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懒惰的图像绘制

发布于 2024-11-03 04:11:32 字数 402 浏览 0 评论 0原文

我有一个需要按需绘制到图形上下文中的对象，但是，内容需要时间来渲染，并且在调用对象绘制方法时可能不可用。

这一般是如何实现的？存储对请求绘制的图形上下文或视图的引用，并在对象内部表示完全渲染时延迟将其绘制到那里？

或者是否有其他标准的可可机制来处理这个问题（例如 NSImage 在使用 NSURL 初始化时执行延迟绘制）？

澄清：

我在 MacOS 上，而不是 iOS
使用一些 NSViews -setNeedsDisplay: 不是我正在寻找的答案（NSImage 不依赖-setNeedsDisplay:）

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把梦留给海 2024-11-10 04:11:32

您应该考虑使用 NSOperation;如果您有一个特定的对象进行渲染，则可以是 NSInvocableOperation；如果渲染足够简单，可以放入单个函数中，则可以是 NSBlockOperation。

如果您可以在实际到达视图的 drawRect: 之前开始渲染，那么就这样做（也许您的应用程序委托在启动时就开始该过程）。否则，检查drawRect:中内容是否可用；如果没有，则开始操作并继续另一张图。当渲染对象完成其工作时，它可能会发布通知，或者，如果您给它一个返回视图的引用，请调用 setNeedsDisplay:

按照最后一句的思路，您还可以考虑您的渲染对象能够返回部分渲染的图像。我不确定渲染的性质，但可能会在某些点（每 n 个循环的末尾，或每 n 行像素）获得结果，将其填充到单独的 NSImage< /code> 与最终图像的大小相同（如有必要，请在末尾进行填充），并使该部分图像可用于视图进行绘制。

更新：NSImage 不“依赖”setNeedsDisplay: 或具有视图引用，因为它不代表屏幕的一部分。它所做的只是包含图像的数据；它只能在视图内绘制自己，然后“显示”——实际上绘制在屏幕上。当您使用 initByReferencingURL: 时，它会存储 URL，然后当另一个对象（例如包含图像并需要显示的视图）向它请求其内容时，它会执行其本来应该执行的操作如果您使用了 initWithURL:，即打开文件并将其内容读入内存，则完成。但它并不懒惰地绘制；它仅在视图正在绘制时绘制到需要它的视图中。

子类化 NSImage 来实现您自己的延迟加载或延迟渲染可能并不容易；它使用我认为是类簇，这就是为什么我建议使用一个包含并返回 NSImage 的“渲染器”对象。

自定义视图的drawRect：

- (void)drawRect:(NSRect)dirtyRect {
    NSLog(@"Entered: %@", NSStringFromSelector(_cmd));
    // Use a nice big image of the Milky Way -- this is about 5MB
    NSImage * lazyImage = [[[NSImage alloc] initByReferencingURL:
                            [NSURL URLWithString:@"http://www.eso.org/public/archives/images/original/milkyway.jpg"]]
                           autorelease];
    NSLog(@"Image instantiated.");
    [lazyImage drawInRect:[self bounds] fromRect:NSZeroRect operation:NSCompositeCopy fraction:1.0];
    NSLog(@"Image drawn");  // 2 minutes later; sometimes 3 in my testing
    [[NSColor yellowColor] set];
    [[NSBezierPath bezierPathWithRect:NSInsetRect([self bounds], 4, 4)] stroke];
    NSLog(@"Bezier path drawn; exiting drawRect.");
}

由此输出的日志；请注意，实例化非常快，但加载和绘制需要两分钟，在此期间没有绘制任何内容，应用程序也没有执行任何其他操作（旋转沙滩球）：

2011-04-27 21:33:00.899 SetNeedsDisplay[80162:a0b] Entered: drawRect:
2011-04-27 21:33:00.901 SetNeedsDisplay[80162:a0b] Image instantiated.
2011-04-27 21:34:57.911 SetNeedsDisplay[80162:a0b] Image drawn.
2011-04-27 21:34:57.912 SetNeedsDisplay[80162:a0b] Bezier path drawn; exiting drawRect.

You should look into using NSOperation; either an NSInvocationOperation, if you have a specific object doing the rendering, or NSBlockOperation, if the rendering is simple enough to fit into a single function.

If you can start rendering before you actually get to your view's drawRect:, then do that (maybe your app delegate starts the process right at launch). Otherwise, check in the drawRect: whether the content is available yet; if not, start the operation and continue with the other drawing. When the rendering object finishes its work, it will probably either post a notification or, if you give it a reference back to the view, call setNeedsDisplay:

Along the lines of your last sentence, you could also consider your rendering object being able to return a partially-rendered image. I'm not certain of the nature of your rendering, but it may be possible to get the results at certain points (the end of every n loops, or every n lines of pixels), stuff that into a separate NSImage of the same size as the final image (padding at the end if necessary), and make this partial image available to the view for drawing.

UPDATE: An NSImage doesn't "rely on" setNeedsDisplay: or having a view reference because it doesn't represent a piece of the screen. All it does is contain the data for an image; it can only draw itself inside a view, which is then "displayed" -- actually painted on the screen. When you use initByReferencingURL:, it stores the URL, and then when another object (like a view that contains the image and needs to be displayed) asks it for its contents, it does what it would've done if you had used initWithURL:, which is open the file and read its contents into memory. It doesn't draw lazily, though; it only draws into the view that wants it, when that view is drawing.

Subclassing NSImage to implement your own lazy loading or lazy rendering may not be easy; it uses helper classes that I believe are part of a class cluster, which is why I suggest having a "renderer" object which contains and returns an NSImage.

A custom view's drawRect:

- (void)drawRect:(NSRect)dirtyRect {
    NSLog(@"Entered: %@", NSStringFromSelector(_cmd));
    // Use a nice big image of the Milky Way -- this is about 5MB
    NSImage * lazyImage = [[[NSImage alloc] initByReferencingURL:
                            [NSURL URLWithString:@"http://www.eso.org/public/archives/images/original/milkyway.jpg"]]
                           autorelease];
    NSLog(@"Image instantiated.");
    [lazyImage drawInRect:[self bounds] fromRect:NSZeroRect operation:NSCompositeCopy fraction:1.0];
    NSLog(@"Image drawn");  // 2 minutes later; sometimes 3 in my testing
    [[NSColor yellowColor] set];
    [[NSBezierPath bezierPathWithRect:NSInsetRect([self bounds], 4, 4)] stroke];
    NSLog(@"Bezier path drawn; exiting drawRect.");
}

The log output from this; notice that the instantiation is very quick, but the loading and drawing takes two minutes, during which nothing is drawn and the app does nothing else (spinning beach ball):

2011-04-27 21:33:00.899 SetNeedsDisplay[80162:a0b] Entered: drawRect:
2011-04-27 21:33:00.901 SetNeedsDisplay[80162:a0b] Image instantiated.
2011-04-27 21:34:57.911 SetNeedsDisplay[80162:a0b] Image drawn.
2011-04-27 21:34:57.912 SetNeedsDisplay[80162:a0b] Bezier path drawn; exiting drawRect.

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