我这里遇到了一个问题。在我的应用程序中，有一个视图显示文档缩略图。由于加载缩略图会使主线程变慢，因此我寻找解决方法，最终采用了NSOperation来创建缩略图。
我正在显示一个带有空缩略图框架和相应活动指示器的视图，告诉用户“等待中，请稍候”。但是这个过程太长了，以至于我考虑加上电梯音乐使等待更愉快x_X。
我设置了一个NSOperationQueue，最大并发操作数为10。设置这个值在一定程度上有助于加载，但只是微不足道的帮助。加载单个缩略图仍然需要大约6秒钟，而奇怪的是，加载10张缩略图也需要同样长的时间。以下是操作本身的代码。
@class ThumbnailView;
@protocol LoadThumbnailOperationDelegate;
@interface LoadThumbnailOperation : NSOperation {
NSString *key;
id<LoadThumbnailOperationDelegate> delegate;
@private
CGSize _size;
CGPDFDocumentRef _docRef;
UIImage * _image;
NSInteger _page;
}
@property (nonatomic,retain) NSString * key;
@property (nonatomic,assign) id<LoadThumbnailOperationDelegate>delegate;
-(id)initWithPage:(NSInteger)page operationKey:(NSString *)opKey fromDocRef:(CGPDFDocumentRef)docRef size:(CGSize)size delegate:(id<LoadThumbnailOperationDelegate>)aDelegate;
-(NSInteger)getPage ;
@protocol LoadThumbnailOperationDelegate <NSObject>
-(void)operation:(LoadThumbnailOperation*)operation finishedLoadingThumbnail:(UIImage*)image;
@end
@interface LoadThumbnailOperation (private)
-(UIImage*)makeThumbnailForPage:(NSInteger)page;
@end
@implementation LoadThumbnailOperation
@synthesize key;
@synthesize delegate;
-(id)initWithPage:(NSInteger)page operationKey:(NSString *)opKey fromDocRef:(CGPDFDocumentRef)docRef size:(CGSize)size delegate:(id<LoadThumbnailOperationDelegate>)aDelegate{
self = [super init];
if (self) {
self.key = opKey;
_docRef = docRef;
CGPDFDocumentRetain(_docRef);
_size = size;
_page = page;
self.delegate = delegate;
}
return self;
}
-(void)main {
#if DEBUG
NSLog( @"LoadThumbnailOperaiton.m -> main key:%@",key);
#endif
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc]init];
if (![self isCancelled]) {
_image = [self makeThumbnailForPage:_page];
[_image retain];
}
if(![self isCancelled]){
if ([delegate respondsToSelector:@selector(operation:finishedLoadingThumbnail:)]) {
[delegate operation:self finishedLoadingThumbnail:_image];
}
}
[pool release];
}
-(void)dealloc {
[key release];
CGPDFDocumentRelease(_docRef);
[_image release];
[super dealloc];
}
#pragma mark -
#pragma mark graphics
-(UIImage*) makeThumbnailForPage :(NSInteger) page {
#if DEBUG
NSLog( @"LoadThumbnailOperaiton.m -> makeThumbnailForPage:%d",page);
#endif
CGPDFPageRef pdfPage = CGPDFDocumentGetPage(_docRef, page);
if (pdfPage !=NULL){
CGPDFPageRetain(pdfPage);
}else {
NSAssert (pdfPage==NULL,@"pdf page NULL");
}
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(NULL,
_size.width,
_size.height,
8, /* bits per component*/
_size.width * 4, /* bytes per row */
colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
CGContextClipToRect(context, CGRectMake(0, 0, _size.width,_size.height));
CGRect pdfPageRect = CGPDFPageGetBoxRect(pdfPage, kCGPDFMediaBox);
CGRect contextRect = CGContextGetClipBoundingBox(context);
CGAffineTransform transform =aspectFit(pdfPageRect, contextRect);
CGContextConcatCTM(context, transform);
CGContextDrawPDFPage(context, pdfPage);
/*
create bitmap context
*/
CGImageRef image = CGBitmapContextCreateImage(context);
CGContextRelease(context);
UIImage *uiImage = [[UIImage alloc]initWithCGImage:image];
// clean up
[uiImage autorelease];
CGImageRelease(image);
CGPDFPageRelease(pdfPage);
return uiImage;
}
-(NSInteger)getPage {
return _page;
}
@end
这是视图控制器上添加已加载的图片的委托方法。
注意：此应用仅适用于iPad。
感谢您的帮助。