objective-c - 缩放包含UIView时，UIImage/UIImageView重绘

Question

我的iPad应用程序有一个导航，其中我显示不同页面的屏幕截图，并且由于我想一次显示多个屏幕截图，因此我将容器缩放到原始屏幕截图(1024x768)的大约24％。

- (void) loadView
{
    // get landscape screen frame
    CGRect screenFrame = [UIScreen mainScreen].bounds;
    CGRect landscapeFrame = CGRectMake(0, 0, screenFrame.size.height, screenFrame.size.width);

    UIView *view = [[UIView alloc] initWithFrame:landscapeFrame];
    view.backgroundColor = [UIColor grayColor];

    self.view = view;

    // add container view for 2 images
    CGRect startFrame = CGRectMake(-landscapeFrame.size.width/2, 0, landscapeFrame.size.width*2, landscapeFrame.size.height);
    container = [[UIView alloc] initWithFrame:startFrame];
    container.backgroundColor = [UIColor whiteColor];

    // add image 1 (1024x768)
    UIImage *img1 = [UIImage imageNamed:@"01.jpeg"];
    UIImageView *img1View = [[UIImageView alloc] initWithImage:img1];
    [container addSubview:img1View];

    // add image 2 (1024x768)
    UIImage *img2 = [UIImage imageNamed:@"02.jpeg"];
    UIImageView *img2View = [[UIImageView alloc] initWithImage:img2];

    // move img2 to the right of img1
    CGRect newFrame = img2View.frame;
    newFrame.origin.x = 1024.0;
    img2View.frame = newFrame;

    [container addSubview:img2View];

    // scale to 24%
    container.transform = CGAffineTransformMakeScale(0.24, 0.24);

    [self.view addSubview:container];
}

但是当我用“小”文字缩放图像时，它看起来像这样:

我必须使用大屏幕截图，因为如果用户点击图像，则图像应缩放至100％且清晰可见。

有没有一种方法可以在不影响性能的情况下(平滑)缩放图像(动态)？
拥有两个版本就足够了:一个为全像素版本，另一个为24％版本。

Answer 1

缩小后的图像显得cr脚的原因是它在OpenGL中进行了缩放，而OpenGL使用的是快速但低质量的线性插值。您可能知道，UIView构建在CALayer之上，而CALayer又是OpenGL纹理的一种包装。由于该层的内容驻留在视频卡中，因此CALayer可以在GPU上发挥其所有魔力，而与CPU是否忙于加载网站，是否阻止磁盘访问等无关。我之所以仅提及这一点，是因为注意图层内部纹理中的实际内容很有用。在您的情况下，UIImageView的图层在其纹理上具有完整的1024x768位图图像，并且不受容器的转换的影响:UIImageView内的CALayer不会看到它会变成(请看一下。)246x185 -screen并重新缩放其位图，它只允许OpenGL执行其操作并在每次更新显示时缩小位图。

为了获得更好的缩放比例，我们需要在CoreGraphics中而不是在OpenGL中进行缩放。这是一种实现方法:

- (UIImage*)scaleImage:(UIImage*)image by:(float)scale
{
    CGSize size = CGSizeMake(image.size.width * scale, image.size.height * scale);
    UIGraphicsBeginImageContextWithOptions(size, YES, 0.0);
    [image drawInRect:CGRectMake(0, 0, size.width, size.height)];
    UIImage *imageCopy = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return imageCopy;
}

- (void)loadView
{
    // get landscape screen frame
    CGRect screenFrame = [UIScreen mainScreen].bounds;
    CGRect landscapeFrame = CGRectMake(0, 0, screenFrame.size.height, screenFrame.size.width);

    UIView *view = [[UIView alloc] initWithFrame:landscapeFrame];
    view.backgroundColor = [UIColor grayColor];

    self.view = view;

    // add container view for 2 images
    CGRect startFrame = CGRectMake(-landscapeFrame.size.width/2, 0, landscapeFrame.size.width*2, landscapeFrame.size.height);
    container = [[UIView alloc] initWithFrame:startFrame];
    container.backgroundColor = [UIColor whiteColor];

    // add image 1 (1024x768)
    UIImage *img1 = [UIImage imageNamed:@"01.png"];
    img1View = [[TapImageView alloc] initWithFrame:CGRectMake(0, 0, 1024, 768)];
    img1View.userInteractionEnabled = YES; // important!
    img1View.image = [self scaleImage:img1 by:0.24];
    [container addSubview:img1View];

    // add image 2 (1024x768)
    UIImage *img2 = [UIImage imageNamed:@"02.png"];
    img2View = [[TapImageView alloc] initWithFrame:CGRectMake(1024, 0, 1024, 768)];
    img2View.userInteractionEnabled = YES;
    img2View.image = [self scaleImage:img2 by:0.24];
    [container addSubview:img2View];

    // scale to 24% and layout subviews
    zoomed = YES;
    container.transform = CGAffineTransformMakeScale(0.24, 0.24);

    [self.view addSubview:container];
}

- (void)viewTapped:(id)sender
{
    zoomed = !zoomed;

    [UIView animateWithDuration:0.5 animations:^
    {
        if ( zoomed )
        {
            container.transform = CGAffineTransformMakeScale(0.24, 0.24);
        }
        else
        {
            img1View.image = [UIImage imageNamed:@"01.png"];
            img2View.image = [UIImage imageNamed:@"02.png"];
            container.transform = CGAffineTransformMakeScale(1.0, 1.0);
        }
    }
    completion:^(BOOL finished)
    {
        if ( zoomed )
        {
            UIImage *img1 = [UIImage imageNamed:@"01.png"];
            img1View.image = [self scaleImage:img1 by:0.24];
            UIImage *img2 = [UIImage imageNamed:@"02.png"];
            img2View.image = [self scaleImage:img2 by:0.24];
        }
    }];
}

这是TapImageView，它是UIImageView的子类，它通过在响应者链上发送操作来告诉我们何时点击它:

@interface TapImageView : UIImageView
@end

@implementation TapImageView

- (void)touchesEnded:(NSSet*)touches withEvent:(UIEvent*)event
{
    [[UIApplication sharedApplication] sendAction:@selector(viewTapped:) to:nil from:self forEvent:event];
}

@end