设置图像属性时，UIImageView 图像不会明显更新

UIImageView image does not update visibly when image property is set(设置图像属性时，UIImageView 图像不会明显更新)

问题描述

我有一个 UIImageView，它的用户交互是真实的，我给了一个点击手势识别器，它的动作处理程序如下:

I have a UIImageView whose user interaction is true and to which I have given a tap gesture recognizer, whose action handler is as follows:

@IBAction func tap(_ sender:UITapGestureRecognizer) {
    let iv = sender.view as! UIImageView
    let im = iv.image!
    let im2 = UIGraphicsImageRenderer(size:im.size).image { _ in
        UIColor.red.setFill()
        UIBezierPath(rect: CGRect(origin:.zero, size:im.size)).fill()
    }
    iv.image = im2
}

我希望当我点击图像视图时显示的图像会被纯红色图像替换.这在我运行 Xcode 9.4 的 High Sierra 机器上运行良好.但在我运行 Xcode 9.2 的 Sierra MacBook 上，没有明显的反应.

I expect the image displayed, when I tap the image view, to be replaced by a solid red image. This works fine on my High Sierra machine running Xcode 9.4. But on my Sierra MacBook running Xcode 9.2, nothing visibly happens.

这很奇怪.通过在调试器中暂停，我可以看到新图像正在正确构建:

It's weird. By pausing in the debugger, I can see that the new image is being constructed correctly:

图像正在被替换，但图像视图没有被重绘.添加像 setNeedsDisplay 这样的调用没有任何作用.

The image is being replaced, but the image view isn't being redrawn. Adding calls like setNeedsDisplay does nothing.

此外，如果我随后将图像视图的图像替换为 不同 图像，我会看到 红色 图像！

Moreover, if I then proceed to replace the image view's image with a different image, I see the red image!

    iv.image = im2
    delay(0.5) {
        iv.image = im // causes im2 to appear!
    }

某种幕后缓存显然会导致图像视图在其显示中落后一个图像.

Some sort of behind-the-scenes caching is evidently causing the image view to get behind in its display by one image.

有人能解释一下吗?这可能是 iOS 本身的一个错误，特别是在 9.2 中；如何解决它?(显然，我们可以批量替换另一个图像视图，但这并不能告诉我们缓存发生了什么.)

Can anyone shed light on this? It's presumably a bug in iOS itself, and perhaps in 9.2 specifically; how would one work around it? (Obviously one could substitute another image view wholesale, but that wouldn't tell us what's going on with the caching.)

推荐答案

这似乎是一种解决方法:

This seems to be a workaround:

iv.image = im2
delay(0.05) {
    iv.image = nil
    iv.image = im2
}

但是太可怕了...省略任何这些分配，或将 delay 减少到零(例如，通过调用 DispatchQueue.main.async 代替)，导致解决方法失败.

But what a horror... Omitting any of those assignments, or reducing the delay to zero (e.g. by calling DispatchQueue.main.async instead), causes the workaround to fail.

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