iphone - 如何以编程方式减少图像的内存大小

Question

我正在开发一个应用程序。我正在从服务器获取图像。我想将这些图像用作 UIView 的背景。当我直接将这些图像用作 uiview 的背景时，UIView 加载速度非常慢。因此，在将该图像设置为 UIView 的背景之前，我想检查图像大小并减小图像的大小。所以请告诉我如何找出并更改图像的大小。我使用下面的代码将图像设置为 UIView

的背景

 UIGraphicsBeginImageContext(self.view.frame.size);
[[UIImage imageNamed:@"image.png"] drawInRect:self.view.bounds];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
 self.view.backgroundColor = [UIColor colorWithPatternImage:image];

drawrect 方法需要更多的时间来执行。

Answer 1

这是一个函数，您可以将其放入类别或辅助类中，以编程方式调整 UIImage 的大小。我在网站的某个地方找到了它，但我现在找不到确切的帖子。

请记住，这需要一些时间来处理。理想情况下，如果您希望在以后使用同一图像时获得性能提升，您应该只执行一次。

这里是:

+ (UIImage *)imageByScalingProportionallyToSize:(CGSize)targetSize withOriginalImage:(UIImage *)origin {

    UIImage *sourceImage = origin;
    UIImage *newImage = nil;

    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;

    CGFloat targetWidth = targetSize.width;
    CGFloat targetHeight = targetSize.height;

    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;

    CGPoint thumbnailPoint = CGPointMake(0.0,0.0);

    if (CGSizeEqualToSize(imageSize, targetSize) == NO) {

        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;

        if (widthFactor < heightFactor) 
            scaleFactor = widthFactor;
        else
            scaleFactor = heightFactor;

        scaledWidth  = width * scaleFactor;
        scaledHeight = height * scaleFactor;

        // center the image

        if (widthFactor < heightFactor) {
            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5; 
        } else if (widthFactor > heightFactor) {
            thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
        }
    }


    // this is actually the interesting part:

    UIGraphicsBeginImageContext(targetSize);

    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width  = scaledWidth;
    thumbnailRect.size.height = scaledHeight;

    [sourceImage drawInRect:thumbnailRect];

    newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    if(newImage == nil) NSLog(@"could not scale image");

    return newImage ;
}

可以这样调用:

UIImage *mySmallImage = [ImageHelper imageByScalingProportionallyToSize:CGSizeMake(128,128) withOriginalImage:(UIImage *)myLargeImage];