iPhone: 对象沿路径动画

这里我将展示如何在UIView上让对象沿着路径动画，我将创建路径并画到UIView上让你能都看见，并沿相同的路径来做动画。

我在添加到屏幕的UIView完成所有的这些…

首先，我们在屏幕上画一条曲线

//This draws a quadratic bezier curved line right across the screen
- ( void ) drawACurvedLine {
//Create a bitmap graphics context, you will later get a UIImage from this
UIGraphicsBeginImageContext(CGSizeMake(320,460));
CGContextRef ctx = UIGraphicsGetCurrentContext();

//Set variables in the context for drawing
CGContextSetLineWidth(ctx, 1.5);
CGContextSetStrokeColorWithColor(ctx, [UIColor whiteColor].CGColor);

//Set the start point of your drawing
CGContextMoveToPoint(ctx, 10, 10);
//The end point of the line is 310,450 .... i'm also setting a reference point of 10,450
//A quadratic bezier curve is drawn using these coordinates - experiment and see the results.
CGContextAddQuadCurveToPoint(ctx, 10, 450, 310, 450);
//Add another curve, the opposite of the above - finishing back where we started
CGContextAddQuadCurveToPoint(ctx, 310, 10, 10, 10);

//Draw the line
CGContextDrawPath(ctx, kCGPathStroke);

//Get a UIImage from the current bitmap context we created at the start and then end the image context
UIImage *curve = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

//With the image, we need a UIImageView
UIImageView *curveView = [[UIImageView alloc] initWithImage:curve];
//Set the frame of the view - which is used to position it when we add it to our current UIView
curveView.frame = CGRectMake(1, 1, 320, 460);
curveView.backgroundColor = [UIColor clearColor];
[self addSubview:curveView];
}

现在我我创建了一个关键帧动画，并添加一个和我们话得线一样的路径。我们还将画一个圈，并沿着路径做动画:

- (void) animateCicleAlongPath {
//Prepare the animation - we use keyframe animation for animations of this complexity
CAKeyframeAnimation *pathAnimation = [CAKeyframeAnimation animationWithKeyPath:@"position"];
//Set some variables on the animation
pathAnimation.calculationMode = kCAAnimationPaced;
//We want the animation to persist - not so important in this case - but kept for clarity
//If we animated something from left to right - and we wanted it to stay in the new position,
//then we would need these parameters
pathAnimation.fillMode = kCAFillModeForwards;
pathAnimation.removedOnCompletion = NO;
pathAnimation.duration = 5.0;
//Lets loop continuously for the demonstration
pathAnimation.repeatCount = 1000;

//Setup the path for the animation - this is very similar as the code the draw the line
//instead of drawing to the graphics context, instead we draw lines on a CGPathRef
CGPoint endPoint = CGPointMake(310, 450);
CGMutablePathRef curvedPath = CGPathCreateMutable();
CGPathMoveToPoint(curvedPath, NULL, 10, 10);
CGPathAddQuadCurveToPoint(curvedPath, NULL, 10, 450, 310, 450);
CGPathAddQuadCurveToPoint(curvedPath, NULL, 310, 10, 10, 10);

//Now we have the path, we tell the animation we want to use this path - then we release the path
pathAnimation.path = curvedPath;
CGPathRelease(curvedPath);

//We will now draw a circle at the start of the path which we will animate to follow the path
//We use the same technique as before to draw to a bitmap context and then eventually create
//a UIImageView which we add to our view
UIGraphicsBeginImageContext(CGSizeMake(20,20));
CGContextRef ctx = UIGraphicsGetCurrentContext();
//Set context variables
CGContextSetLineWidth(ctx, 1.5);
CGContextSetFillColorWithColor(ctx, [UIColor greenColor].CGColor);
CGContextSetStrokeColorWithColor(ctx, [UIColor whiteColor].CGColor);
//Draw a circle - and paint it with a different outline (white) and fill color (green)
CGContextAddEllipseInRect(ctx, CGRectMake(1, 1, 18, 18));
CGContextDrawPath(ctx, kCGPathFillStroke);
UIImage *circle = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

UIImageView *circleView = [[UIImageView alloc] initWithImage:circle];
circleView.frame = CGRectMake(1, 1, 20, 20);
[self addSubview:circleView];

//Add the animation to the circleView - once you add the animation to the layer, the animation starts
[circleView.layer addAnimation:pathAnimation forKey:@"moveTheSquare"];
}

要让所有的都跑起来，你可以使用init函数：

- (id)initWithFrame:(CGRect)frame {
if (self = [super initWithFrame:frame]) {
[self drawACurvedLine];
[self animateCicleAlongPath];
}
return self;
}

在你的ViewController中像这样写

- (void)viewDidLoad {
UIView *customView = [[Canvas alloc] initWithFrame:CGRectMake(0, 0, 320, 460)];
customView.backgroundColor = [UIColor blackColor];
[self.view addSubview:customView];
[customView release];
[super viewDidLoad];
}

还有，不要忘记添加 Quartz 引用:

#import <QuartzCore/QuartzCore.h>

我确定有很多更好的方式来做这个事，例如使用CALayers 和添加CGImage 到layers。但是那是一些我没有尝试的东西。上面的例子应该足够让你开始沿着路径做动画。



这里是工程拷贝http://blog.devedup.com/wp-content/uploads/2010/06/AnimateAlongAPath.zip





（译者水平有限，欢迎指正。）