Problems with view controllers going into the background of UITabBarController - ios

Question

I have a UITabBarController with 3 view controllers connected to it. Everything works fine apart from when you switch from one view to another, the new view takes a moment to reload what it was doing. I know this isn't such a big deal but it just makes the app look a bit sloppy. Does anyone know how I can keep the app running in the background, or something so that it doesn't have to reload each time.
As you might have noticed I'm very new to Objective-C so I can understand if I'm being unclear but any help is really appreciated!
EDIT: FOR DAVID
This is the code for the stopwatch in the .m file:
#implementation StopwatchViewController
- (BOOL)prefersStatusBarHidden
{
return YES;
}
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self) {
// Custom initialization
}
return self;
}
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view from its nib.
isCounting = false;
}
- (IBAction)startOrStop:(id)sender
{
if (self->isCounting == false) {
self->isCounting = true;
[self startStopwatch];
} else {
self->isCounting = false;
[self stopStopwatch];
}
}
- (void)startStopwatch
{
[startStopButton setTitle:#"STOP" forState:UIControlStateNormal];
[self performSelector:#selector(stopwatch) withObject:self afterDelay:1.0];
}
- (IBAction)resetStopwatch:(id)sender
{
[self reset];
}
- (void)stopwatch
{
if (self->isCounting == false) {
return;
} else {
NSInteger hourInt = [hourLabel.text intValue];
NSInteger minuteInt = [minuteLabel.text intValue];
NSInteger secondInt = [secondLabel.text intValue];
if (secondInt == 59) {
secondInt = 0;
if (minuteInt == 59) {
minuteInt = 0;
if (hourInt == 23) {
hourInt = 0;
} else {
hourInt += 1;
}
} else {
minuteInt += 1;
}
} else {
secondInt += 1;
}
NSString *hourString = [NSString stringWithFormat:#"%02d", hourInt];
NSString *minuteString = [NSString stringWithFormat:#"%02d", minuteInt];
NSString *secondString = [NSString stringWithFormat:#"%02d", secondInt];
hourLabel.text = hourString;
minuteLabel.text = minuteString;
secondLabel.text = secondString;
CGRect hourFrame = self->hourBar.frame;
CGRect minuteFrame = self->minuteBar.frame;
CGRect secondFrame = self->secondBar.frame;
if ((NSInteger)hourFrame.size.height != hourInt) { // check if we need to modify
hourFrame.origin.y -= ((hourInt * 10.0) - hourFrame.size.height);
hourFrame.size.height = (hourInt * 10.0);
self->hourBar.frame = hourFrame;
}
if ((NSInteger)minuteFrame.size.height != minuteInt) { // check if we need to modify
minuteFrame.origin.y -= ((minuteInt * 4.0) - minuteFrame.size.height);
minuteFrame.size.height = (minuteInt * 4.0);
self->minuteBar.frame = minuteFrame;
}
if ((NSInteger)secondFrame.size.height != secondInt) { // check if we need to modify
secondFrame.origin.y -= ((secondInt * 4.0) - secondFrame.size.height);
secondFrame.size.height = (secondInt * 4.0);
self->secondBar.frame = secondFrame;
}
[NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:#selector(stopwatch) userInfo:nil repeats:NO];
}
}
- (void)stopStopwatch
{
[startStopButton setTitle:#"START" forState:UIControlStateNormal];
}
- (void)reset
{
[startStopButton setTitle:#"START" forState:UIControlStateNormal];
self->isCounting = false;
hourLabel.text = #"00";
minuteLabel.text = #"00";
secondLabel.text = #"00";
CGRect hourFrame = self->hourBar.frame;
CGRect minuteFrame = self->minuteBar.frame;
CGRect secondFrame = self->secondBar.frame;
hourFrame.size.height = 0;
minuteFrame.size.height = 0;
secondFrame.size.height = 0;
hourFrame.origin.y = 321.0;
minuteFrame.origin.y = 321.0;
secondFrame.origin.y = 321.0;
self->hourBar.frame = hourFrame;
self->minuteBar.frame = minuteFrame;
self->secondBar.frame = secondFrame;
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#end
SECOND EDIT FOR DAVID:
Changed the main parts of the code to look like this:
- (void)viewWillDisappear:(BOOL)animated
{
[super viewWillDisappear:YES];
[self swapFrames];
}
- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:YES];
[self updateBars];
}
- (void)stopwatch
{
if (self->isCounting == false) {
return;
} else {
hourInt = [hourLabel.text intValue];
minuteInt = [minuteLabel.text intValue];
secondInt = [secondLabel.text intValue];
if (secondInt == 59) {
secondInt = 0;
if (minuteInt == 59) {
minuteInt = 0;
if (hourInt == 23) {
hourInt = 0;
} else {
hourInt += 1;
}
} else {
minuteInt += 1;
}
} else {
secondInt += 1;
}
NSString *hourString = [NSString stringWithFormat:#"%02d", hourInt];
NSString *minuteString = [NSString stringWithFormat:#"%02d", minuteInt];
NSString *secondString = [NSString stringWithFormat:#"%02d", secondInt];
hourLabel.text = hourString;
minuteLabel.text = minuteString;
secondLabel.text = secondString;
[self swapFrames];
[self updateBars];
[NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:#selector(stopwatch) userInfo:nil repeats:NO];
}
}
- (void)updateBars
{
if ((NSInteger)hourFrame.size.height != hourInt) { // check if we need to modify
hourFrame.origin.y -= ((hourInt * 10.0) - hourFrame.size.height);
hourFrame.size.height = (hourInt * 10.0);
self->hourBar.frame = hourFrame;
}
if ((NSInteger)minuteFrame.size.height != minuteInt) { // check if we need to modify
minuteFrame.origin.y -= ((minuteInt * 4.0) - minuteFrame.size.height);
minuteFrame.size.height = (minuteInt * 4.0);
self->minuteBar.frame = minuteFrame;
}
if ((NSInteger)secondFrame.size.height != (secondInt * 4.0)) { // check if we need to modify
secondFrame.origin.y -= ((secondInt * 4.0) - secondFrame.size.height);
secondFrame.size.height = (secondInt * 4.0);
self->secondBar.frame = secondFrame;
}
}
- (void)swapFrames
{
hourFrame = self->hourBar.frame;
minuteFrame = self->minuteBar.frame;
secondFrame = self->secondBar.frame;
}
I separated the code so that just before the view appear it should update the bars. However, it did not work. I did some investigating by printing out the values of some of the variables at certain points. It appears that in viewWillAppear, secondBar.frame (and minuteBar, etc.) has updated to the correct height. However, in viewDidAppear, that value is reset to 0. This does not happen to secondInt, or secondFrame. Somewhere between those two methods the frame is reset but I cannot figure it out.

Answer 1

From what I understand, the frames for your properties self.hourBar, self.minuteBar and self.secondBar are set to 0 when you switch between tabs, and only update them every second.
If this is indeed the case, just set them to their correct values in your viewControllers viewWillAppear: method (assign them to some property in viewWillDisappear:).
As a sidenote, you seem to be coming from C++. The "->" notation is very uncommon for Objective-C, since properties are accessed with ".", and their corresponding instance variable with "->". Using the arrow notation will not call the auto-synthesised getter/setter methods of properties!
Also, is there a specific reason why you always create new NSTimer objects instead of setting repeats: to yes? Creating a timer and adding it to a runloop (which scheduledTimerWith:... does) is a relatively costly operation.