not be full
+                [imageView setImage:nil];
             }
         }
-        
-        [self setNeedsLayout];
     }
 }
 
@@ -117,6 +103,18 @@ - (id)initWithStyle:(UITableViewCellStyle)style reuseIdentifier:(NSString *)reus
     if (self != nil) {
         _tapGestureRecognizer = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(_tapGestureRecognizerStateDidChange)];
         [self addGestureRecognizer:_tapGestureRecognizer];
+
+        NSInteger photosPerRow = [[self class] photosPerRow];
+        NSMutableArray *imageViews = [[NSMutableArray alloc] initWithCapacity:photosPerRow];
+
+        for (NSInteger i = 0; i < photosPerRow; i++) {
+            UIImageView *imageView = [[UIImageView alloc] init];
+            [imageView setContentMode:UIViewContentModeScaleAspectFill];
+            [imageViews addObject:imageView];
+            [self.contentView addSubview:imageView];
+        }
+
+        _imageViews = [imageViews copy];
     }
 
     return self;
@@ -139,14 +137,12 @@ - (void)layoutSubviews {
     CGFloat outerPadding = [[self class] outerPadding];
 
     CGRect imageViewFrame = CGRectMake(outerPadding, outerPadding, FICDPhotoSquareImageSize.width, FICDPhotoSquareImageSize.height);
-    
-    UIView *contentView = [self contentView];
+
     NSInteger count = [_photos count];
 
     for (NSInteger i = 0; i < count; i++) {
         UIImageView *imageView = [_imageViews objectAtIndex:i];
         [imageView setFrame:imageViewFrame];
-        [contentView addSubview:imageView];
 
         imageViewFrame.origin.x += imageViewFrame.size.width + innerPadding;
     }

FastImageCache/FastImageCacheDemo/Classes/FICDViewController.m

@@ -95,6 +95,7 @@ - (void)loadView {
         [_tableView setDelegate:self];
         [_tableView setAutoresizingMask:(UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight)];
         [_tableView setSeparatorStyle:UITableViewCellSeparatorStyleNone];
+        [_tableView registerClass:[FICDPhotosTableViewCell class] forCellReuseIdentifier:[FICDPhotosTableViewCell reuseIdentifier]];
 
         CGFloat tableViewCellOuterPadding = [FICDPhotosTableViewCell outerPadding];
         [_tableView setContentInset:UIEdgeInsetsMake(0, 0, tableViewCellOuterPadding, 0)];
@@ -461,13 +462,9 @@ - (NSInteger)tableView:(UITableView*)tableView numberOfRowsInSection:(NSInteger)
 - (UITableViewCell*)tableView:(UITableView*)table cellForRowAtIndexPath:(NSIndexPath*)indexPath {
     NSString *reuseIdentifier = [FICDPhotosTableViewCell reuseIdentifier];
 
-    FICDPhotosTableViewCell *tableViewCell = (FICDPhotosTableViewCell *)[table dequeueReusableCellWithIdentifier:reuseIdentifier];
-    if (tableViewCell == nil) {
-        tableViewCell = [[FICDPhotosTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:reuseIdentifier];
-        [tableViewCell setBackgroundColor:[table backgroundColor]];
-        [tableViewCell setSelectionStyle:UITableViewCellSelectionStyleNone];
-    }
-    
+    FICDPhotosTableViewCell *tableViewCell = (FICDPhotosTableViewCell *)[table dequeueReusableCellWithIdentifier:reuseIdentifier forIndexPath:indexPath];
+    tableViewCell.selectionStyle = UITableViewCellSeparatorStyleNone;
+
     [tableViewCell setDelegate:self];
     [tableViewCell setImageFormatName:_imageFormatName];