iOS如何设置滚动文字边缘渐变

设计上有一个这种文字滚动到屏幕外的时候,滚动到边缘进出场需要添加渐变透明度的需求 ,这个以前没有做过于是网上找了找

iOS文字颜色渐变透明 可惜它这个只支持半边的渐变,方向有上下或者左右啊,已经不能满足需求了,它的原理是设置渐变的maskLayer

于是,照猫画虎,写了两个渐变layer当成subLayer加上去,效果还行,就是颜色需要再调整调整

    CGFloat SCREEN_WIDTH = UIScreen.mainScreen.bounds.size.width;
    CGFloat SCREEN_HEIGHT = UIScreen.mainScreen.bounds.size.height;
    NSString *text = @"\t壬戌之秋，七月既望，\r\n苏子与客泛舟游于赤壁之下。\r\n清风徐来，水波不兴。\r\n举酒属客，诵明月之诗，\r\n歌窈窕之章。少焉，月出于东山之上，\r\n徘徊于斗牛之间。\r\n白露横江，水光接天。纵一苇之所如，\r\n凌万顷之茫然。浩浩乎如冯虚御风，\t\r\n而不知其所止；飘飘乎如遗世独立，\r\n羽化而登仙。\n\t于是饮酒乐甚，扣舷而歌之。\r\n歌曰：“桂棹兮兰桨，击空明兮溯流光。\r\n渺渺兮予怀，望美人兮天一方。\r\n”客有吹洞箫者，倚歌而和之。\r\n其声呜呜然，如怨如慕，\r\n如泣如诉，余音袅袅，\r\n不绝如缕。舞幽壑之潜蛟，\r\n泣孤舟之嫠妇。\n\t苏子愀然，正襟危坐而问客曰：\r\n“何为其然也？”客曰：“‘月明星稀，乌鹊南飞’，此非曹孟德之诗乎？西望夏口，\r\n东望武昌，山川相缪，\r\n郁乎苍苍，此非孟德之困于周郎者乎？\r\n方其破荆州，下江陵，顺流而东也，舳舻千里，\r\n旌旗蔽空，酾酒临江，横槊赋诗，固一世之雄也，\r\n而今安在哉？况吾与子渔樵于江渚之上，侣鱼虾而友麋鹿，\r\n驾一叶之扁舟，举匏樽以相属。寄蜉蝣于天地，渺沧海之一粟。哀吾生之须臾，\r\n羡长江之无穷。挟飞仙以遨游，抱明月而长终。知不可乎骤得，托遗响于悲风。”\n\t苏子曰：“客亦知夫水与月乎？逝者如斯，而未尝往也；盈虚者如彼，而卒莫消长也。盖将自其变者而观之，则天地曾不能以一瞬；自其不变者而观之，则物与我皆无尽也。而又何羡乎！且夫天地之间，物各有主，苟非吾之所有，虽一毫而莫取。惟江上之清风，与山间之明月，\r\n耳得之而为声，目遇之而成色，\r\n取之无禁，用之不竭，是造物者之无尽藏也，\r\n而吾与子之所共适。”\n\t客喜而笑，洗盏更酌。肴核既尽，\r\n杯盘狼籍。相与枕藉乎舟中，不知东方之既白。";
    
    UITextView *textView = [[UITextView alloc] initWithFrame:CGRectMake(20, 100, SCREEN_WIDTH - 40 , 100)];
    textView.editable = NO;
    textView.text = text;
    textView.contentInset = UIEdgeInsetsMake(50, 0, 0, 0);
    textView.contentOffset = CGPointMake(0, -50);
    textView.backgroundColor = [UIColor whiteColor];
    [self.view addSubview:textView];
    textView.scrollEnabled = NO;
    CGFloat textHeight =  [textView sizeThatFits:CGSizeMake(SCREEN_WIDTH-40, MAXFLOAT)].height;
    textView.scrollEnabled = YES;
    textView.contentSize = CGSizeMake(SCREEN_WIDTH-40, textHeight*2);
    textView.frame = CGRectMake(20, 100, SCREEN_WIDTH - 40 , textHeight);
    
    NSArray *colors = [NSArray arrayWithObjects:
                       (id)[[[UIColor whiteColor] colorWithAlphaComponent:0.5] CGColor],
                       (id)[[[UIColor whiteColor] colorWithAlphaComponent:0.75] CGColor],
                       (id)[[[UIColor whiteColor] colorWithAlphaComponent:1.0] CGColor],
                        nil];
    CAGradientLayer *topLayer = [CAGradientLayer layer];
    topLayer.colors = colors;
    topLayer.startPoint = CGPointMake(0, 1);
    topLayer.endPoint = CGPointMake(0, 0);
    topLayer.frame = CGRectMake(20, 100, SCREEN_WIDTH-40, 50);
    [self.view.layer addSublayer:topLayer];

    CAGradientLayer *bottomLayer = [CAGradientLayer layer];
    bottomLayer.colors = colors;
    bottomLayer.startPoint = CGPointMake(0, 0);
    bottomLayer.endPoint = CGPointMake(0, 1);
    bottomLayer.frame = CGRectMake(20, textHeight+50,SCREEN_WIDTH-40, 50);
    [self.view.layer addSublayer:bottomLayer];