CALayer不呈现内容 [英] CALayer not rendering contents

问题描述

这一个是骗我。我有一个 CALayer 它创建在 UIViewController 的 viewDidLoad 方法类似这样：

This one is stumping me. I have a CALayer which I create in a UIViewController's viewDidLoad method something like this:

- (void)viewDidLoad {

    [super viewDidLoad];

    UIImage* img = [UIImage imageNamed:@"foo.png"];

    _imageLayer = [[CALayer alloc] init];

    _imageLayer.bounds = CGRectMake( 0, 0, img.size.width, img.size.height);
    _imageLayer.position = CGPointMake( self.view.bounds.size.width/2.0, self.view.bounds.size.height/2.0 );

    _imageLayer.contents = (id)img.CGImage;
    _imageLayer.contentsScale = img.scale;

    [self.view.layer addSublayer:_imageLayer];
    [_imageLayer setNeedsDisplay];
}

UIViewController 从一个nib加载并放置到在代码中创建的 UITabBarController 中。

The UIViewController is loaded from a nib and placed into a UITabBarController that was created in code.

我的问题是<当 UIViewController 变为可见时，c $ c> CALayer 不显示其 UIImage 我可以使 CALayer 渲染 UIImage 的唯一方法是将其内容设置为 UIViewController 可见之后，例如 viewDidAppear：方法中的

My problem is that the CALayer does not render its UIImage contents when the UIViewController becomes visible. The only way I can make the CALayer to render the UIImage is by setting its contents to the UIImage after the UIViewController is made visible, such as in the viewDidAppear: method.

我真的更愿意做所有的设置在 viewDidLoad 方法？）。如何获取 CALayer 以呈现其内容？

I would really prefer to do all my set up in the viewDidLoad method (isn't that what's it’s for?). How do I get the CALayer to render its contents?

推荐答案

不需要在图层或超级视图上 setNeedsDisplay 。

You don't need that setNeedsDisplay on the layer, or the superview.

setNeedsDisplay 告诉图层或视图它需要重绘其内容。 （对于视图，它将调用 drawRect：来获取新内容;对于CALayer，它将询问其委托）。注意，这仅影响视图或图层本身，

setNeedsDisplay tells a layer or view that it needs to redraw its contents. (For a view, it will call drawRect: to get new contents; for a CALayer, it will ask its delegate.) Note that this affects only the view or layer itself, not its children.

由于您直接提供图层的内容，因此您不希望CA丢弃该内容并要求替换。

Since you are providing the content for the layer directly, you don't want CA to discard that content and ask for a replacement.

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