iphone - 在小框架中显示大图像 UIImageView 仍然消耗大量内存？

Question

我有几个大图像，大小约为 1.5 MB。我使用 UIViewContentModeScaleFit 在 UIImageView 中显示它们中的每一个。

UIImageViews 的帧大小仅为 150 * 150。

我的问题是

我知道如果我全屏显示大图像，内存会大大增加。

但是如果它们在小的 UIImageView 中，它们仍然会消耗内存吗？

谢谢

Answer 1

UIImage 和 UIImageView 是不同的东西。每个 UIImageView 都知道显示其关联的 UIImage 的尺寸。 UIImage 没有如何显示自身或如何用于显示的概念，因此仅更改 UIImageView 的大小这一行为不会对UIImage。因此它对总内存使用没有影响。

您可能想要做的是使用 Core Graphics 获取 UIImage 并生成其 150x150 版本作为新的 UIImage，然后将其推送到 UIImageView.

要执行缩放，应使用如下代码(按我输入的方式编写，因此未进行彻底检查)即可完成工作:

#include <math.h>

- (UIImage *)scaledImageForImage:(UIImage *)srcImage toSize:(CGSize)maxSize
{
    // pick the target dimensions, as though applying
    // UIViewContentModeScaleAspectFit; seed some values first
    CGSize sizeOfImage = [srcImage size];
    CGSize targetSize; // to store the output size

    // logic here: we're going to scale so as to apply some multiplier
    // to both the width and height of the input image. That multiplier
    // is either going to make the source width fill the output width or
    // it's going to make the source height fill the output height. Of the
    // two possibilities, we want the smaller one, since the larger will
    // make the other axis too large
    if(maxSize.width / sizeOfImage.width < maxSize.height / sizeOfImage.height)
    {
        // we'll letter box then; scaling width to fill width, since
        // that's the smaller scale of the two possibilities
        targetSize.width = maxSize.width;

        // height is the original height adjusted proportionally
        // to match the proportional adjustment in width
        targetSize.height = 
                      (maxSize.width / sizeOfImage.width) * sizeOfImage.height;
    }
    else
    {
        // basically the same as the above, except that we pillar box
        targetSize.height = maxSize.height;
        targetSize.width = 
                     (maxSize.height / sizeOfImage.height) * sizeOfImage.width;
    }

    // images can be integral sizes only, so round up
    // the target size and width, then construct a target
    // rect that centres the output image within that size;
    // this all ensures sub-pixel accuracy
    CGRect targetRect;

    // store the original target size and round up the original
    targetRect.size = targetSize;
    targetSize.width = ceilf(targetSize.width);
    targetSize.height = ceilf(targetSize.height);

    // work out how to centre the source image within the integral-sized
    // output image
    targetRect.origin.x = (targetSize.width - targetRect.size.height) * 0.5f;
    targetRect.origin.y = (targetSize.height - targetRect.size.width) * 0.5f;

    // now create a CGContext to draw to, draw the image to it suitably
    // scaled and positioned, and turn the thing into a UIImage

    // get a suitable CoreGraphics context to draw to, in RGBA;
    // I'm assuming iOS 4 or later here, to save some manual memory
    // management.
    CGColorSpaceRef colourSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context  = CGBitmapContextCreate(
           NULL,
           targetSize.width, targetSize.height,
           8, targetSize.width * 4,
           colourSpace, 
           kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast);
    CGColorSpaceRelease(colourSpace);

    // clear the context, since it may currently contain anything.
    CGContextClearRect(context, 
                  CGRectMake(0.0f, 0.0f, targetSize.width, targetSize.height));

    // draw the given image to the newly created context
    CGContextDrawImage(context, targetRect, [srcImage CGImage]);

    // get an image from the CG context, wrapping it as a UIImage
    CGImageRef cgImage = CGBitmapContextCreateImage(context);
    UIImage *returnImage = [UIImage imageWithCGImage:cgImage];

    // clean up
    CGContextRelease(context);
    CGImageRelease(cgImage);

    return returnImage;
}

显然，我通过大量注释使其看起来很复杂，但实际上只有 23 行。