iphone - 异步文档中的图像

Question

我需要从 NSDocumentDirectory 异步读取图像到多个 uiimageview，这样它就不会阻塞 UI。我知道我可以在后台使用执行选择器来加载 uiimage，但是我如何才能将它与动态 uiimageview 相关联？

Answer 1

一种方便的方法是使用 block ，例如:

[self loadFullImageAt:imagePath completion:^(UIIMage * image){
    self.imageView.image = image;
}];

将图像作为数据加载的位置(因为 UIImage 否则会延迟加载图像数据 - 当您首次访问它时)。在后台线程中解压缩图像也是一个好主意，这样当我们第一次使用图像时主线程就不必这样做。

- (void)loadFullImageAt:(NSString *)imageFilePath completion:(MBLoaderCompletion)completion {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        NSData *imageData = [NSData dataWithContentsOfFile:imageFilePath];
        UIImage *image = nil;
        if (imageData) {
            image = [[[UIImage alloc] initWithData:imageData] decodedImage];
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            completion(image);
        });
    });
}

回调定义为:

typedef void (^MBLoaderCompletion)(UIImage *image);

下面是一个实现解压代码的UIImage类:

UIIMage+Decode.h

#import <UIKit/UIKit.h>


@interface UIImage (Decode)

- (UIImage *)decodedImage;

@end

UIIMage+Decode.m

#import "UIImage+Decode.h"


@implementation UIImage (Decode)

- (UIImage *)decodedImage {
    CGImageRef imageRef = self.CGImage;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(NULL,
                                                 CGImageGetWidth(imageRef),
                                                 CGImageGetHeight(imageRef),
                                                 8,
                                                 // Just always return width * 4 will be enough
                                                 CGImageGetWidth(imageRef) * 4,
                                                 // System only supports RGB, set explicitly
                                                 colorSpace,
                                                 // Makes system don't need to do extra conversion when displayed.
                                                 kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);
    CGColorSpaceRelease(colorSpace);
    if (!context) return nil;

    CGRect rect = (CGRect){CGPointZero,{CGImageGetWidth(imageRef), CGImageGetHeight(imageRef)}};
    CGContextDrawImage(context, rect, imageRef);
    CGImageRef decompressedImageRef = CGBitmapContextCreateImage(context);
    CGContextRelease(context);

    UIImage *decompressedImage = [[UIImage alloc] initWithCGImage:decompressedImageRef scale:self.scale orientation:self.imageOrientation];
    CGImageRelease(decompressedImageRef);
    return decompressedImage;
}

@end

此处提供的示例代码假设我们使用的是 ARC。