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
然后是9.3系统iPhone5的真机效果
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：
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- (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：
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- (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：
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- (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)); // 获取需要高亮的链接CGRect，并填充背景色 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);} |
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：
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- (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：
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- (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]; // 视频播放时，m_pixelFormatType=kCVPixelFormatType_32BGRA // 其他用途，如视频压缩，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]; // 要确保nominalFrameRate>0，之前出现过android拍的0帧视频 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); // 解码图片 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：
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- (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、触摸交互是覆盖了以下方法实现：
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- (void)touchesCancelled:(NSSet<uitouch *> *)touches withEvent:(UIEvent *)event- (void)touchesCancelled:(NSSet<uitouch *> *)touches withEvent:(UIEvent *)event- (void)touchesEnded:(NSSet<uitouch *> *)touches withEvent:(UIEvent *)event</uitouch *></uitouch *></uitouch *> |
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
本文作者：伯乐在线 - Hawk0620