iOS 中更改图像尺寸的几种方法

iOS 中有多种方法能够更改图片的尺寸，各种方法有不同的性能。

使用UIGraphicsBeginImageContext

UIKit 框架中提供了UIGraphicsBeginImageContext() 和 UIGraphicsGetImageFromCurrentImageContext()方法来更改图像尺寸。

+ (UIImage *)scaleImage:(UIImage *)image toSize:(CGSize)size {
    UIGraphicsBeginImageContext(size);
    [image drawInRect:CGRectMake(0, 0, size.width, size.height)];
    UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return scaledImage;
}

使用CGBitmapContextCreate & CGContextDrawImage

Core Graphics / Quartz 2D 同样提供了 APIs 来处理图片尺寸调整。

+ (UIImage *)scaleImage:(UIImage *)image toSize:(CGSize)size {
    CGImageRef cgImage = image.CGImage;
    size_t bitsPerComponent = CGImageGetBitsPerComponent(cgImage);
    size_t bytesPerRow = CGImageGetBytesPerRow(cgImage);
    CGColorSpaceRef colorSpace = CGImageGetColorSpace(cgImage);
    CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(cgImage);
    
    CGContextRef context = CGBitmapContextCreate(nil, size.width, size.height, bitsPerComponent, bytesPerRow, colorSpace, bitmapInfo);
    CGContextSetInterpolationQuality(context, kCGInterpolationHigh);
    CGContextDrawImage(context, CGRectMake(0, 0, size.width, size.height), cgImage);
    
    UIImage *scaredImage = [UIImage imageWithCGImage:CGBitmapContextCreateImage(context)];

    CGContextRelease(context);
    CGImageRelease(cgImage);

    return scaledImage;
}