iOS 图片压缩

发帖的时候需要用到从相册中选取图片然后上传，由于每次上传图片的最大数量为9张，所以需要对图片进行压缩。开始时用了以前经常用的压缩的方法：

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1. //压缩图片质量  
2. +(UIImage *)reduceImage:(UIImage *)image percent:(float)percent  
3. {  
4.     NSData *imageData = UIImageJPEGRepresentation(image, percent);  
5.     UIImage *newImage = [UIImage imageWithData:imageData];  
6.     return newImage;  
7. }  
8. //压缩图片尺寸  
9. + (UIImage*)imageWithImageSimple:(UIImage*)image scaledToSize:(CGSize)newSize  
10. {  
11.     // Create a graphics image context  
12.     UIGraphicsBeginImageContext(newSize);  
13.     // new size  
14.     [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];  
15.     // Get the new image from the context  
16.     UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();  
17.       
18.     // End the context  
19.     UIGraphicsEndImageContext();  
20.     // Return the new image.  
21.     return newImage;  
22. }  

//压缩图片质量
+(UIImage *)reduceImage:(UIImage *)image percent:(float)percent
{
    NSData *imageData = UIImageJPEGRepresentation(image, percent);
    UIImage *newImage = [UIImage imageWithData:imageData];
    return newImage;
}
//压缩图片尺寸
+ (UIImage*)imageWithImageSimple:(UIImage*)image scaledToSize:(CGSize)newSize
{
    // Create a graphics image context
    UIGraphicsBeginImageContext(newSize);
    // new size
    [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
    // Get the new image from the context
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
    
    // End the context
    UIGraphicsEndImageContext();
    // Return the new image.
    return newImage;
}

上面的方法比较常见，可是需要加载到内存中来处理图片，当图片数量多了的时候就会收到内存警告，程序崩溃。研究半天终于在一篇博客中找到了解决方法：

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1. static size_t getAssetBytesCallback(voidvoid *info, voidvoid *buffer, off_t position, size_t count) {  
2.     ALAssetRepresentation *rep = (__bridge id)info;  
3.       
4.     NSError *error = nil;  
5.     size_t countRead = [rep getBytes:(uint8_t *)buffer fromOffset:position length:count error:&error];  
6.       
7.     if (countRead == 0 && error) {  
8.         // We have no way of passing this info back to the caller, so we log it, at least.  
9.         NDDebug(@"thumbnailForAsset:maxPixelSize: got an error reading an asset: %@", error);  
10.     }  
11.       
12.     return countRead;  
13. }  
14.   
15. static void releaseAssetCallback(voidvoid *info) {  
16.     // The info here is an ALAssetRepresentation which we CFRetain in thumbnailForAsset:maxPixelSize:.  
17.     // This release balances that retain.  
18.     CFRelease(info);  
19. }  
20.   
21. // Returns a UIImage for the given asset, with size length at most the passed size.  
22. // The resulting UIImage will be already rotated to UIImageOrientationUp, so its CGImageRef  
23. // can be used directly without additional rotation handling.  
24. // This is done synchronously, so you should call this method on a background queue/thread.  
25. - (UIImage *)thumbnailForAsset:(ALAsset *)asset maxPixelSize:(NSUInteger)size {  
26.     NSParameterAssert(asset != nil);  
27.     NSParameterAssert(size > 0);  
28.       
29.     ALAssetRepresentation *rep = [asset defaultRepresentation];  
30.       
31.     CGDataProviderDirectCallbacks callbacks = {  
32.         .version = 0,  
33.         .getBytePointer = NULL,  
34.         .releaseBytePointer = NULL,  
35.         .getBytesAtPosition = getAssetBytesCallback,  
36.         .releaseInfo = releaseAssetCallback,  
37.     };  
38.       
39.     CGDataProviderRef provider = CGDataProviderCreateDirect((voidvoid *)CFBridgingRetain(rep), [rep size], &callbacks);  
40.     CGImageSourceRef source = CGImageSourceCreateWithDataProvider(provider, NULL);  
41.       
42.     CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(source, 0, (__bridge CFDictionaryRef) @{  
43.                                                                                                       (NSString *)kCGImageSourceCreateThumbnailFromImageAlways : @YES,  
44.                                                                                                       (NSString *)kCGImageSourceThumbnailMaxPixelSize : [NSNumber numberWithInt:size],  
45.                                                                                                       (NSString *)kCGImageSourceCreateThumbnailWithTransform : @YES,  
46.                                                                                                       });  
47.     CFRelease(source);  
48.     CFRelease(provider);  
49.       
50.     if (!imageRef) {  
51.         return nil;  
52.     }  
53.       
54.     UIImage *toReturn = [UIImage imageWithCGImage:imageRef];  
55.       
56.     CFRelease(imageRef);  
57.       
58.     return toReturn;  
59. }  

static size_t getAssetBytesCallback(void *info, void *buffer, off_t position, size_t count) {
    ALAssetRepresentation *rep = (__bridge id)info;
    
    NSError *error = nil;
    size_t countRead = [rep getBytes:(uint8_t *)buffer fromOffset:position length:count error:&error];
    
    if (countRead == 0 && error) {
        // We have no way of passing this info back to the caller, so we log it, at least.
        NDDebug(@"thumbnailForAsset:maxPixelSize: got an error reading an asset: %@", error);
    }
    
    return countRead;
}

static void releaseAssetCallback(void *info) {
    // The info here is an ALAssetRepresentation which we CFRetain in thumbnailForAsset:maxPixelSize:.
    // This release balances that retain.
    CFRelease(info);
}

// Returns a UIImage for the given asset, with size length at most the passed size.
// The resulting UIImage will be already rotated to UIImageOrientationUp, so its CGImageRef
// can be used directly without additional rotation handling.
// This is done synchronously, so you should call this method on a background queue/thread.
- (UIImage *)thumbnailForAsset:(ALAsset *)asset maxPixelSize:(NSUInteger)size {
    NSParameterAssert(asset != nil);
    NSParameterAssert(size > 0);
    
    ALAssetRepresentation *rep = [asset defaultRepresentation];
    
    CGDataProviderDirectCallbacks callbacks = {
        .version = 0,
        .getBytePointer = NULL,
        .releaseBytePointer = NULL,
        .getBytesAtPosition = getAssetBytesCallback,
        .releaseInfo = releaseAssetCallback,
    };
    
    CGDataProviderRef provider = CGDataProviderCreateDirect((void *)CFBridgingRetain(rep), [rep size], &callbacks);
    CGImageSourceRef source = CGImageSourceCreateWithDataProvider(provider, NULL);
    
    CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(source, 0, (__bridge CFDictionaryRef) @{
                                                                                                      (NSString *)kCGImageSourceCreateThumbnailFromImageAlways : @YES,
                                                                                                      (NSString *)kCGImageSourceThumbnailMaxPixelSize : [NSNumber numberWithInt:size],
                                                                                                      (NSString *)kCGImageSourceCreateThumbnailWithTransform : @YES,
                                                                                                      });
    CFRelease(source);
    CFRelease(provider);
    
    if (!imageRef) {
        return nil;
    }
    
    UIImage *toReturn = [UIImage imageWithCGImage:imageRef];
    
    CFRelease(imageRef);
    
    return toReturn;
}

采用上面的方法之后内存占用率很低！