iOS 图片保存到沙盒中尺寸变大问题

前段时间在做项目的时候遇到图片保存到沙盒中尺寸变大问题？？？
尺寸变大？为什么之前没有注意到？
虽然用其他方法给暂时解决了问题，但是一直存有疑惑。今天闲来无事，把之前的问题重新拿出来探究一下。

第一步，将一张图片直接保存到沙盒中进行测试。

[UIImageJPEGRepresentation(image, 1) writeToFile:[NSHomeDirectory() stringByAppendingPathComponent:@"Documents/1.jpg"] atomically:YES];
[UIImageJPEGRepresentation(image, 0.6) writeToFile:[NSHomeDirectory() stringByAppendingPathComponent:@"Documents/2.jpg"] atomically:YES];
[UIImagePNGRepresentation(image) writeToFile:[NSHomeDirectory() stringByAppendingPathComponent:@"Documents/3.png"] atomically:YES];

发现在沙盒中三张图片的尺寸都没有改变。

第二步，探索问题。
因为我之前是将图片重新绘制，然后试用writeToFile方法保存图片到沙盒的，有可能是因为流程上的问题？之所以说是流程上的问题，是因为之前重新绘制的图片拿出来后查看了图片的尺寸并没有变大，但是保存的时候就会变大。

// 4 方式
    UIGraphicsBeginImageContextWithOptions(image.size, NO, 1);
    [image drawInRect:CGRectMake(0, 0, image.size.width, image.size.height)];
    UIImage *image4 = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [self writeImageWithImage:image4 Name:@"4"];
    
    // 5 方式
    UIGraphicsBeginImageContextWithOptions(image.size, YES, 1);
    [image drawInRect:CGRectMake(0, 0, image.size.width, image.size.height)];
    UIImage *image5 = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [self writeImageWithImage:image5 Name:@"5"];
    
    // 6 方式 -> 图片尺寸会变大
    UIGraphicsBeginImageContextWithOptions(image.size, NO, 0);
    [image drawInRect:CGRectMake(0, 0, image.size.width, image.size.height)];
    UIImage *image6 = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [self writeImageWithImage:image6 Name:@"6"];
    
    // 7 方式 -> 图片尺寸会变大
    UIGraphicsBeginImageContextWithOptions(image.size, YES, 0);
    [image drawInRect:CGRectMake(0, 0, image.size.width, image.size.height)];
    UIImage *image7 = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [self writeImageWithImage:image7 Name:@"7"];
    
    // 8 方式
    UIGraphicsBeginImageContext(image.size);
    [image drawInRect:CGRectMake(0, 0, image.size.width, image.size.height)];
    UIImage *image8 = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [self writeImageWithImage:image8 Name:@"8"];

- (void)writeImageWithImage:(UIImage *)image Name:(NSString *)imagename {
    [UIImageJPEGRepresentation(image, 1) writeToFile:[NSHomeDirectory() stringByAppendingPathComponent:[NSString stringWithFormat:@"Documents/%@.jpg",imagename]] atomically:YES];
    [UIImageJPEGRepresentation(image, 0.6) writeToFile:[NSHomeDirectory() stringByAppendingPathComponent:[NSString stringWithFormat:@"Documents/%@_2.jpg",imagename]] atomically:YES];
    [UIImagePNGRepresentation(image) writeToFile:[NSHomeDirectory() stringByAppendingPathComponent:[NSString stringWithFormat:@"Documents/%@.png",imagename]] atomically:YES];
}

经过测试发现，方式6和方式7会使图片变大，变大的比例和UIScreen的scale一致。他们之间的共性就是UIGraphicsBeginImageContextWithOptions方法的第三个参数传入的值都为0.猜想有可能是因为我们设置为0时，它会按照UIScreen的scale来进行绘制。

进一步猜想，这个UIGraphicsBeginImageContextWithOptions的scale会造成什么样的后果呢？

// 9 方式
    UIGraphicsBeginImageContextWithOptions(image.size, YES, 0.5);
    [image drawInRect:CGRectMake(0, 0, image.size.width, image.size.height)];
    UIImage *image9 = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [self writeImageWithImage:image9 Name:@"9"];
    
    // 10 方式
    UIGraphicsBeginImageContextWithOptions(image.size, YES, 2);
    [image drawInRect:CGRectMake(0, 0, image.size.width, image.size.height)];
    UIImage *image10 = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [self writeImageWithImage:image10 Name:@"10"];

经过测试，image.size打印是原始尺寸，写入沙盒的是根据scale改变后的尺寸，重新从沙盒中加载的image尺寸也是改变后的尺寸。
具体底层是什么原因造成这样的现象目前还不清楚，如果有明白的，欢迎帮忙解惑。