1.全屏图片设置

大致思路是判断屏幕高度来选择不同图片，用一个分类封装起来直接调用

typedef NS_ENUM(NSInteger, thisDeviceClass) {
    
    thisDeviceClass_iPhone5,
    thisDeviceClass_iPhone6,
    thisDeviceClass_iPhone6plus,
    thisDeviceClass_iPhoneX,
    
    thisDeviceClass_unknown
};

thisDeviceClass currentDeviceClass();

@interface UIImage (DeviceSpecificMedia)

+ (instancetype )imageForDeviceWithName:(NSString *)fileName;

@end

#import "UIImage+DeviceSpecificMedia.h"

thisDeviceClass currentDeviceClass() {
    
    CGFloat greaterPixelDimension = (CGFloat) fmaxf(((float)[[UIScreen mainScreen]bounds].size.height),
                                                    ((float)[[UIScreen mainScreen]bounds].size.width));
    
    switch ((NSInteger)greaterPixelDimension) {
        case 568:
            return thisDeviceClass_iPhone5;
            break;
        case 667:
            return thisDeviceClass_iPhone6;
            break;
        case 736:
            return thisDeviceClass_iPhone6plus;
            break;
        case 812:
            return thisDeviceClass_iPhoneX;
            break;
        default:
            return thisDeviceClass_unknown;
            break;
    }
}

@implementation UIImage (DeviceSpecificMedia)

+ (NSString *)magicSuffixForDevice
{
    switch (currentDeviceClass()) {
        case thisDeviceClass_iPhone5:
            return @"-568h";
            break;
        case thisDeviceClass_iPhone6:
            return @"-667h"; //or some other arbitrary string..
            break;
        case thisDeviceClass_iPhone6plus:
            return @"-736h";
            break;
            
        case thisDeviceClass_iPhoneX:
            return @"-812h";
            break;
            
        case thisDeviceClass_unknown:
        default:
            return @"";
            break;
    }
}

+ (instancetype )imageForDeviceWithName:(NSString *)fileName
{
    UIImage *result = nil;
    NSString *nameWithSuffix = [fileName stringByAppendingString:[UIImage magicSuffixForDevice]];
    
    result = [UIImage imageNamed:nameWithSuffix];
    if (!result) {
        result = [UIImage imageNamed:fileName];
    }
    return result;
}

@end

2.渐变色图片生成
首先用CAGradientLayer生成渐变的图层：

+ (CAGradientLayer *)layerFromStartColor:(UIColor *)startColor stopColor:(UIColor *)stopColor frame:(CGRect)frame
{
    CAGradientLayer *gradientLayer = [CAGradientLayer layer];
    gradientLayer.colors = @[(__bridge id)startColor.CGColor,(__bridge id)stopColor.CGColor];
    gradientLayer.locations = @[@0.0,@1.0];
    gradientLayer.startPoint = CGPointMake(0, 0);
    gradientLayer.endPoint = CGPointMake(1.0, 0);
    gradientLayer.frame = frame;
    
    return gradientLayer;
}

这里我简化了一下，因为一般只用简单的首尾两种颜色渐变，当然可以设置多种颜色。

然后将图层(CALayer)转换成图片(UIImage)

+ (UIImage *)imageFromLayer:(CALayer *)layer
{
    UIGraphicsBeginImageContextWithOptions(layer.frame.size, NO, 0);
    
    [layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *outputImage = UIGraphicsGetImageFromCurrentImageContext();
    
    UIGraphicsEndImageContext();
    
    return outputImage;
}