When I add a UITextView to a custom UITableViewCell on StoryBoard I always receive the error:
bool _WebTryThreadLock(bool), 0x7c523c0: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...
Related
In my iPhone app, I am occasionally seeing a crash caused by tableView:cellForRowAtIndexPath: being called on a background thread.
Obviously, this should not be happening. I'm not calling it, my object is a delegate to a UITableView and the foundation is calling it - the only thing I see in the stack for the thread in question is
-_WebTryThreadLock(bool)
-_dequeuReusableViewOfType
-tableView:cellForRowAtIndexPath:
-_createPreparedCellForGlobalRow:withIndexPath
-_pthread_qathread
The crash occurs in WebTryThreadLock - no surprise really, it's not threadsafe and should not be called off the main thread.
But how do I figure out WHY my tableView delegate is being called on a background thread?
I wonder - if I were to call [tableView reloadData] on a background thread, would that do it?
I always thought it would just dispatch the call on the main thread regardless. I'm not sure I am even doing that, but I might be, and I will check for that, but really, shouldn't UIKit check for that and call the delegate methods on the main thread anyway?
You can't call [tableView reloadData] on a secondary thread. You can't call any UIKit stuff on a secondary thread (with a few exceptions, such as UIImage). This includes all tableView methods, including straight getters and setters. It doesn't matter whether it is rendering related or not.
It might be interesting to sprinkle some of these around your view controller:
if (![NSThread isMainThread]) {
NSLog(#"Huh?");
}
I'm pretty sure UIKit/IOS would not decide on it's own to call a table view delegate method on a background thread. Do you have any dispatch_async, detachNewThreadSelector, performSelectorInBackground?
I have a custom tableview cell, which contains some object UIImageView, UILabel, and a UITextView. All object, except the UITextView works fine, but when I try to change the text on the textView:
myTextView.text = #"Some string";
the app crashes with this error:
Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...
Any suggestion is very appreciated.
Thanks
The error sounds like you are setting the text property from a code block that is not executing on the main thread. Check to make sure that any code you have that modifies UIKit objects does so while on the main thread. Example below:
dispatch_async(dispatch_get_main_queue(), ^{
// Set text here
myTextView.text = #"Some string";
});
**bool _WebTryThreadLock(bool),** 0x4c7a7e0: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...
I m getting an above error when i do this,
[alertForSavingText dismissWithClickedButtonIndex:ALERT_CANCEL animated:YES];
This is my custom alert with textField inside. Error comes only when I switch the application from background to foreground also it occurs when there is some text present in custom alert textField & keyboard is also visible.
Any suggestion.
Please check whether the current thread is main thread. If it is main thread, dismiss it as you have given. Else, if it is secondary thread, dismiss the alert view in main thread by calling performSelectorInMainThread:... method
At the moment, I'm getting data for the next view from the server.
While this goes on, I am showing the user a loading indicator.
After the data have been loaded, the main thread gets notified and loads the nib for the new view.
My question is:
Because I already have this loading mechanism, can I safely load the nib's in the working thread and just push them from within the main thread?
UIKit is not thread safe, so I suspect it should be pretty unsafe loading the nib in a secondary thread. Just loading the nib in a UIViewController and displaying it from the main thread could work for you and may break elsewhere.
I suppose you know about performSelectorOnMainThread:withObject:waitUntilDone to dispatch a method from a secondary thread to the main thread. You could try and use this from your secondary thread when you want to load the nib.
EDIT:
what about preloading the nib while the working thread is retrieving the data, so that when the working thread has done, the main thread has only to display the nib views?
General Description:
To start with what works, I have a UITableView which has been placed onto an Xcode-generated view using Interface Builder. The view's File Owner is set to an Xcode-generated subclass of UIViewController. To this subclass I have added working implementations of numberOfSectionsInTableView: tableView:numberOfRowsInSection: and tableView:cellForRowAtIndexPath: and the Table View's dataSource and delegate are connected to this class via the File Owner in Interface Builder.
The above configuration works with no problems. The issue occurs when I want to move this Table View's dataSource and delegate-implementations out to a separate class, most likely because there are other controls on the View besides the Table View and I'd like to move the Table View-related code out to its own class. To accomplish this, I try the following:
Create a new subclass of UITableViewController in Xcode
Move the known-good implementations of numberOfSectionsInTableView:, tableView:numberOfRowsInSection: and tableView:cellForRowAtIndexPath: to the new subclass
Drag a UITableViewController to the top level of the existing XIB in InterfaceBuilder, delete the UIView/UITableView that are automatically created for this UITableViewController, then set the UITableViewController's class to match the new subclass
Remove the previously-working UITableView's existing dataSource and delegate connections and connect them to the new UITableViewController
When complete, I do not have a working UITableView. I end up with one of three outcomes which can seemingly happen at random:
When the UITableView loads, I get a runtime error indicating I am sending tableView:cellForRowAtIndexPath: to an object which does not recognize it
When the UITableView loads, the project breaks into the debugger without error
There is no error, but the UITableView does not appear
With some debugging and having created a basic project just to reproduce this issue, I am usually seeing the 3rd option above (no error but no visible table view). I added some NSLog calls and found that although numberOfSectionsInTableView: and numberOfRowsInSection: are both getting called, cellForRowAtIndexPath: is not. I am convinced I'm missing something really simple and was hoping the answer may be obvious to someone with more experience than I have. If this doesn't turn out to be an easy answer I would be happy to update with some code or a sample project. Thanks for your time!
Complete steps to reproduce:
Create a new iPhone OS, View-Based Application in Xcode and call it TableTest
Open TableTestViewController.xib in Interface Builder and drag a UITableView onto the provided view surface.
Connect the UITableView's dataSource and delegate-outlets to File's Owner, which should already represent the TableTestViewController-class. Save your changes
Back in Xcode, add the following code to TableTestViewController.m:
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
NSLog(#"Returning num sections");
return 1;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
NSLog(#"Returning num rows");
return 1;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
NSLog(#"Trying to return cell");
static NSString *CellIdentifier = #"Cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:CellIdentifier] autorelease];
}
cell.text = #"Hello";
NSLog(#"Returning cell");
return cell;
}
Build and Go, and you should see the word Hello appear in the UITableView
Now to attempt to move this UITableView's logic out to a separate class, first create a new file in Xcode, choosing UITableViewController subclass and calling the class TableTestTableViewController
Remove the above code snippet from TableTestViewController.m and place it into TableTestTableViewController.m, replacing the default implementation of these three methods with ours.
Back in Interface Builder within the same TableTestViewController.xib-file, drag a UITableViewController into the main IB window and delete the new UITableView object that automatically came with it
Set the class for this new UITableViewController to TableTestTableViewController
Remove the dataSource and delegate bindings from the existing, previously-working UITableView and reconnect the same two bindings to the new TableTestTableViewController we created.
Save changes, Build and Go, and if you're getting the results I'm getting, note the UITableView no longer functions properly
Solution:
With some more troubleshooting and some assistance from the iPhone Developer Forums, I've documented a solution! The main UIViewController subclass of the project needs an outlet pointing to the UITableViewController instance. To accomplish this, simply add the following to the primary view's header (TableTestViewController.h):
#import "TableTestTableViewController.h"
and
IBOutlet TableTestTableViewController *myTableViewController;
Then, in Interface Builder, connect the new outlet from File's Owner to TableTestTableViewController in the main IB window. No changes are necessary in the UI part of the XIB. Simply having this outlet in place, even though no user code directly uses it, resolves the problem completely. Thanks to those who've helped and credit goes to BaldEagle on the iPhone Developer Forums for finding the solution.
I followed your steps, recreated the project and ran into the same problem. Basically you are almost there. There are 2 things missing (once fixed it works):
You need to connect the tableView of the TableTestTableViewController to the UITableView you have on the screen. As I said before because it is not IBOutlet you can override the tableView property and make it and IBOutlet:
#interface TableTestTableViewController : UITableViewController {
UITableView *tableView;
}
#property (nonatomic, retain) IBOutlet UITableView *tableView;
Next thing is to add a reference to the TableTestTableViewController and retain it in the TableTestViewController. Otherwise your TableTestTableViewController may be released (after loading the nib with nothing hanging on to it.) and that is why you are seeing the erratic results, crashes or nothing showing. To do that add:
#interface TableTestViewController : UIViewController {
TableTestTableViewController *tableViewController;
}
#property (nonatomic, retain) IBOutlet TableTestTableViewController *tableViewController;
and connect that in the Interface Builder to the TableTestTableViewController instance.
With the above this worked fine on my machine.
Also I think it would be good to state the motivation behind all this (instead of just using the UITableViewController with its own UITableView). In my case it was to use other views that just the UITableView on the same screenful of content. So I can add other UILabels or UIImages under UIView and show the UITableView under them or above them.
I just spent many hours pulling my hair out trying to figure out why a UITableView wouldn't show up when when I had it embedded in a separate nib instead of in the main nib. I finally found your discussion above and realized that it was because my UITableViewController wasn't being retained! Apparently the delegate and datasource properties of UITableView are not marked "retain" and so my nib was loading but the controller was getting tossed... And due to the wonders of objective-c I got no error messages at all from this... I still don't understand why it didn't crash. I know that I've seen "message sent to released xxx" before... why wasn't it giving me one of those?!?
I think most developers would assume that structure that they build in an interface builder would be held in some larger context (the Nib) and not subject to release. I guess I know why they do this.. so that the iPhone can drop and reload parts of the nib on low memory. But man, that was hard to figure out.
Can someone tell me where I should have read about that behavior in the docs?
Also - about hooking up the view. First, if you drag one in from the UI builder you'll see that they hook up the view property (which is an IBOutlet) to the table view. It's not necessary to expose the tableView, that seems to get set internally. In fact it doesn't even seem to be necessary to set the view unless you want viewDidLoad notification. I've just broken the view connection between my uitableview and uitableviewcontroller (only delegate and datasource set) and it's apparently working fine.
Yes for some reason (please chime in if anybody knows why...) tableView property of the UITableViewController is not exposed as an IBOutlet even though it is a public property. So when you use Interface Builder, you can't see that property to connect to your other UITableView. So in your subclass, you can create a tableView property marked as an IBOutlet and connect that.
This all seems hacky and a workaround to me, but it seems to be the only way to separate a UITableViewController's UITableView and put it somewhere else in UI hierarchy. I ran into the same issue when I tried to design view where there are things other than the UITableView and that was the way I solved it... Is this the right approach???
I was able to make this work. I built a really nice dashboard with 4 TableViews and a webview with video. The key is having the separate tableView controllers and the IBOutlets to the other tableview controllers defined in the view controller. In UIB you just need to connect the other tableview controllers to the file owner of the view controller. Then connect the tables to the corresponding view controllers for the datasource and delegate.