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The second parameter is the compression coefficient. Its setting does not guarantee the size of the image, because the size of the compressed image is also related to the content of the image. For example, if the color of your image is similar, the size of the compressed image will be smaller. Do you care about WiFi compression? If you need to upload, you can compress two formats of pictures, high quality and low quality pictures, upload high quality pictures with wifi, upload low quality pictures without wifi?

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UIImageJPEGRepresentation(self.photo, 0.5);
For each image compression, in fact, there is a minimum value, and no matter how small the compression coefficient is, it will not help.
If it's still too big, you can only cut the picture.

- (NSData *)imageWithImage:(UIImage*)image
          scaledToSize:(CGSize)newSize;
{
    UIGraphicsBeginImageContext(newSize);
    [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return UIImageJPEGRepresentation(newImage, 0.8);
}

You can make a for loop and keep approaching the size you want.

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- (UIImage *)compressImage:(UIImage *)image toMaxFileSize:(NSInteger)maxFileSize {
    CGFloat compression = 0.9f;
    CGFloat maxCompression = 0.1f;
    NSData *imageData = UIImageJPEGRepresentation(image, compression);
    while ([imageData length] > maxFileSize && compression > maxCompression) {
        compression -= 0.1;
        imageData = UIImageJPEGRepresentation(image, compression);
    }
    
    UIImage *compressedImage = [UIImage imageWithData:imageData];
    return compressedImage;
}

Picture compression is actually two concepts.
1. If the volume of the "pressed" file becomes smaller, but the number of pixels remains unchanged and the length and width of the file remain unchanged, the quality may decrease.
2. The size of the "shrink" file decreases, that is, the number of pixels decreases. As the length and width of the file become smaller, the volume of the file will also decrease.

This UI Image JPEG Representation (image, 0.0) is a function of 1. This [sourceImage drawInRect:CGRectMake(0,0,targetWidth, targetHeight)] is a function of 2.

So, you have to use the two together to meet the needs, or you blindly use 1, resulting in blurred pictures, but the size is still very large.

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I have a picture, maybe 14M or more, I want to compress it to less than 500K, UI Image JPEG Representation (image, 0.0), even if the compression coefficient is set to the minimum, it is still larger than 500K after compression; then what method can compress any size of picture to the specified byte size, ignoring the requirement of image clarity? The landlord did not solve this problem, solve it!

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-(UIImage *) imageCompressForWidth:(UIImage *)sourceImage targetWidth:(CGFloat)defineWidth
{
    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;
    CGFloat targetWidth = defineWidth;
    CGFloat targetHeight = (targetWidth / width) * height;
    UIGraphicsBeginImageContext(CGSizeMake(targetWidth, targetHeight));
    [sourceImage drawInRect:CGRectMake(0,0,targetWidth,  targetHeight)];
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}

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Combine image quality with image size reduction:

/**
 Press picture quality

 @param image image
 @return Data
 */
+ (NSData *)zipImageWithImage:(UIImage *)image
{
    if (!image) {
        return nil;
    }
    CGFloat maxFileSize = 32*1024;
    CGFloat compression = 0.9f;
    NSData *compressedData = UIImageJPEGRepresentation(image, compression);
    while ([compressedData length] > maxFileSize) {
        compression *= 0.9;
        compressedData = UIImageJPEGRepresentation([[self class] compressImage:image newWidth:image.size.width*compression], compression);
    }
    return compressedData;
}

/**
 *  Scale this picture in equal proportion
 *
 *  @param newImageWidth The width of the zoomed image in pixels
 *
 *  @return self-->(image)
 */
+ (UIImage *)compressImage:(UIImage *)image newWidth:(CGFloat)newImageWidth
{
    if (!image) return nil;
    float imageWidth = image.size.width;
    float imageHeight = image.size.height;
    float width = newImageWidth;
    float height = image.size.height/(image.size.width/width);
    
    float widthScale = imageWidth /width;
    float heightScale = imageHeight /height;
    
    // Create a bitmap context
    // And set it to the context currently in use
    UIGraphicsBeginImageContext(CGSizeMake(width, height));
    
    if (widthScale > heightScale) {
        [image drawInRect:CGRectMake(0, 0, imageWidth /heightScale , height)];
    }
    else {
        [image drawInRect:CGRectMake(0, 0, width , imageHeight /widthScale)];
    }
    
    // Create a resized image from the current context
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    // Get the current context out of the stack
    UIGraphicsEndImageContext();
    
    return newImage;
    
}