基于 CoreText 实现的高性能 UITableView

引起UITableView卡顿比较常见的原因有cell的层级过多、cell中有触发离屏渲染的代码（譬如：cornerRadius、maskToBounds 同时使用）、像素是否对齐、是否使用UITableView自动计算cell高度的方法等。本文将从cell层级出发，以一个仿朋友圈的demo来讲述如何让列表保持顺滑，项目的源码可在文末获得。不可否认的是，过早的优化是魔鬼，请在项目出现性能瓶颈再考虑优化。

首先看看reveal上页面层级的效果图

1、绘制文本

使用core text可以将文本绘制在一个CGContextRef上，最后再通过UIGraphicsGetImageFromCurrentImageContext()生成图片，再将图片赋值给cell.contentView.layer，从而达到减少cell层级的目的。

绘制普通文本（譬如用户昵称）在context上，相关注释在代码里：

- (void)drawInContext:(CGContextRef)context withPosition:(CGPoint)p andFont:(UIFont *)font andTextColor:(UIColor *)color andHeight:(float)height andWidth:(float)width lineBreakMode:(CTLineBreakMode)lineBreakMode {

    CGSize size = CGSizeMake(width, height);

    // 翻转坐标系

    CGContextSetTextMatrix(context,CGAffineTransformIdentity);

    CGContextTranslateCTM(context,0,height);

    CGContextScaleCTM(context,1.0,-1.0);

    NSMutableDictionary * attributes = [StringAttributes attributeFont:font andTextColor:color lineBreakMode:lineBreakMode];

    // 创建绘制区域（路径）

    CGMutablePathRef path = CGPathCreateMutable();

    CGPathAddRect(path,NULL,CGRectMake(p.x, height-p.y-size.height,(size.width),(size.height)));

    // 创建AttributedString

    NSMutableAttributedString *attributedStr = [[NSMutableAttributedString alloc] initWithString:self attributes:attributes];

    CFAttributedStringRef attributedString = (__bridge CFAttributedStringRef)attributedStr;

    // 绘制frame

    CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)attributedString);

    CTFrameRef ctframe = CTFramesetterCreateFrame(framesetter, CFRangeMake(0,0),path,NULL);

    CTFrameDraw(ctframe,context);

    CGPathRelease(path);

    CFRelease(framesetter);

    CFRelease(ctframe);

    [[attributedStr mutableString] setString:@""];

    CGContextSetTextMatrix(context,CGAffineTransformIdentity);

    CGContextTranslateCTM(context,0, height);

    CGContextScaleCTM(context,1.0,-1.0);

}

绘制朋友圈内容文本（带链接）在context上，这里我还没有去实现文本多了会折叠的效果，与上面普通文本不同的是这里需要创建带链接的AttributeString和CTLineRef的逐行绘制：

- (NSMutableAttributedString *)highlightText:(NSMutableAttributedString *)coloredString{

    // 创建带高亮的AttributedString

    NSString* string = coloredString.string;

    NSRange range = NSMakeRange(0,[string length]);

    NSDataDetector *linkDetector = [NSDataDetector dataDetectorWithTypes:NSTextCheckingTypeLink error:nil];

    NSArray *matches = [linkDetector matchesInString:string options:0 range:range];

    for(NSTextCheckingResult* match in matches) {

        [self.ranges addObject:NSStringFromRange(match.range)];

        UIColor *highlightColor = UIColorFromRGB(0x297bc1);

        [coloredString addAttribute:(NSString*)kCTForegroundColorAttributeName

                              value:(id)highlightColor.CGColor range:match.range];

    }

    return coloredString;

}

- (void)drawFramesetter:(CTFramesetterRef)framesetter

       attributedString:(NSAttributedString *)attributedString

              textRange:(CFRange)textRange

                 inRect:(CGRect)rect

                context:(CGContextRef)c {

    CGMutablePathRef path = CGPathCreateMutable();

    CGPathAddRect(path, NULL, rect);

    CTFrameRef frame = CTFramesetterCreateFrame(framesetter, textRange, path, NULL);

    CGFloat ContentHeight = CGRectGetHeight(rect);

    CFArrayRef lines = CTFrameGetLines(frame);

    NSInteger numberOfLines = CFArrayGetCount(lines);

    CGPoint lineOrigins[numberOfLines];

    CTFrameGetLineOrigins(frame, CFRangeMake(0, numberOfLines), lineOrigins);

    // 遍历每一行

    for (CFIndex lineIndex = 0; lineIndex < numberOfLines; lineIndex++) {

        CGPoint lineOrigin = lineOrigins[lineIndex];

        CTLineRef line = CFArrayGetValueAtIndex(lines, lineIndex);

        CGFloat descent = 0.0f, ascent = 0.0f, lineLeading = 0.0f;

        CTLineGetTypographicBounds((CTLineRef)line, &ascent, &descent, &lineLeading);

        CGFloat penOffset = (CGFloat)CTLineGetPenOffsetForFlush(line, NSTextAlignmentLeft, rect.size.width);

        CGFloat y = lineOrigin.y - descent - self.font.descender;

        // 设置每一行位置

        CGContextSetTextPosition(c, penOffset + self.xOffset, y - self.yOffset);

        CTLineDraw(line, c);

        // CTRunRef同一行中文本的不同样式，包括颜色、字体等，此处用途为处理链接高亮

        CFArrayRef runs = CTLineGetGlyphRuns(line);

        for (int j = 0; j < CFArrayGetCount(runs); j++) {

            CGFloat runAscent, runDescent, lineLeading1;

            CTRunRef run = CFArrayGetValueAtIndex(runs, j);

            NSDictionary *attributes = (__bridge NSDictionary*)CTRunGetAttributes(run);

            // 判断是不是链接

            if (!CGColorEqualToColor((__bridge CGColorRef)([attributes valueForKey:@"CTForegroundColor"]), self.textColor.CGColor)) {

                CFRange range = CTRunGetStringRange(run);

                float offset = CTLineGetOffsetForStringIndex(line, range.location, NULL);

                // 得到链接的CGRect

                CGRect runRect;

                runRect.size.width = CTRunGetTypographicBounds(run, CFRangeMake(0,0), &runAscent, &runDescent, &lineLeading1);

                runRect.size.height = self.font.lineHeight;

                runRect.origin.x = lineOrigin.x + offset+ self.xOffset;

                runRect.origin.y = lineOrigin.y;

                runRect.origin.y -= descent + self.yOffset;

                // 因为坐标系被翻转，链接正常的坐标需要通过CGAffineTransform计算得到

                CGAffineTransform transform = CGAffineTransformMakeTranslation(0, ContentHeight);

                transform = CGAffineTransformScale(transform, 1.f, -1.f);

                CGRect flipRect = CGRectApplyAffineTransform(runRect, transform);

                // 保存是链接的CGRect

                NSRange nRange = NSMakeRange(range.location, range.length);

                self.framesDict[NSStringFromRange(nRange)] = [NSValue valueWithCGRect:flipRect];

                // 保存同一条链接的不同CGRect，用于点击时背景色处理

                for (NSString *rangeString in self.ranges) {

                    NSRange range = NSRangeFromString(rangeString);

                    if (NSLocationInRange(nRange.location, range)) {

                        NSMutableArray *array = self.relationDict[rangeString];

                        if (array) {

                            [array addObject:NSStringFromCGRect(flipRect)];

                            self.relationDict[rangeString] = array;

                        } else {

                            self.relationDict[rangeString] = [NSMutableArray arrayWithObject:NSStringFromCGRect(flipRect)];

                        }

                    }

                }

            }

        }

    }

    CFRelease(frame);

    CFRelease(path);

}

上述方法运用起来就是：

这样就完成了文本的显示。

2、显示图片

图片包括用户头像和朋友圈的内容，这里只是将CALayer添加到contentView.layer上，具体做法是继承了CALayer，实现部分功能。

通过链接显示图片：

- (void)setContentsWithURLString:(NSString *)urlString {

    self.contents = (__bridge id _Nullable)([UIImage imageNamed:@"placeholder"].CGImage);

    @weakify(self)

    SDWebImageManager *manager = [SDWebImageManager sharedManager];

    [manager downloadImageWithURL:[NSURL URLWithString:urlString]

                          options:SDWebImageCacheMemoryOnly

                         progress:nil

                        completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {

                            if (image) {

                                @strongify(self)

                                dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

                                    if (!_observer) {

                                        _observer = CFRunLoopObserverCreateWithHandler(kCFAllocatorDefault, kCFRunLoopBeforeWaiting | kCFRunLoopExit, false, POPAnimationApplyRunLoopOrder, ^(CFRunLoopObserverRef observer, CFRunLoopActivity activity) {

                                            self.contents = (__bridge id _Nullable)(image.CGImage);

                                        });

                                        if (_observer) {

                                            CFRunLoopAddObserver(CFRunLoopGetMain(), _observer,  kCFRunLoopCommonModes);

                                        }

                                    }

                                });

                                self.originImage = image;

                            }

                        }];

}

其他比较简单就不展开。

3、显示小视频

之前的一篇文章简单讲了怎么自己做一个播放器，这里就派上用场了。而显示小视频封面图片的CALayer同样在显示小视频的时候可以复用。

这里使用了NSOperationQueue来保障播放视频的流畅性，具体继承NSOperation的VideoDecodeOperation相关代码如下：

解码图片是因为UIImage在界面需要显示的时候才开始解码，这样可能会造成主线程的卡顿，所以在子线程对其进行解压缩处理。

具体的使用：

4、其他

1、触摸交互是覆盖了以下方法实现：

- (void)touchesCancelled:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event

- (void)touchesCancelled:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event

- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event

2、页面上FPS的测量是使用了YYKit项目中的YYFPSLabel。

3、测试数据是微博找的，其中小视频是Gif快手。

本文的代码在https://github.com/hawk0620/PYQFeedDemo