When retrieving an image asynchronously, preheat the data so that the data is paged in and ready to go when the image is eventually drawn.

Author: Mallory Paine

FastImageCache/FICImageCache.m

@@ -163,7 +163,7 @@ - (BOOL)_retrieveImageForEntity:(id <FICEntity>)entity withFormatName:(NSString
         imageExists = YES;
 
         dispatch_async([FICImageCache dispatchQueue], ^{
-            UIImage *image = [imageTable newImageForEntityUUID:entityUUID sourceImageUUID:sourceImageUUID];
+            UIImage *image = [imageTable newImageForEntityUUID:entityUUID sourceImageUUID:sourceImageUUID preheatData:YES];
 
             if (completionBlock != nil) {
                 dispatch_async(dispatch_get_main_queue(), ^{
@@ -172,7 +172,7 @@ - (BOOL)_retrieveImageForEntity:(id <FICEntity>)entity withFormatName:(NSString
             }
         });
     } else {
-        UIImage *image = [imageTable newImageForEntityUUID:entityUUID sourceImageUUID:sourceImageUUID];
+        UIImage *image = [imageTable newImageForEntityUUID:entityUUID sourceImageUUID:sourceImageUUID preheatData:NO];
         imageExists = image != nil;
 
         dispatch_block_t completionBlockCallingBlock = ^{
@@ -338,7 +338,7 @@ - (void)_processImage:(UIImage *)image forEntity:(id <FICEntity>)entity imageTab
         dispatch_async([FICImageCache dispatchQueue], ^{
             [imageTable setEntryForEntityUUID:entityUUID sourceImageUUID:sourceImageUUID imageDrawingBlock:imageDrawingBlock];
 
-            UIImage *resultImage = [imageTable newImageForEntityUUID:entityUUID sourceImageUUID:sourceImageUUID];
+            UIImage *resultImage = [imageTable newImageForEntityUUID:entityUUID sourceImageUUID:sourceImageUUID preheatData:NO];
 
             if (completionBlocks != nil) {
                 dispatch_async(dispatch_get_main_queue(), ^{

FastImageCache/FICImageTable.h

@@ -119,7 +119,7 @@ extern NSString *const FICImageTableScreenScaleKey;
  @note If either the entity UUID or the source image UUID doesn't match the corresponding UUIDs in the entry data, then something has changed. The entry data is deleted for the
  provided entity UUID, and `nil` is returned.
  */
-- (UIImage *)newImageForEntityUUID:(NSString *)entityUUID sourceImageUUID:(NSString *)sourceImageUUID;
+- (UIImage *)newImageForEntityUUID:(NSString *)entityUUID sourceImageUUID:(NSString *)sourceImageUUID preheatData:(BOOL)preheatData;
 
 /**
  Deletes image entry data in the image table.

FastImageCache/FICImageTable.m

@@ -299,7 +299,7 @@ - (void)setEntryForEntityUUID:(NSString *)entityUUID sourceImageUUID:(NSString *
     }
 }
 
-- (UIImage *)newImageForEntityUUID:(NSString *)entityUUID sourceImageUUID:(NSString *)sourceImageUUID {
+- (UIImage *)newImageForEntityUUID:(NSString *)entityUUID sourceImageUUID:(NSString *)sourceImageUUID preheatData:(BOOL)preheatData {
     UIImage *image = nil;
 
     if (entityUUID != nil && sourceImageUUID != nil) {
@@ -345,6 +345,10 @@ - (UIImage *)newImageForEntityUUID:(NSString *)entityUUID sourceImageUUID:(NSStr
                     NSString *message = [NSString stringWithFormat:@"*** FIC Error: %s could not create a new CGImageRef for entity UUID %@.", __PRETTY_FUNCTION__, entityUUID];
                     [[FICImageCache sharedImageCache] _logMessage:message];
                 }
+                
+                if (image != nil && preheatData) {
+                    [entryData preheat];
+                }
             }
         }
 

FastImageCache/FICImageTableEntry.h

@@ -54,6 +54,8 @@ typedef struct {
 
 @property (nonatomic, readonly) FICImageTableChunk *imageTableChunk;
 
+- (void)preheat;
+
 ///----------------------------------------
 /// @name Initializing an Image Table Entry
 ///----------------------------------------

FastImageCache/FICImageTableEntry.m

@@ -108,4 +108,15 @@ - (void)flush {
     }
 }
 
+- (void)preheat {
+    int pageSize = [FICImageTable pageSize];
+    void *bytes = [self bytes];
+    NSUInteger length = [self length];
+    
+    // Read a byte off of each VM page to force the kernel to page in the data
+    for (NSUInteger i = 0; i < length; i += pageSize) {
+        *((volatile uint8_t *)bytes + i);
+    }
+}
+
 @end