使用maskView来实现下面的效果:

效果图
首先我们要知道,maskView的定义,这是一个视图的遮罩视图,要实现以上的效果需要和CAGradientLayer配合使用,话不多说,直接上代码.
为了方便使用,对这种效果进行了封装,类名为LayerContentView,继承于UIView.

LayerContentView

LayerContentView.h文件
#import <UIKit/UIKit.h>
@interface LayerContentView : UIView
@property (nonatomic, strong) UILabel *label;
//遮罩视图
@property (nonatomic, strong) UIView *contentMaskView;
@property (nonatomic, strong) NSString *text;
//向右动画
- (void)fadeRight;
@end

LayerContentView.m文件
#import "LayerContentView.h"

@implementation LayerContentView
@synthesize text = _text;

- (instancetype)initWithFrame:(CGRect)frame {
    if ([super initWithFrame:frame]) {
        //创建label
        [self creatLabel:self.bounds];
        //创建遮罩图层
        [self creatMaskView:self.bounds];
    }
    return self;
}

- (void)creatLabel:(CGRect)frame {
    _label = [[UILabel alloc] initWithFrame:frame];
    _label.font = [UIFont boldSystemFontOfSize:20];
    _label.textColor = [UIColor redColor];
    _label.textAlignment = NSTextAlignmentCenter;
    [self addSubview:_label];
}

- (void)creatMaskView:(CGRect)frame {
    _contentMaskView = [[UIView alloc] initWithFrame:frame];
    //首先要创建出渐变的layer(核心代码)
    CAGradientLayer *gradientLayer = [CAGradientLayer layer];
    gradientLayer.frame = frame;
    gradientLayer.colors = @[(__bridge id)[UIColor clearColor].CGColor, (__bridge id)[UIColor blackColor].CGColor, (__bridge id)[UIColor clearColor].CGColor];
    gradientLayer.locations = @[@(0.01), @(0.1), @(0.9), @(0.99)];
    gradientLayer.startPoint = CGPointMake(0, 0);
    gradientLayer.endPoint = CGPointMake(1, 0);
    //将gradientLayer添加到我们的遮罩的view的layer上
    [_contentMaskView.layer addSublayer:gradientLayer];
    //设置self的遮罩视图为_contentMaskView
    self.maskView = _contentMaskView;
}

//动画
- (void)fadeRight {
    [UIView animateWithDuration:2.0 delay:0.0 options:UIViewAnimationOptionCurveEaseOut animations:^{
        CGRect frame = _contentMaskView.frame;
        frame.origin.x += frame.size.width;
        _contentMaskView.frame = frame;
    } completion:^(BOOL finished) {
        NSLog(@"完成");
        //不需要循环动画的可以将finished的block中的代码注释掉(也就是下面的代码)
        [UIView animateWithDuration:2.0 delay:0.0 options:UIViewAnimationOptionCurveEaseOut animations:^{
            CGRect frame = _contentMaskView.frame;
            frame.origin.x -= frame.size.width;
            _contentMaskView.frame = frame;
        } completion:^(BOOL finished) {
            [self fadeRight];
        }];
    }];
}

- (void)setText:(NSString *)text {
    _text = text;
    _label.text = text;
}

- (NSString *)text {
    return _text;
}
@end

//控制器中的调用
#import "ThirdViewController.h"
#import "LayerContentView.h"

@interface ThirdViewController ()
@property (nonatomic, strong) LayerContentView *layerContentView;
@end

@implementation ThirdViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    self.title = @"maskView";
    self.view.backgroundColor = [UIColor whiteColor];
    _layerContentView = [[LayerContentView alloc] initWithFrame:CGRectMake(0, 200, iPhoneWidth, 30)];
    _layerContentView.text = @"西河老伯iOS开发";
    [self.view addSubview:_layerContentView];
    //设置动画开始
    [_layerContentView fadeRight];
}

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