生成符合图片大小及长宽限制的缩略图片

iOS中，经常需要在列表显示图片的缩略图。在分享图片到微信和QQ好友时，不仅有长宽限制，而且有大小限制。

本函数使用大小缩小，JPEG压缩的方式，递归生成符合指定大小，长宽限制的UIImage缩略图。

//Generate a preview image which conforms the defined size and scale limit
+ (UIImage *)genPreviewImageByOriginalImage:(UIImage *)sourceImage sizeLimit:(float)sizeLimit widthLimit:(float)widthLimit heightLimit:(float) heightLimit{
CGFloat width = roundf(sourceImage.size.width/sourceImage.size.height*heightLimit);
//按照最大位深32位来计算
//预估图片尺寸＝图片长度*图片宽度*位深／8
float maximumWidth = sizeLimit/4/heightLimit;

if (widthLimit > width) {
width = widthLimit;
}
if (maximumWidth < width) {
width = maximumWidth;
}

UIGraphicsBeginImageContext(CGSizeMake(width, heightLimit));
[sourceImage drawInRect:CGRectMake(0, 0, width, heightLimit)];
UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

NSData *previewImageData = UIImagePNGRepresentation(scaledImage);
QZLOG_DEBUG(@"length of previewImageData when using PNGRepresentation is %ld and using JPEGRepresentation is:%ld",[previewImageData length]);
float lengthOfImage = [previewImageData length];
if (sizeLimit < lengthOfImage) {
static float compressRatioStep = 0.2;
static float maximumQuality = 1.0;
static float lowestQuality = 0.0;
for (float compressRatio = maximumQuality;
lowestQuality <= compressRatio;
compressRatio -= compressRatioStep) {
@autoreleasepool {
previewImageData = UIImageJPEGRepresentation(scaledImage, compressRatio);
lengthOfImage = [previewImageData length];
if (sizeLimit > lengthOfImage) {
scaledImage = [UIImage imageWithData:previewImageData];
break;
}
}
}
}

if (sizeLimit < lengthOfImage) {
NSLog(@"debug preview image data length:%.2f is larger than %.2f.so reduce size again",lengthOfImage,sizeLimit);
return [self genPreviewImageByOriginalImage:scaledImage sizeLimit:sizeLimit widthLimit:widthLimit heightLimit:(heightLimit - 40)?(heightLimit - 40):40];
} else {
NSLog(@"debug preview image data length:%.2f is within length limit:%f",lengthOfImage,sizeLimit);
}

return scaledImage;
}

其中：

UIImageJPEGRepresentation(scaledImage, compressRatio);

的compressRatio参数，由1变为0，表示逐渐加大压缩比，0的压缩比最高，得到的图片大小最小。

虽然是递归操作，但是根据参数的限制，在生成分享到微信的缩略图时，基本上不会出现3次以上的递归，效率还是OK的。