從上到下：
CAGradientLayer *gradient = [CAGradientLayer layer];
gradient.frame = self.view.bounds;
gradient.colors = [NSArray arrayWithObjects:(id)[[UIColor colorWithRed:74/255.0 green:198/255.0 blue:228/255.0 alpha:1] CGColor], (id)[[UIColor colorWithRed:104/255.0 green:215/255.0 blue:223/255.0 alpha:1] CGColor], (id)[[UIColor colorWithRed:107/255.0 green:217/255.0 blue:222/255.0 alpha:1] CGColor], (id)[[UIColor colorWithRed:156/255.0 green:238/255.0 blue:241/255.0 alpha:1] CGColor],nil];
[topView.layer insertSublayer:gradient atIndex:0];

從左到右
[nav.navigationBar.layer insertSublayer:[self gradientLayer] atIndex:0];

• (CAGradientLayer *)gradientLayer {
  CAGradientLayer *gradientLayer = [[CAGradientLayer alloc]init];
  // CGColor是無法放入數(shù)組中的，必須要轉(zhuǎn)型。
  gradientLayer.colors = @[
  (__bridge id)[UIColor colorWithRed:210 / 255.0 green:135 / 255.0 blue:35/255.0 alpha:1.0].CGColor,
  (__bridge id)[UIColor colorWithRed:219 / 255.0 green:83/ 255.0 blue:30 / 255.0 alpha:1.0].CGColor,
  (__bridge id)[UIColor colorWithRed:219 / 255.0 green:83 / 255.0 blue:35/255.0 alpha:1.0].CGColor,
  ];
  // 顏色分割線
  gradientLayer.locations = @[@0, @0.8,@1.5];
  // 顏色漸變的起點(diǎn)和終點(diǎn)，范圍為 (0~1.0, 0~1.0)
  gradientLayer.startPoint = CGPointMake(0, 0);
  gradientLayer.endPoint = CGPointMake(1.0, 0);
  gradientLayer.frame = CGRectMake(0, -20,[UIScreen mainScreen].bounds.size.width,64);
  return gradientLayer;
  }

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