NSCell vs NSView: when many controls are needed

问题:

I am aware that Apple is deprecating the use of NSCell in favour of NSView (see AppKit 10.10 release notes). It was previously recommended that NSCell be used for performance reasons when many controls were needed.

I have spent considerable time implementing a custom control that required many subViews and the performance using NSView-type subViews was not good. See related stackoverflow discussion What are the practical limits in terms of number of NSView-type instances you can have in a window? I was struggling with 1000-2000 in-memory objects (which doesn't seem a lot). What is the actual reason for this limitation?

One thing that confuses me on the above is View-based Cocoa NSTableViews. You can create tableViews with more than 1000-2000 cells and they don't seem to have poor loading and scrolling performance? If each of the cells is an NSView then how is this achieved?

If there are practical limits, then what are Apple thinking when they say they are deprecating usage of NSCell's? I am sure they are aware that some controls need a large number of subViews.

Further, an (probably outdated) Apple Developer Guide give the following explanation for the difference between NSView & NSCell which I need explained further:

"Because cells are lighter-weight than controls, in terms of inherited data and behavior, it is more efficient to use a multi-cell control rather than multiple controls."

Inherited data: this would surely only cause "bloat" if the data was being used => and it would only be used it you needed it?

Inherited behavior: methods that you don't use in a class/object surely can't cause any overhead ?

What is the real difference between the lightweight NSCell versus the heavyweight NSView other than that it just seems to be conventionally accepted? (I would really like to know.)

回答1:

One thing that confuses me on the above is View-based Cocoa NSTableViews. You can create tableViews with more than 1000-2000 cells and they don't seem to have poor loading and scrolling performance? If each of the cells is an NSView then how is this achieved?

NSTableViews re-use views. The only views actually generated are those that are visible, plus maybe a row of views above and a row below the visible area. When scrolling the table, the object value associated with a given row of views is changed to that the views in the row will display different content

回答2:

A brief and incomplete answer:

NSCells are about drawing state, and not much else. NSViews must draw, but also must maintain and update all kinds of other information, such as their layout, responding to user input events, etc.

Consider the amount of calculation that must happen if you resize a view that contains many hundreds of subviews: every single subview position and size must be maintained according to the existing constraints for the view layout. That alone is quickly a very large amount of processing.

NSCells, in contrast, don't exist in a layout in that way. Their only job is to draw information in a given rectangle whenever asked to do so.