最近做的項目有一個需求，就是讓地球在圓圈里面從右到左運動，而且在運動的過程中不能出現(xiàn)空白，就是一直平滑的運動。
剛開始可把我給愁死了，首先竟然傻傻的想到用關鍵幀動畫，結果搞了一下午也沒搞出來。第二天就換了一種思路，但是會出現(xiàn)卡頓的情況。后來經過大BOSS的指導，一會兒的功夫就搞出來了。請允許我小小的汗顏一下。
接下來言歸正傳，先看看效果圖：

animation.gif

用到的圖片是這樣的：

image_earth_5.png

原理大概是這樣的：這是倆張相同的圖片結合起來的，每個UIImageView的大小是圓環(huán)的二倍。第一個UIImageView的起始位置是圓環(huán)的最左面，第二個UIImageView的起始位置是第一個UIImageView的最右邊。每張圖片的動畫距離是UIImageView的寬度。當?shù)谝粡圲IImageView移動結束，第二個UIImageView移動到一半，此時第一個UIImageView重新開始運動，就不會出現(xiàn)圖片突然出現(xiàn)，感覺像是跳了一下的感覺。大概就是這個意思，讀者能看明白吧。下面就是代碼啦！

@property(nonatomic, strong) UIView *circleView;
@property(nonatomic, strong) UIImageView *frontEarthImg;
@property(nonatomic, strong) UIImageView *behindEarthImg;
@property(nonatomic, strong) UIButton *bottomBtn ;

-(void) setupUI {
    _circleView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, RYScreenWidth * 0.6 , RYScreenWidth * 0.6)];
    _circleView.center = self.view.center;
    _circleView.backgroundColor = UIColorFromRGB(0x4ba0ef);
    _circleView.clipsToBounds = YES;
    _circleView.layer.masksToBounds = YES;
    _circleView.layer.cornerRadius = _circleView.frame.size.width /2;
    [self.view addSubview:_circleView];
    UIImage *image = [UIImage imageNamed:@"image_earth.png"];
    CGFloat earthWidth = RYScreenWidth * 0.6 * 2;
    CGFloat earthHeight = earthWidth * image.size.height / image.size.width ;
    CGFloat imgY = (earthWidth /2 - earthHeight)/2;
    _frontEarthImg = [[UIImageView alloc ] initWithFrame:CGRectMake(0, imgY, earthWidth, earthHeight)];
    _frontEarthImg.image = image;
    [_circleView addSubview:_frontEarthImg];
    _behindEarthImg = [[UIImageView alloc ] initWithFrame:CGRectMake(earthWidth, imgY, earthWidth, earthHeight)];
    _behindEarthImg.image = image;
    [_circleView addSubview:_behindEarthImg];
    _bottomBtn = [UIButton buttonWithType:UIButtonTypeCustom];
    _bottomBtn.backgroundColor = UIColorFromRGB(0x4ba0ef);
    _bottomBtn.frame = CGRectMake(RYScreenWidth * 0.3, RYScreenHeight * 0.7 , RYScreenWidth * 0.4 , 40);
    [_bottomBtn setTitle:@"開始動畫" forState:UIControlStateNormal];
    [_bottomBtn setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
    [_bottomBtn addTarget:self action:@selector(bottomBtnClick) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:_bottomBtn];
}

-(void) bottomBtnClick {
    if ([_bottomBtn.titleLabel.text isEqualToString:@"開始動畫"]) {
        [_bottomBtn setTitle:@"停止動畫" forState:UIControlStateNormal];
        [self startAnimation];
    } else if ([_bottomBtn.titleLabel.text isEqualToString:@"停止動畫"]){
        [_bottomBtn setTitle:@"開始動畫" forState:UIControlStateNormal];
        [self stopAnimation];
    }
}

-(void)startAnimation {
    CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"position.x"];
    animation.repeatCount = MAXFLOAT;
    animation.fillMode= kCAFillModeForwards;
    animation.removedOnCompletion = NO;
    animation.duration = 8.0;
    animation.toValue = [NSNumber numberWithFloat:-1 * _frontEarthImg.frame.size.width];
    [_frontEarthImg.layer addAnimation:animation forKey:nil];
    animation.toValue = [NSNumber numberWithFloat:0];
    [_behindEarthImg.layer addAnimation:animation forKey:nil];
}

-(void)stopAnimation {
    [_frontEarthImg.layer removeAllAnimations];
    [_behindEarthImg.layer removeAllAnimations];
}

寫的不對的地方請多多指教。