snapshotViewAfterScreenUpdates 创建一个空白图像 [英] snapshotViewAfterScreenUpdates creating a blank image

问题描述

我正在尝试使用以下代码创建一些复合 UIImage 对象:

I'm trying to create some composite UIImage objects with this code:

someImageView.image = [ImageMaker coolImage];

ImageMaker:

ImageMaker:

- (UIImage*)coolImage {
    UIView *composite = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 400, 400)];
    UIImageView *imgView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"coolImage"]]; //This is a valid image - can be viewed when debugger stops here
    [composite addSubview:imgView];

    UIView *snapshotView = [composite snapshotViewAfterScreenUpdates:YES];
//at this point snapshotView is just a blank image
    UIImage *img = [self imageFromView:snapshotView];
    return img;

}

- (UIImage *)imageFromView:(UIView *)view
{
    UIGraphicsBeginImageContextWithOptions(view.bounds.size, YES, 0.0);
    [view drawViewHierarchyInRect:view.bounds afterScreenUpdates:NO];
    UIImage * img = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return img;
}

我只得到一个空白的黑色图像.我该如何解决?

I just get back a blank black image. How can I fix?

推荐答案

Supplying YES for -snapshotViewAfterScreenUpdates: 意味着它需要回到 runloop 来实际绘制图片.如果您提供 NO，它将立即尝试，但如果您的视图不在屏幕上或尚未绘制到屏幕上，则快照将为空.

Supplying YES for -snapshotViewAfterScreenUpdates: means it needs a trip back to the runloop to actually draw the image. If you supply NO, it will try immediately, but if your view is off screen or otherwise hasn't yet drawn to the screen, the snapshot will be empty.

为了可靠地获取图像:

- (void)withCoolImage:(void (^)(UIImage *))block {
    UIView *composite = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 400, 400)];
    UIImageView *imgView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"coolImage"]]; //This is a valid image - can be viewed when debugger stops here
    [composite addSubview:imgView];

    UIView *snapshotView = [composite snapshotViewAfterScreenUpdates:YES];

    // give it a chance to update the screen…
    dispatch_async(dispatch_get_main_queue(), ^
    {
        // … and now it'll be a valid snapshot in here
        if(block)
        {
            block([self imageFromView:snapshotView]);
        }
    });
}

你会像这样使用它:

[someObject withCoolImage:^(UIImage *image){
    [self doSomethingWithImage:image];
}];

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