不清晰寫法

- (UIImage *)reSizeImage:(UIImage *)image toSize:(CGSize)reSize

{

     UIGraphicsBeginImageContext(reSize);

     [image drawInRect:CGRectMake(0, 0, reSize.width, reSize.height)];

     UIImage *reSizeImage = UIGraphicsGetImageFromCurrentImageContext();

     UIGraphicsEndImageContext();

     return reSizeImage;

}

修改過(guò)后

-(UIImage *)reSizeImage:(UIImage *)image toSize:(CGSize)reSize
{

    //instead of UIGraphicsBeginImageContext(reSize);

    UIGraphicsBeginImageContextWithOptions(reSize, YES, 0); 

    [image drawInRect:CGRectMake(0, 0, reSize.width, reSize.height)];

    UIImage *reSizeImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return reSizeImage;

}

補(bǔ)充說(shuō)明

void UIGraphicsBeginImageContextWithOptions(CGSize size, BOOL opaque, CGFloat scale);
size——同UIGraphicsBeginImageContext
opaque——透明開(kāi)關(guān)，如果圖形完全不用透明，設(shè)置為YES以優(yōu)化位圖的存儲(chǔ)。
scale—–縮放因子：設(shè)為0后，系統(tǒng)就會(huì)自動(dòng)設(shè)置正確的比例了