引起UITableView卡頓比較常見的原因有cell的層級過多、cell中有觸發離屏渲染的代碼（譬如：cornerRadius、maskToBounds 同時使用）、像素是否對齊、是否使用UITableView自動計算cell高度的方法等。本文將從cell層級出發，以一個仿朋友圈的demo來講述如何讓列表保持順滑，項目的源碼可在文末獲得。不可否認的是，過早的優化是魔鬼，請在項目出現性能瓶頸再考慮優化。

首先看看reveal上頁面層級的效果圖

然後是9.3系統iPhone5的真機效果

1、繪制文本

使用core text可以將文本繪制在一個CGContextRef上，最後再通過UIGraphicsGetImageFromCurrentImageContext()生成圖片，再將圖片賦值給cell.contentView.layer，從而達到減少cell層級的目的。

繪制普通文本（譬如用戶昵稱）在context上，相關注釋在代碼裡：

- (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);
    // 翻轉坐標系
    CGContextSetTextMatrix(context,CGAffineTransformIdentity);
    CGContextTranslateCTM(context,0,height);
    CGContextScaleCTM(context,1.0,-1.0);
    NSMutableDictionary * attributes = [StringAttributes attributeFont:font andTextColor:color lineBreakMode:lineBreakMode];
    // 創建繪制區域（路徑）
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathAddRect(path,NULL,CGRectMake(p.x, height-p.y-size.height,(size.width),(size.height)));
    // 創建AttributedString
    NSMutableAttributedString *attributedStr = [[NSMutableAttributedString alloc] initWithString:self attributes:attributes];
    CFAttributedStringRef attributedString = (__bridge CFAttributedStringRef)attributedStr;
    // 繪制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);
}

繪制朋友圈內容文本（帶鏈接）在context上，這裡我還沒有去實現文本多了會折疊的效果，與上面普通文本不同的是這裡需要創建帶鏈接的AttributeString和CTLineRef的逐行繪制：

- (NSMutableAttributedString *)highlightText:(NSMutableAttributedString *)coloredString{
    // 創建帶高亮的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);
    // 遍歷每一行
    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;
        // 設置每一行位置
        CGContextSetTextPosition(c, penOffset + self.xOffset, y - self.yOffset);
        CTLineDraw(line, c);
        // CTRunRef同一行中文本的不同樣式，包括顏色、字體等，此處用途為處理鏈接高亮
        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);
            // 判斷是不是鏈接
            if (!CGColorEqualToColor((__bridge CGColorRef)([attributes valueForKey:@"CTForegroundColor"]), self.textColor.CGColor)) {
                CFRange range = CTRunGetStringRange(run);
                float offset = CTLineGetOffsetForStringIndex(line, range.location, NULL);
                // 得到鏈接的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;
                // 因為坐標系被翻轉，鏈接正常的坐標需要通過CGAffineTransform計算得到
                CGAffineTransform transform = CGAffineTransformMakeTranslation(0, ContentHeight);
                transform = CGAffineTransformScale(transform, 1.f, -1.f);
                CGRect flipRect = CGRectApplyAffineTransform(runRect, transform);
                // 保存是鏈接的CGRect
                NSRange nRange = NSMakeRange(range.location, range.length);
                self.framesDict[NSStringFromRange(nRange)] = [NSValue valueWithCGRect:flipRect];
                // 保存同一條鏈接的不同CGRect，用於點擊時背景色處理
                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);
}

上述方法運用起來就是：

- (void)fillData:(CGContextRef)context {
    [self.nickname drawInContext:context withPosition:(CGPoint){kTextXOffset, kSpec} andFont:kNicknameFont
                    andTextColor:UIColorFromRGB(0x556c95) andHeight:self.nicknameSize.height
                        andWidth:self.nicknameSize.width lineBreakMode:kCTLineBreakByTruncatingTail];
    [self.drawer setText:self.contentString context:context contentSize:self.contentSize
         backgroundColor:[UIColor whiteColor] font:kContentTextFont textColor:[UIColor blackColor]
                   block:nil xOffset:kTextXOffset yOffset:kSpec * 2 + self.nicknameSize.height];
}
- (void)fillContents:(NSArray *)array {
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(self.size.width, self.size.height), YES, 0);
    CGContextRef context = UIGraphicsGetCurrentContext();
    [UIColorFromRGB(0xffffff) set];
    CGContextFillRect(context, CGRectMake(0, 0, self.size.width, self.size.height));
    // 獲取需要高亮的鏈接CGRect，並填充背景色
    if (array) {
        for (NSString *string in array) {
            CGRect rect = CGRectFromString(string);
            [UIColorFromRGB(0xe5e5e5) set];
            CGContextFillRect(context, rect);
        }
    }
    [self fillData:context];
    UIImage *temp = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    self.contentView.layer.contents = (__bridge id _Nullable)(temp.CGImage);
}

這樣就完成了文本的顯示。

2、顯示圖片

圖片包括用戶頭像和朋友圈的內容，這裡只是將CALayer添加到contentView.layer上，具體做法是繼承了CALayer，實現部分功能。

通過鏈接顯示圖片：

- (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;
                            }
                        }];
}

其他比較簡單就不展開。

3、顯示小視頻

之前的一篇文章簡單講了怎麼自己做一個播放器，這裡就派上用場了。而顯示小視頻封面圖片的CALayer同樣在顯示小視頻的時候可以復用。

這裡使用了NSOperationQueue來保障播放視頻的流暢性，具體繼承NSOperation的VideoDecodeOperation相關代碼如下：

- (void)main {
    @autoreleasepool {
        if (self.isCancelled) {
            _newVideoFrameBlock = nil;
            _decodeFinishedBlock = nil;
            return;
        }
        AVURLAsset *asset = [AVURLAsset URLAssetWithURL:[[NSURL alloc] initFileURLWithPath:self.filePath] options:nil];
        NSError *error;
        AVAssetReader* reader = [[AVAssetReader alloc] initWithAsset:asset error:&error];
        if (error) {
            return;
        }
        NSArray* videoTracks = [asset tracksWithMediaType:AVMediaTypeVideo];
        AVAssetTrack* videoTrack = [videoTracks objectAtIndex:0];
        // 視頻播放時，m_pixelFormatType=kCVPixelFormatType_32BGRA
        // 其他用途，如視頻壓縮，m_pixelFormatType=kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange
        int m_pixelFormatType = kCVPixelFormatType_32BGRA;
        NSDictionary* options = [NSDictionary dictionaryWithObject:[NSNumber numberWithInt: (int)m_pixelFormatType]
                                                            forKey:(id)kCVPixelBufferPixelFormatTypeKey];
        AVAssetReaderTrackOutput* videoReaderOutput = [[AVAssetReaderTrackOutput alloc]
                initWithTrack:videoTrack outputSettings:options];
        [reader addOutput:videoReaderOutput];
        [reader startReading];
        // 要確保nominalFrameRate>0，之前出現過android拍的0幀視頻
        if (self.isCancelled) {
            _newVideoFrameBlock = nil;
            _decodeFinishedBlock = nil;
            return;
        }
        while ([reader status] == AVAssetReaderStatusReading && videoTrack.nominalFrameRate > 0) {
            if (self.isCancelled) {
                _newVideoFrameBlock = nil;
                _decodeFinishedBlock = nil;
                return;
            }
            CMSampleBufferRef sampleBuffer = [videoReaderOutput copyNextSampleBuffer];
            CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
            // Lock the base address of the pixel buffer
            CVPixelBufferLockBaseAddress(imageBuffer, 0);
            // Get the number of bytes per row for the pixel buffer
            size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
            // Get the pixel buffer width and height
            size_t width = CVPixelBufferGetWidth(imageBuffer);
            size_t height = CVPixelBufferGetHeight(imageBuffer);
            //Generate image to edit`
            unsigned char* pixel = (unsigned char *)CVPixelBufferGetBaseAddress(imageBuffer);
            CGColorSpaceRef colorSpace=CGColorSpaceCreateDeviceRGB();
            CGContextRef context=CGBitmapContextCreate(pixel, width, height, 8, bytesPerRow, colorSpace,
                                                       kCGBitmapByteOrder32Little|kCGImageAlphaPremultipliedFirst);
            if (context != NULL) {
                CGImageRef imageRef = CGBitmapContextCreateImage(context);
                CVPixelBufferUnlockBaseAddress(imageBuffer, 0);
                CGColorSpaceRelease(colorSpace);
                CGContextRelease(context);
                // 解碼圖片
                size_t width = CGImageGetWidth(imageRef);
                size_t height = CGImageGetHeight(imageRef);
                size_t bitsPerComponent = CGImageGetBitsPerComponent(imageRef);
                // CGImageGetBytesPerRow() calculates incorrectly in iOS 5.0, so defer to CGBitmapContextCreate
                size_t bytesPerRow = 0;
                CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
                CGColorSpaceModel colorSpaceModel = CGColorSpaceGetModel(colorSpace);
                CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(imageRef);
                if (colorSpaceModel == kCGColorSpaceModelRGB) {
                    uint32_t alpha = (bitmapInfo & kCGBitmapAlphaInfoMask);
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wassign-enum"
                    if (alpha == kCGImageAlphaNone) {
                        bitmapInfo &= ~kCGBitmapAlphaInfoMask;
                        bitmapInfo |= kCGImageAlphaNoneSkipFirst;
                    } else if (!(alpha == kCGImageAlphaNoneSkipFirst || alpha == kCGImageAlphaNoneSkipLast)) {
                        bitmapInfo &= ~kCGBitmapAlphaInfoMask;
                        bitmapInfo |= kCGImageAlphaPremultipliedFirst;
                    }
#pragma clang diagnostic pop
                }
                CGContextRef context = CGBitmapContextCreate(NULL, width, height, bitsPerComponent,
                                                             bytesPerRow, colorSpace, bitmapInfo);
                CGColorSpaceRelease(colorSpace);
                if (!context) {
                    if (self.newVideoFrameBlock) {
                        dispatch_async(dispatch_get_main_queue(), ^{
                            if (self.isCancelled) {
                                _newVideoFrameBlock = nil;
                                _decodeFinishedBlock = nil;
                                return;
                            }
                            self.newVideoFrameBlock(imageRef, self.filePath);
                            CGImageRelease(imageRef);
                        });
                    }
                } else {
                    CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, width, height), imageRef);
                    CGImageRef inflatedImageRef = CGBitmapContextCreateImage(context);
                    CGContextRelease(context);
                    if (self.newVideoFrameBlock) {
                        dispatch_async(dispatch_get_main_queue(), ^{
                            if (self.isCancelled) {
                                _newVideoFrameBlock = nil;
                                _decodeFinishedBlock = nil;
                                return;
                            }
                            self.newVideoFrameBlock(inflatedImageRef, self.filePath);
                            CGImageRelease(inflatedImageRef);
                        });
                    }
                    CGImageRelease(imageRef);
                }
                if(sampleBuffer) {
                    CMSampleBufferInvalidate(sampleBuffer);
                    CFRelease(sampleBuffer);
                    sampleBuffer = NULL;
                } else {
                    break;
                }
            }
            [NSThread sleepForTimeInterval:CMTimeGetSeconds(videoTrack.minFrameDuration)];
        }
        if (self.isCancelled) {
            _newVideoFrameBlock = nil;
            _decodeFinishedBlock = nil;
            return;
        }
        if (self.decodeFinishedBlock) {
            self.decodeFinishedBlock(self.filePath);
        }
    }
}

解碼圖片是因為UIImage在界面需要顯示的時候才開始解碼，這樣可能會造成主線程的卡頓，所以在子線程對其進行解壓縮處理。

具體的使用：

- (void)playVideoWithFilePath:(NSString *)filePath_ type:(NSString *)type {
    @weakify(self)
    [[VideoPlayerManager shareInstance] decodeVideo:filePath_
                              withVideoPerDataBlock:^(CGImageRef imageData, NSString *filePath) {
                                  @strongify(self)
                                  if ([type isEqualToString:@"video"]) {
                                      if ([filePath isEqualToString:self.filePath]) {
                                          [self.sources.firstObject
                                                  setContents:(__bridge id _Nullable)(imageData)];
                                      }
                                  }
                              } decodeFinishBlock:^(NSString *filePath){
                [self playVideoWithFilePath:filePath type:type];
            }];
}

4、其他

1、觸摸交互是覆蓋了以下方法實現：

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event

2、頁面上FPS的測量是使用了YYKit項目中的YYFPSLabel。

3、測試數據是微博找的，其中小視頻是Gif快手。

本文的代碼在https://github.com/hawk0620/PYQFeedDemo

本文作者：伯樂在線 - Hawk0620