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cocoaappkit

Updating contents of multiple NSTextView objects in a single operation

The database application I am working on can have a window with multiple NSTextView elements for displaying and editing data. When the current spot in the database is repositioned, all of the NSTextView objects in the window need to be updated with new contents. This is done with a loop that scans each object and checks to see if it needs to be updated. If it does, the new value is calculated, then updated by using the [NSTextView setString:] method. Here is a simplified version of the code involved.

for formObject in formObjectsInWindow {
    NSTextView * objectTextView = [formObject textView];
    NSString * updatedValue = [formObject calculateValue];
    [objectTextView setString: updatedValue];
}

This works, but if there are a lot of objects, it is somewhat slow. Probably related, the display does not update all at once, you can actually see a "ripple" as the objects are updated, as illustrated in this movie (this movie has been slowed down to 1/4 speed to make the ripple effect more pronounced, but it is definitely visible at full speed).

enter image description here

If you've gotten this far, you might suspect that the calculateValue method is slow, but that isn't the problem. In other places the same code is used and runs at tens of thousands of operations per second. Also, this delay only occurs during update operations, it doesn't occur when the window is first opened, even though the same calculations are required at that time. Here is an example. Notice that when I switch back to the detail view all the NSTextView objects update instantaneously, even though the record changed and all of the values are different.

enter image description here

My suspicion is that the [NSTextView setString:] method is updating the off-screen buffer, then immediately copying that to the on-screen buffer, so that this double buffering is happening over and over again for each item, causing the delay and ripple. If so, I'm sure there must be some way to prevent this so that the screen is only updated at the end after all of the values have been updated. It's probably something simple that I am missing, but I'm afraid I am stumped as to how this is supposed to be done.

By the way, this application does not use layer-backed views, and is not linked against the QuartzCore framework.

Solution

  • I brought up this question with Apple engineers at the WWDC 2018 labs. It turns out the problem is that the setString: method does not mark the NSTextView object as needing display. The system does eventually notice that the text has changed and updates the display, but this happens in an asynchronous process, hence the "ripple" effect. So the workaround is simply to add a call to setNeedsDisplay after calling setString.

    [objectTextView setString: updatedValue]
[objectTextView setNeedsDisplay:YES];

    Adding this one line of code fixed the problem, no more ripple effect.

    I'm told that this is actually a bug, so hopefully this extra line won't be needed in future versions of macOS. As requested, a radar has been filed (41273611 if any Apple engineers are reading this).