前段時(shí)間在做項(xiàng)目的時(shí)候遇到圖片保存到沙盒中尺寸變大問題？？？
尺寸變大？為什么之前沒有注意到？
雖然用其他方法給暫時(shí)解決了問題，但是一直存有疑惑。今天閑來無事，把之前的問題重新拿出來探究一下。

第一步，將一張圖片直接保存到沙盒中進(jìn)行測試。

[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];

發(fā)現(xiàn)在沙盒中三張圖片的尺寸都沒有改變。

第二步，探索問題。
因?yàn)槲抑笆菍D片重新繪制，然后試用writeToFile方法保存圖片到沙盒的，有可能是因?yàn)榱鞒躺系膯栴}？之所以說是流程上的問題，是因?yàn)橹爸匦吕L制的圖片拿出來后查看了圖片的尺寸并沒有變大，但是保存的時(shí)候就會變大。

// 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 -> 圖片尺寸會變大
    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 -> 圖片尺寸會變大
    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];
}

經(jīng)過測試發(fā)現(xiàn)，方式6和方式7會使圖片變大，變大的比例和UIScreen的scale一致。他們之間的共性就是UIGraphicsBeginImageContextWithOptions方法的第三個(gè)參數(shù)傳入的值都為0.猜想有可能是因?yàn)槲覀冊O(shè)置為0時(shí)，它會按照UIScreen的scale來進(jìn)行繪制。

進(jìn)一步猜想，這個(gè)UIGraphicsBeginImageContextWithOptions的scale會造成什么樣的后果呢？

// 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"];

經(jīng)過測試，image.size打印是原始尺寸，寫入沙盒的是根據(jù)scale改變后的尺寸，重新從沙盒中加載的image尺寸也是改變后的尺寸。
具體底層是什么原因造成這樣的現(xiàn)象目前還不清楚，如果有明白的，歡迎幫忙解惑。