当前位置：文江博客话题详情

绘制矩形与布局子视图 - NSView - Cocoa touch

发布于 2024-09-24 12:10:57 字数 104 浏览 0 评论 0原文

我找不到一个合适的答案来解释这两个函数之间的区别。每个人什么时候接到电话，其中一个与另一个有何不同？

例如，我不能只在drawrect 中布局我的视图吗？

谢谢

原文

分享到QQ

分享到微博

如果你对这篇内容有疑问，欢迎到本站社区发帖提问参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。

发布评论

需要登录才能够评论，你可以免费注册一个本站的账号。

转瞬即逝 2024-10-01 12:10:57

如果设置了“需要布局”标志，则从 -layoutIfNeeded 调用 -layoutSubviews（使用 -setNeedsLayout，或在视图边界更改时自动）。使用它来定位~~您的视图~~ [编辑：使用它来定位子视图]。

如果设置了“需要显示”标志（使用-setNeedsDisplay），则从-displayIfNeeded调用-drawRect：，或者如果设置则自动调用>view.contentMode = UIViewContentModeRedraw）。

-layoutIfNeeded 和 -displayIfNeeded 都会在绘制到屏幕之前由 UIKit/CoreAnimation 自动调用；你很少需要直接打电话给他们。

您可以将子视图放置在 -drawRect: 中（您甚至可以添加子视图！），但这是不明智的：

默认情况下，不会在边界更改时自动调用 -setNeedsDisplay 。
实现 -drawRect: 会降低性能（UIKit/CoreAnimation 必须为您创建位图支持的图形上下文）；仅当您需要执行自定义绘图时才这样做。
您需要在-drawRect:中重绘视图。画画很贵。移动视图很便宜。
UIKit/CoreAnimation 可能会执行布局过程，然后进行绘图过程。 CoreAnimation 可以使用布局信息来决定需要绘制哪些视图（例如，它可以忽略被不透明子视图、离屏视图或 ClipsToBounds=YES 视图边界之外的子视图遮挡的视图；或者它只能绘制大视野）。如果在绘制过程中移动视图，CoreAnimation 可能无法正确绘制它们。

编辑：当我醒来时，还有一些更多细节：

“显示”和“绘制”之间有什么区别？显示是通过-[CALayer显示]来完成的；默认实现是（大约）

如果图层的委托响应 -displayLayer:，则调用 [self.delegate displayLayer:self]。 -displayLayer: 应该将 layer.content 设置为（例如）CGImage，
否则，如果图层的委托响应 -drawLayer:inContext:，设置一个位图支持的上下文，调用[self.delegate drawLayer:self inContext:context]，并将输出保存到layer.content（输出实际上是CABackingStore，可能是私有 API）
否则，请勿更改 layer.content。

视图是图层的委托，因此您可以改为实现 -[MyView displayLayer:] ，并执行一些有趣的操作，例如

self.layer.contents = (id)([UIImage imageNamed:@" foo"].CGImage) （这大致就是 UIImageView 所做的）
无操作，以防止任何“绘制”。如果您已经子类化了 UIToolbar 并希望为其提供透明背景，这可能会很有用。（这也会阻止创建 CGContext/CABackingStore。）
在不影响性能的情况下移动子视图（但由于上述原因，这仍然不是一个好主意）。

-layoutSubviews is called from -layoutIfNeeded if the "layout needed" flag was set (using -setNeedsLayout, or automatically when the view bounds changes). Use it for positioning ~~your view~~ [EDIT: Use it for positioning subviews].

-drawRect: is called from -displayIfNeeded if the "display needed" flag was set (using -setNeedsDisplay, or automatically if you set view.contentMode = UIViewContentModeRedraw).

Both -layoutIfNeeded and -displayIfNeeded are called automatically by UIKit/CoreAnimation before things are drawn to screen; you rarely need to call them directly.

You can position your subviews in -drawRect: (you can even add subviews!), but this is unwise:

By default, -setNeedsDisplay is not called automatically on a bounds change.
Implementing -drawRect: reduces performance (UIKit/CoreAnimation has to create a bitmap-backed graphics context for you); only do so if you need to perform custom drawing.
You need to redraw the view in -drawRect:. Drawing is expensive. Moving views around is cheap.
UIKit/CoreAnimation probably does a layout pass followed by a drawing pass. CoreAnimation can use layout information to decide what views need drawing (e.g. it could ignore views obscured by opaque subviews, off-screen views, or subviews outside the bounds of a clipsToBounds=YES view; or it could only draw a sub-rect of a large view). If you move views during the drawing pass, CoreAnimation might not draw them correctly.

EDIT: And some more detail when I'm awake:

What's the difference between "display" and "draw"? Displaying is done by-[CALayer display]; the default implementation is (approximately)

If the layer's delegate responds to -displayLayer:, call [self.delegate displayLayer:self]. -displayLayer: is supposed to set layer.content to (e.g.) a CGImage,
Otherwise, if the layer's delegate responds to -drawLayer:inContext:, set up a bitmap-backed context, call [self.delegate drawLayer:self inContext:context], and save the output to layer.content (the output is actually a CABackingStore, which is presumably a private API)
Otherwise, don't change layer.content.

The view is the layer's delegate, so you can implement -[MyView displayLayer:] instead, and do interesting stuff like

self.layer.contents = (id)([UIImage imageNamed:@"foo"].CGImage) (which is roughly what UIImageView does)
A no-op, to prevent any "drawing". This might be useful if you've subclassed e.g. UIToolbar and want to give it a transparent background. (This also prevents creation of the CGContext/CABackingStore.)
Move subviews around without the performance penalty (but it's still not a good idea for the reasons above).

回复收藏 0 原文