UITextView won't scroll while background thread busy - ios

Question

I'm running mathematical computation in a background thread. Attempting to post results in real time in a UITextView. However the results don't show up until the background thread completes. Why not?
I kick off a method in the background,
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^() {
[self v2];
});
The background thread method is of the form,
- (void) v2 {
NSString *result;
// ... loop a bunch of times generating lots of results
for (bunch of stuff to compute) {
// If using dispatch_async, nothing is displayed until this method finishes
// If dispatch_sync then it does display and update
result = [self computeNextValue];
dispatch_async(dispatch_get_main_queue(), ^() {
textView.text = result;
});
} // end computation
}
This actually hadn't been much of a problem until I started trying to scroll the view. Painfully slow. So I created a NSTimer to periodically scroll the UITextView. However, even though the timer popped and the method is run to request the scroll, the UITextView doesn't scroll until the background method completes.

Answer 1

First, make sure that you are using weakSelf rather than self within the block.
__weak MyClass weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^() {
[weakSelf v2];
});
...
__weak MyClass weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^() {
weakSelf.textView.text = result;
});
But that won't cause the delayed update. I am very suspect of the queue's priority, DISPATCH_QUEUE_PRIORITY_LOW. Try using DISPATCH_QUEUE_PRIORITY_DEFAULT instead.

Answer 2

I think I figured it out, at least empirically.
It looks like setting text on a UITextView or programmatically initiating scrolling sets up an animation sequence thats run in another thread. What seems to be happening is that the requests to set the text of the UITextView is coming in faster than the ability of UITextView to setup the animation. When the property changes again, it apparently cancels the earlier animation that it was setting up on another thread. As I continue to flood it with change requests, its never able to finish what it wants to do before the value of 'text' has changed again.
My solution involves multiple steps.
In my original approach I was setting textView.text very rapidly to some new value. I set up a timer to periodically request the UITextView to scroll.
In my modification, I calculate the new value of the result string, but do not set it to the UITextView. Instead the text is set on the textview periodically based on the timer. This allows the text view to catch up.
However, I noticed that this still wasn't reliable. If I happened to set the text again while it was still scrolling, weird effects would occur, such as a very slow scroll. It seems that the scroll animation and repeated settings of text were still causing a problem.
So solve this problem, I created a property to indicate if the view is scrolling. Set the view controller as the UITextField delegate. When I request the view to scroll, I set the flag to indicate its scrolling. Only update the content and request scroll if its not already scrolling. Ends up working great. Doesn't matter how fast I set the timer, it ends up waiting appropriately.
// ViewController.h
#property BOOL isViewScrolling;
// ViewController.m
// initialize property in viewDidLoad
- (void)viewDidLoad {
self.isViewScrolling = FALSE;
textView.delegate = self;
self.timer = [NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:#selector(scrollIt) userInfo:nil repeats:TRUE];
}
- (void)scrollIt {
NSLog(#"scrollit thread=%d", [[NSThread currentThread]isMainThread]);
if (!self.isViewScrolling) {
textView.text = self.result;
NSRange range = NSMakeRange(textView.text.length - 1, 1);
self.isViewScrolling = TRUE;
[textView scrollRangeToVisible:range];
}
}
// UITextView delegate
- (void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView {
NSLog(#"Animation stopped");
self.isViewScrolling = FALSE;
}

Answer 3

Try changing the predefined dispatch queue string to end with background.
__weak MyClass weakSelf = self;
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^() {
weakSelf.textView.text = result;
});
Also you should add a UIActivityIndicator and start animating at the beginning of the operation, then stop animating after the textview.text field has been updated.
It's a nice feature to have to show the user there is a process currently being done
Also I would stay away from NSThread as I have read a few forums and docs from apple that emphasize the use of GCD and block operations.