Is it possible to update a UITextView from a background thread?

RickSDK · July 2012

My code crashes when I try updating the UITextView from the background thread. updates to UITextField and UILabels work just fine. No problem at all, but for some reason the text view doesn't work.

I've search the web and it seems many people have run into this problem, but I haven't found any clear solutions.

BrianSlick · July 2012

UI stuff must be done on the main thread. You're apparently getting lucky with the labels and text fields.

RickSDK · July 2012

hmmm.... that's what they say. But my app updates tons of UI elements on bg with no problems at all. labels, textfields, imageviews, buttons. All no problem. But as soon as I try to update the UITextView it crashes.

RickSDK · July 2012

now that I think about it, that's can't be true about UI elements. A progress bar view is a UI element and it wouldn't make any sense at all to update that on the main thread. the only useful way to update it is on a background thread.

BrianSlick · July 2012

You do the UI change on the main thread, even for a progress bar. Perhaps you aren't coming from a background thread as often as you may think. Often times the crash message will specifically say that the UI was updated from a background thread.

RLScott · July 2012

RickSDK;441501 said:
now that I think about it, that's can't be true about UI elements. A progress bar view is a UI element and it wouldn't make any sense at all to update that on the main thread. the only useful way to update it is on a background thread.

No, Brian is right. Even a progress bar should only be updated by the main thread. The event that triggers the update may very well come from a background thread, but the proper way to connect that background event and the updating of the progress bar is through one of the many inter-thread synchronization methods such as "run selector on main thread". I forgot the exact method name.

Duncan C · July 2012

RLScott;441503 said:
No, Brian is right. Even a progress bar should only be updated by the main thread. The event that triggers the update may very well come from a background thread, but the proper way to connect that background event and the updating of the progress bar is through one of the many inter-thread synchronization methods such as "run selector on main thread". I forgot the exact method name.

performSelectorOnMainThread:withObject:waitUntilDone:.

GCD has much more flexible methods that let you perform a block on the main thread, along with all the advantages of blocks.

To the OP: Everybody is speaking truth. Do not update the UI on a background thread.

From a background thread, this line:

label.text = someString;

which can also be written as

[label setText: someString];

Should be written as:

[label performSelectorOnMainThread: @selector(setText:)
  withObject: someString
  waitUntilDone: FALSE];

RickSDK · July 2012

ok great. I was able to get duncan's code to work. thanks!

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Is it possible to update a UITextView from a background thread?

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