View jumps back after setting center in iOS 8 - objective-c

Question

I’m beginner with Xcode, and i have problem with counter in my app.
When i put timer to count points the player(paddle) jump in the place where it was
in first time(in the middle)
everytime when timer count point.
So what i should do to keep paddle where it is?
Is there something other way to count points?
Its doesnt matter how to control player with swipe or g-sensor,
but in this example i control it with swipe.
And this problem appears only in ios 8.0> , not in ios 7.
Here some code:
.h file:
int ScoreNumber;
#interface ViewController : UIViewController{
IBOutlet UILabel *ScoreLabel;
NSTimer *Timer;
}
#property (nonatomic, strong)IBOutlet UIImageView *paddle;
#property (nonatomic) CGPoint paddleCenterPoint;
#end
.m file:
#implementation ViewController
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{
UITouch *touch = [touches anyObject];
CGPoint touchLocation = [touch locationInView:self.view];
CGFloat yPoint = self.paddleCenterPoint.y;
CGPoint paddleCenter = CGPointMake(touchLocation.x, yPoint);
self.paddle.center = paddleCenter;
}
-(void)Score{
ScoreNumber = ScoreNumber +1;
ScoreLabel.text = [NSString stringWithFormat:#"%i", ScoreNumber];
}
-(void)viewDidLoad {
Timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:#selector(Score) userInfo:nil repeats:YES];
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
}

Answer 1

In iOS 8, autolayout is king, and setting center doesn't change the layout constraints. This has nothing to do with the timer. It has to do with changing the label's text. When that happens, it triggers a layout of the whole view, and that reasserts the constraints (via layoutIfNeeded). Even though you're probably not setting any constraints in IB, Xcode will automatically insert them during the build.
There are several solutions. First, you could insert the paddle programmatically in viewDidLoad, which would bypass the constraints system. For this specific problem, that's probably ok (and it might even be how I'd do it personally). Something like this:
#interface ViewController ()
#property (nonatomic, strong) UIView *paddle;
#end
#implementation ViewController
- (void)viewDidLoad {
self.paddle = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 200, 50)];
self.paddle.backgroundColor = [UIColor blueColor];
[self.view addSubview:self.paddle];
}
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [touches anyObject];
CGPoint touchLocation = [touch locationInView:self.view];
self.paddle.center = CGPointMake(touchLocation.x, self.paddle.center.y);
}
#end
But how can we solve this without throwing away constraints? Well, we could just modify the constraint. Add your paddle view in IB, and add constraints for the left, bottom, width, and height. Then create an IB outlet for the horizontal and width constraints.
Edit the Horizontal Space Constraint to remove "Relative to margin" from both the first and second items:
Now, you can modify the constraint rather than the center:
#interface ViewController ()
#property (nonatomic, weak) IBOutlet UIView *paddle;
#property (weak, nonatomic) IBOutlet NSLayoutConstraint *paddleHorizontalConstraint;
#property (weak, nonatomic) IBOutlet NSLayoutConstraint *paddleWidthConstraint;
#end
#implementation ViewController
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [touches anyObject];
CGPoint touchLocation = [touch locationInView:self.view];
CGFloat width = self.paddleWidthConstraint.constant;
self.paddleHorizontalConstraint.constant = touchLocation.x - width/2;
}
#end
You need to read the width constraint because the frame or bounds may not be correct until viewDidLayoutSubviews: is called. But you don't want to update the constraint there, since that'll force another layout.