iOS图片压缩

做上传图片功能,特别是类似于微信,QQ里面,发布9张图片, 少不了碰到一个问题,就是图片压缩问题.我研究了这个问题,发现网上普遍的方法是如下

//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,
我测试过上面方法,上面方法真不好用,真的不推荐大家用.那么我推荐下面这个方法:

static size_t getAssetBytesCallback(voidvoid *info, voidvoid *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.
NSLog(@"thumbnailForAsset:maxPixelSize: got an error reading an asset: %@", error);
}

return countRead;
}

static void releaseAssetCallback(voidvoid *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. <pre code_snippet_id="1747779" snippet_file_name="blog_20160706_1_5109449" name="code" class="objc">//压缩时调用该放发即可，最好在子线程调用
- (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((voidvoid *)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！