图片解码方法

采用SDWebImage中的实现方法：

+ (UIImage *)decodedImageWithImage:(UIImage *)image {

    if (image.images) {

        // Do not decode animated images

        return image;

    }

    CGImageRef imageRef = image.CGImage;

    CGSize imageSize =CGSizeMake(CGImageGetWidth(imageRef),CGImageGetHeight(imageRef));

    CGRect imageRect = (CGRect){.origin =CGPointZero, .size = imageSize};

    CGColorSpaceRef colorSpace =CGColorSpaceCreateDeviceRGB();

    CGBitmapInfo bitmapInfo =CGImageGetBitmapInfo(imageRef);

    int infoMask = (bitmapInfo &kCGBitmapAlphaInfoMask);

    BOOL anyNonAlpha = (infoMask ==kCGImageAlphaNone ||

            infoMask == kCGImageAlphaNoneSkipFirst ||

            infoMask == kCGImageAlphaNoneSkipLast);

    // CGBitmapContextCreate doesn't support kCGImageAlphaNone with RGB.

    // https://developer.apple.com/library/mac/#qa/qa1037/_index.html

    if (infoMask ==kCGImageAlphaNone && CGColorSpaceGetNumberOfComponents(colorSpace) >1) {

        // Unset the old alpha info.

        bitmapInfo &= ~kCGBitmapAlphaInfoMask;

        // Set noneSkipFirst.

        bitmapInfo |= kCGImageAlphaNoneSkipFirst;

    }

            // Some PNGs tell us they have alpha but only 3 components. Odd.

    elseif (!anyNonAlpha && CGColorSpaceGetNumberOfComponents(colorSpace) ==3) {

        // Unset the old alpha info.

        bitmapInfo &= ~kCGBitmapAlphaInfoMask;

        bitmapInfo |= kCGImageAlphaPremultipliedFirst;

    }

    // It calculates the bytes-per-row based on the bitsPerComponent and width arguments.

    CGContextRef context =CGBitmapContextCreate(NULL,

            imageSize.width,

            imageSize.height,

            CGImageGetBitsPerComponent(imageRef),

            0,

            colorSpace,

            bitmapInfo);

    CGColorSpaceRelease(colorSpace);

    // If failed, return undecompressed image

    if (!context)return image;

    CGContextDrawImage(context, imageRect, imageRef);

    CGImageRef decompressedImageRef =CGBitmapContextCreateImage(context);

    CGContextRelease(context);

    UIImage *decompressedImage = [UIImageimageWithCGImage:decompressedImageRefscale:image.scaleorientation:image.imageOrientation];

    CGImageRelease(decompressedImageRef);

    return decompressedImage;

}

简单说就是：

根据当前输入image的CGImageRef；

根据CGImageRef得到CGSize；

根据CGImageRef生成CGBitmapInfo；

生成一个颜色空间；

根据以上值，生成一个CGBitmap画布，然后draw 一个CGRect大小的CGImageRef;

根据以上画布，得到一个新的CGImageRef;

根据以上CGImageRef生成一个UIImage，返回；