从 CGBitmapContext 创建 CGImage 并添加到 UIImageView

Question

我正在尝试通过创建 CGBitMapContext 来创建 UICollectionViewCell 的快照。我并不完全清楚如何执行此操作或如何使用相关类，但经过一番研究，我编写了以下方法，该方法从我的 UICollectionViewCell 子类中调用：

- (void)snapShotOfCell
{
    float scaleFactor = [[UIScreen mainScreen] scale];
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(NULL, self.frame.size.width * scaleFactor, self.frame.size.height * scaleFactor, 8, self.frame.size.width * scaleFactor * 4, colorSpace, kCGImageAlphaPremultipliedFirst);

    CGImageRef image = CGBitmapContextCreateImage(context);
    UIImage *snapShot = [[UIImage alloc]initWithCGImage:image];

    UIImageView *imageView = [[UIImageView alloc]initWithFrame:self.frame];
    imageView.image = snapShot;
    imageView.opaque = YES;
    [self addSubview:imageView];

     CGImageRelease(image);
     CGContextRelease(context);
     CGColorSpaceRelease(colorSpace);
}

结果是图像没有出现。调试后，我可以确定我有一个有效的（非零）上下文、CGImage、UIImage 和 UIImageView，但屏幕上什么也没有出现。谁能告诉我我错过了什么？

Answer 1

您可以将此作为类别添加到 UIView 中，任何视图都可以访问它

- (UIImage*) snapshot
{
    UIGraphicsBeginImageContextWithOptions(self.frame.size, YES /*opaque*/, 0 /*auto scale*/);
    [self.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage* image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

那么你只需要从你的单元格对象中执行[self addSubview:[[UIImageView alloc] initWithImage:self.snapshot]]。

[编辑]

如果需要异步渲染（完全可以理解），这可以使用调度队列来实现。我认为这会起作用：

typedef void(^ImageOutBlock)(UIImage* image);

- (void) snapshotAsync:(ImageOutBlock)block
{
    CGFloat scale = [[UIScreen mainScreen] scale];
    CALayer* layer = self.layer;
    CGRect frame = self.frame;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^() {
        CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
        CGContextRef context = CGBitmapContextCreate(NULL, frame.size.width * scaleFactor, frame.size.height * scaleFactor, 8, frame.size.width * scaleFactor * 4, colorSpace, kCGImageAlphaPremultipliedFirst);
        UIGraphicsBeginImageContextWithOptions(frame.size, YES /*opaque*/, scale);
        [layer renderInContext:UIGraphicsGetCurrentContext()];
        UIImage* image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        CGContextRelease(context);
        CGColorSpaceRelease(colorSpace);
        dispatch_async(dispatch_get_main_queue(), ^() {
            block(image);
        });
    });
}

[编辑]

- (void) execute
{
    __weak typeof(self) weakSelf = self;
    [self snapshotAsync:^(UIImage* image) { 
        [weakSelf addSubview:[[UIImageView alloc] initWithImage:image]] 
    }];
}