我有一个UITabBarController，它连接了3个视图控制器。除了从一个视图切换到另一个视图时，一切都很好，新视图需要花一点时间来重新加载它正在做的事情。我知道这不是什么大问题，但它只是让应用看起来有点草率。有没有人知道如何让应用程序在后台运行，或者不需要每次都重新加载。
你可能已经注意到我对Objective-C很新，所以我可以理解我是否不清楚，但是真的很感激任何帮助！
编辑：为大卫
这是.m文件中秒表的代码：
@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
大卫的第二次编辑：
将代码的主要部分更改为：
- (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;
}
我将代码分开，以便在视图出现之前它应该更新条形图。但是，它没有用。我通过在某些点打印出一些变量的值来做一些调查。看来在viewWillAppear中，secondBar.frame（和minuteBar等）已更新到正确的高度。但是，在viewDidAppear中，该值将重置为0.这不会发生在secondInt或secondFrame中。在这两种方法之间，框架被重置，但我无法弄明白。
答案 0 :(得分：1)
根据我的理解，当您在标签之间切换时，属性self.hourBar，self.minuteBar和self.secondBar的框架设置为0，并且每秒只更新一次。
如果确实如此，只需在viewControllers viewWillAppear:方法中将它们设置为正确的值（将它们分配给viewWillDisappear:中的某个属性）。
作为旁注，你似乎来自C ++。 “ - ＆gt;”对于Objective-C，表示法非常罕见，因为使用“。”访问属性，并使用“ - ＆gt;”访问它们对应的实例变量。使用箭头符号将不调用属性的自动合成getter / setter方法！
此外，是否有一个特定原因，您始终创建新的NSTimer对象而不是将repeats:设置为是？创建一个计时器并将其添加到runloop（scheduledTimerWith:...执行）是一个相对昂贵的操作。