snapshotViewAfterScreenUpdates创建空白图像

Question

我正在尝试使用以下代码创建一些复合UIImage对象:

someImageView.image = [ImageMaker coolImage];

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;
}

我刚刚拿回一张空白的黑色图片.我该怎么办？

Answer 1

供应YES用于-snapshotViewAfterScreenUpdates:意味着它需要回一趟到runloop实际绘制图像.如果您提供NO,它将立即尝试,但如果您的视图在屏幕外或尚未绘制到屏幕上,则快照将为空.

要可靠地获取图像:

- (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];
}];