How do I ensure Core Data retrieval between switching tabs on a tab bar controller?

Question

Can someone help me out??

Okay.....my project is going well. Got a small issue of screen/data updating.

I have the standard 5 tab TabBarController working fine. One of my tabs enters data from a text field into an Entity Attribute in Core Data. All is fine so far.

When I switch to another tab, I have the Attribute being read from the database and entered into a picker.

The code works fine but it's when then reading takes place which is the problem.

After entering the data on tab 3 and it being written, I then switch to tab 1 and the picker hasn't updated. It's not until I stop the simulator and run again when the data shows up in picker on tab 1.

At the moment, I have the code for reading the attribute in the database and updating the picker array in the viewDidLoad method.

Is there another method that fires before this so my saved data can be read just before the tab switch and the picker displays?

I've also tried putting the database retrieval code in viewWillAppear method thinking it will fire before the view appears, read my data, update the picker array and then display but no!!!

As I said, the data is being written and retrieved fine....it's just that I have to stop the simulator and start it to fill the picker.

Any thoughts?

Cheers,

Gaz.

Bristol England.

Answer 1

viewWillAppear: (or viewDidAppear:) is probably the correct place for this sort of behavior, please investigate and explain why you do not see the behavior you expected in that case.

viewDidLoad is called when a view controller's view is constructed (either built in loadView or "loaded" from a nib file). Since the view must be loaded before it can be displayed you can count on viewDidLoad being called before a view first appears but not before every time it appears. If the view is already loaded it will not need to be loaded again. If however the application receives a memory warning while a view is not visible it will be unloaded and then loaded again if it needs to be re-displayed. Thus viewDidLoad may be called many times during a controller's life but you cannot rely on it being called very time the view will appear, that's what viewWillAppear/viewDidAppear are for.

Answer 2

......okay......and an update on this question!

The way to update the picker that I used in the viewWillAppear method is to use

[self.projectPicker reloadAllComponents];

(where the projectPicker is the name of your picker.

Make sure you place it at the end of the viewWillAppear method after you've updated your picker arrays!

Gaz.

Answer 3

viewWillAppear method will be called everytime view appears. So that is the correct place to retrieve the data.

Answer 4

.....in the spirit of other newbies using search engines to find threads relating to issues they have, I thought I'd post this final entry to help anyone in the future that might find this!!!

After following the guys advice, I placed breakpoints and stepped through code line by line to establish what code was being executed and when.

I tracked down that the important methods that the picker calls, were only being fired on the app launch. When I switched between tabs, the picker wasn't reloading even though the data was being entered into the picker data array.

It does have a sense of irony because I refined my search after this, searching for "reloading picker view data".

The first search returned me to an old thread on here where a fellow forum member was having the same problem.

One very small line of code had me doing cartwheels!!!

I followed his advice and placed the line.....self.picker.delegate=self; in the viewWillAppear method......everything now updates between tab switches!!

I may have to bow to one of the guru's here to explain why this line makes all the difference.......it's important for me to know as it's been a revelation to me.

(I replaced picker with my pickername)

Can anyone offer a explanation of why this statement made the picker update in terms a newbie can understand??...I'd really appreciate it.....

Gaz.