接到给文字加渐变色的需求后，我和同事一筹莫展，在网上找各种资料，找到两种实现方式：
1、通过把label的layer做为一个渐变图层的mask，即只让文字部分遮住渐变图层
UILabel *label = [[UILabel alloc] initWithFrame:self.view.bounds];
label.textAlignment = NSTextAlignmentCenter;
label.frame = self.view.bounds;
label.backgroundColor = [UIColor clearColor];
label.font = [UIFont systemFontOfSize:20];
label.numberOfLines = 0;
label.text = @"炫彩字体 炫彩字体 炫彩字体 炫彩字体 炫彩字体 炫彩字体";
[self.view addSubview:label];
CAGradientLayer *layer = [[CAGradientLayer alloc] init];
layer.colors = @[(id)[UIColor redColor].CGColor, (id)[UIColor blueColor].CGColor];
layer.startPoint = CGPointZero;
layer.endPoint = CGPointMake(1, 1);
layer.frame = label.bounds;
layer.mask = label.layer; //把文字作为渐变图层的遮罩
[self.view.layer addSublayer:layer];
2、重写UILabel的绘制方法drawTextInRect:，从上下文生成图片，清除上下文内容，再把图片作为整个上下文的mask，最后在上下文里绘制渐变
[super drawTextInRect:rect]; //先绘制
CGContextRef context = UIGraphicsGetCurrentContext();
CGFloat width = rect.size.width;
CGFloat height = rect.size.height;
CGImageRef imageRef = CGBitmapContextCreateImage(context);//获得倒置的图片
CGContextClearRect(context, rect); //清除内容
CGContextTranslateCTM(context, 0, height); // 坐标转换
CGContextScaleCTM(context, 1.0f, -1.0f);
CGContextClipToMask(context, rect, imageRef);//将图片设置为mark
//划渐变色图层
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGGradientRef gradientRef = CGGradientCreateWithColors(colorSpaceRef, (__bridge CFArrayRef)gradientColors, NULL);
CGPoint startPoint = CGPointZero;
CGPoint endPoint = CGPointMake(width, height);;
CGContextDrawLinearGradient(context, gradientRef, startPoint, endPoint, kCGGradientDrawsBeforeStartLocation | kCGGradientDrawsAfterEndLocation);
CGColorSpaceRelease(colorSpaceRef);
CGImageRelease(imageRef);
CGGradientRelease(gradientRef);
毕竟是消息列表，就选择了性能稍微好点的第二种方式。万事大吉，炫彩字体就出来了。
image.png
但是后来发现如果文字中有表情，表情就成为了一个圆形色块，仔细一想应该是把表情的轮廓作为遮罩处理的。
image.png
怎么让表情不作为渐变色的遮罩呢，只想到一个方法，用CoreText绘制。先把当前上下文环境保存起来。首先只绘制普通文字，遇到表情，把表情的位置预留出来，并把表情和其位置信息保存起来供下次绘制。绘制完文字后，取到图片、清除上下文、坐标转换、设置mask，然后绘制渐变。然后恢复之前保存的上下文环境，并取到表情信息再一次绘制表情。
> CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSaveGState(context); //保存上下文环境
NSDictionary *attributes = @{NSFontAttributeName:self.font, NSForegroundColorAttributeName:self.textColor};
NSAttributedString *attributedString = [[NSAttributedString alloc] initWithString:self.text attributes:attributes];
CGFloat width = CGRectGetWidth(rect);
CGFloat height = CGRectGetHeight(rect);
//生成CTFrame
CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((__bridge CFAttributedStringRef)attributedString);
CGPathRef path = CGPathCreateWithRect(CGRectMake(0, 0, width, height), NULL);
CTFrameRef frameRef = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, 0), path, nil);
CFArrayRef CTLines = CTFrameGetLines(frameRef); //获取到每一行
NSInteger lineCount = CFArrayGetCount(CTLines);
// 绘制每一行的普通文字
CGFloat lineTop = 0;
NSMutableArray *emojis = [NSMutableArray array];
for (NSInteger i = 0; i < lineCount; i++) {
CTLineRef line = CFArrayGetValueAtIndex(CTLines, i);
CGFloat ascent, descent, leading;
CTLineGetTypographicBounds(line, &ascent, &descent, &leading);
CGFloat runHeight = ascent + fabs(descent) + fabs(leading);
NSArray *runArray = (__bridge NSArray *)CTLineGetGlyphRuns(line); //获取到每一个glyphRun
CGFloat startX = 0;
for (NSInteger j = 0; j< runArray.count; j++) {
CTRunRef runRef = (__bridge CTRunRef)runArray[j];
CFRange runRange = CTRunGetStringRange(runRef);
NSDictionary * attributes = (NSDictionary *)CTRunGetAttributes(runRef);
UIFont *font = [attributes objectForKey:@"NSFont"];
CGFloat runWidth = CTRunGetTypographicBounds(runRef, CFRangeMake(0, 0), 0, 0, 0);
NSString *str = [self.text substringWithRange:NSMakeRange(runRange.location, runRange.length)];
if ([font.familyName containsString:@"Emoji"]) { //判断是否是emoji表情
CGRect temRect = CGRectMake(startX,lineTop, runWidth, runHeight);
[emojis addObject:@{@"str":str, @"rect":[NSValue valueWithCGRect:temRect]}];
} else {
//这里不加上descent/2，文字会往上偏，这里还不清楚为什么，有大佬给指教下吗
CGRect temRect = CGRectMake(startX,lineTop + descent/2, runWidth, runHeight);
[str drawInRect:temRect withAttributes:attributes];
}
startX +=runWidth;
}
lineTop += runHeight;
}
//释放资源
CFRelease(framesetter);
CFRelease(frameRef);
CGPathRelease(path);
//获取图片
CGImageRef imageRef = CGBitmapContextCreateImage(context);
CGContextClearRect(context, rect);
//坐标转换
CGContextTranslateCTM(context, 0, self.bounds.size.height);
CGContextScaleCTM(context, 1.0f, -1.0f);
//将图片设置为mark
CGContextClipToMask(context, rect, imageRef);
//绘制渐变层
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGGradientRef gradientRef = CGGradientCreateWithColors(colorSpaceRef, (__bridge CFArrayRef)gradientColors, NULL);
CGPoint startPoint = CGPointZero;
CGPoint endPoint = CGPointMake(width, height);
CGContextDrawLinearGradient(context, gradientRef, startPoint, endPoint, kCGGradientDrawsBeforeStartLocation | kCGGradientDrawsAfterEndLocation);
//释放资源
CGColorSpaceRelease(colorSpaceRef);
CGImageRelease(imageRef);
CGGradientRelease(gradientRef);
//恢复上下文环境
CGContextRestoreGState(context);
//绘制表情
for (NSDictionary *emoji in emojis) {
NSString *str = emoji[@"str"];
CGRect rect = [emoji[@"rect"] CGRectValue];
[str drawInRect:rect withAttributes:attributes];
}
由于对CoreText了解有限，实现的也很蹩脚，不过总算能够满足效果了。
重点：后来，同事突然给我说，有一个更简单且完美的方法，直接设置textColor为渐变颜色就行了。
UIGraphicsBeginImageContextWithOptions(label.bounds.size, NO, [UIScreen mainScreen].scale);
CGContextRef context = UIGraphicsGetCurrentContext();
//绘制渐变层
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGGradientRef gradientRef = CGGradientCreateWithColors(colorSpaceRef,
(__bridge CFArrayRef)@[(id)[UIColor redColor].CGColor,(id)[UIColor greenColor].CGColor],
NULL);
CGPoint startPoint = CGPointZero;
CGPoint endPoint = CGPointMake(CGRectGetMaxX(label.bounds), CGRectGetMaxY(label.bounds));
CGContextDrawLinearGradient(context, gradientRef, startPoint, endPoint, kCGGradientDrawsBeforeStartLocation | kCGGradientDrawsAfterEndLocation);
//取到渐变图片
UIImage *gradientImage = UIGraphicsGetImageFromCurrentImageContext();
//释放资源
CGColorSpaceRelease(colorSpaceRef);
CGGradientRelease(gradientRef);
UIGraphicsEndImageContext();
label.textColor = [UIColor colorWithPatternImage:gradientImage];
image.png