YYKit探究之YYImage（一）

1.读取图片数据

+ (nullable YYImage *)imageNamed:(NSString *)name; // no cache!
+ (nullable YYImage *)imageWithContentsOfFile:(NSString *)path;
+ (nullable YYImage *)imageWithData:(NSData *)data;
+ (nullable YYImage *)imageWithData:(NSData *)data scale:(CGFloat)scale;

这4种类方法用于获取图片数据/路径。前三种方法的实现最后都归结为 imageWithData:(NSData *)data scale:(CGFloat)scale 这个方法。让我们看看imageWithData:(NSData *)data scale:(CGFloat)scale 这个方法的实现逻辑。

2.由实例方法演变而来。

+ (YYImage *)imageWithData:(NSData *)data scale:(CGFloat)scale {
    return [[self alloc] initWithData:data scale:scale];
}

具体看实例方法的逻辑。

- (instancetype)initWithData:(NSData *)data scale:(CGFloat)scale {
    if (data.length == 0) return nil;
　　//获取屏幕分辨率，传入解码decoder
    if (scale <= 0) scale = [UIScreen mainScreen].scale;
　　//创建信号量，参数：信号量的初值，如果小于0则会返回NULL，crate的value表示，最多几个资源可访问
    _preloadedLock = dispatch_semaphore_create(1);
    @autoreleasepool {
　　//　　解析图片数据，见2
        YYImageDecoder *decoder = [YYImageDecoder decoderWithData:data scale:scale];
　　//对当前的图片进行解码操作，见3
        YYImageFrame *frame = [decoder frameAtIndex:0 decodeForDisplay:YES];
　　//获取图片第一帧
        UIImage *image = frame.image;
        if (!image) return nil;
　　//Image初始化
        self = [self initWithCGImage:image.CGImage scale:decoder.scale orientation:image.imageOrientation];
        if (!self) return nil;
        _animatedImageType = decoder.type;
　　//　　动图的处理方法
        if (decoder.frameCount > 1) {
            _decoder = decoder;
            _bytesPerFrame = CGImageGetBytesPerRow(image.CGImage) * CGImageGetHeight(image.CGImage);
            _animatedImageMemorySize = _bytesPerFrame * decoder.frameCount;
        }
        self.isDecodedForDisplay = YES;
    }
    return self;
}

2.1，解析图片数据

+ (instancetype)decoderWithData:(NSData *)data scale:(CGFloat)scale {
    if (!data) return nil;
    YYImageDecoder *decoder = [[YYImageDecoder alloc] initWithScale:scale];
    [decoder updateData:data final:YES];
    if (decoder.frameCount == 0) return nil;
    return decoder;
}

这一步主要看updateData里面做了什么

- (BOOL)updateData:(NSData *)data final:(BOOL)final {
    BOOL result = NO;
    //对线程上锁，此时其他线程阻塞等待该线程释放锁
    pthread_mutex_lock(&_lock);
    //这里先判断图片格式YYImageDetectType((__bridge CFDataRef)data)，再根据格式获取图片帧
    result = [self _updateData:data final:final];
//   //执行完后释放锁
    pthread_mutex_unlock(&_lock);
    return result;
}

2.2.获取图片帧数据

　　updateData里面除了判断图片格式，是否是动图，还要更新源（可以是data，也可以是路径），下面以updataSourceAPNG为例，剖析实现过程

[self _updateSourceImageIO]; // decode first frame
    if (_frameCount == 0) return; // png decode failed
    if (!_finalized) return; // ignore multi-frame before finalized
        //_data.bytes 提供存储apng的内存对应的指针，获取动图的数据信息
    yy_png_info *apng = yy_png_info_create(_data.bytes, (uint32_t)_data.length);
    if (!apng) return; // apng decode failed
    if (apng->apng_frame_num == 0 ||
        (apng->apng_frame_num == 1 && apng->apng_first_frame_is_cover)) {
        yy_png_info_release(apng);
        return; // no animation
    }
    if (_source) { // apng decode succeed, no longer need image souce
        CFRelease(_source);
        _source = NULL;
    }

　　其中_data.bytes 提供存储apng的内存对应的指针，获取动图的数据信息，接下来就是对动图各个帧数据处理

　　uint32_t canvasWidth = apng->header.width;
    uint32_t canvasHeight = apng->header.height;
    NSMutableArray *frames = [NSMutableArray new];
    BOOL needBlend = NO;
    uint32_t lastBlendIndex = 0;
　　//PNG动画的逐帧信息返回到Frames数组中
    for (uint32_t i = 0; i < apng->apng_frame_num; i++) {
        _YYImageDecoderFrame *frame = [_YYImageDecoderFrame new];
        [frames addObject:frame];
        
        yy_png_frame_info *fi = apng->apng_frames + i;
        frame.index = i;
        frame.duration = yy_png_delay_to_seconds(fi->frame_control.delay_num, fi->frame_control.delay_den);
        frame.hasAlpha = YES;
        frame.width = fi->frame_control.width;
        frame.height = fi->frame_control.height;
        frame.offsetX = fi->frame_control.x_offset;
        frame.offsetY = canvasHeight - fi->frame_control.y_offset - fi->frame_control.height;
        
        BOOL sizeEqualsToCanvas = (frame.width == canvasWidth && frame.height == canvasHeight);
        BOOL offsetIsZero = (fi->frame_control.x_offset == 0 && fi->frame_control.y_offset == 0);
        frame.isFullSize = (sizeEqualsToCanvas && offsetIsZero);
        
        switch (fi->frame_control.dispose_op) {
            case YY_PNG_DISPOSE_OP_BACKGROUND: {
                frame.dispose = YYImageDisposeBackground;
            } break;
            case YY_PNG_DISPOSE_OP_PREVIOUS: {
                frame.dispose = YYImageDisposePrevious;
            } break;
            default: {
                frame.dispose = YYImageDisposeNone;
            } break;
        }
        switch (fi->frame_control.blend_op) {
            case YY_PNG_BLEND_OP_OVER: {
                frame.blend = YYImageBlendOver;
            } break;
                
            default: {
                frame.blend = YYImageBlendNone;
            } break;
        }
        
        if (frame.blend == YYImageBlendNone && frame.isFullSize) {
            frame.blendFromIndex  = i;
            if (frame.dispose != YYImageDisposePrevious) lastBlendIndex = i;
        } else {
            if (frame.dispose == YYImageDisposeBackground && frame.isFullSize) {
                frame.blendFromIndex = lastBlendIndex;
                lastBlendIndex = i + 1;
            } else {
                frame.blendFromIndex = lastBlendIndex;
            }
        }
        if (frame.index != frame.blendFromIndex) needBlend = YES;
    }
    
    _width = canvasWidth;
    _height = canvasHeight;
    _frameCount = frames.count;
    _loopCount = apng->apng_loop_num;
    _needBlend = needBlend;
    _apngSource = apng;
    dispatch_semaphore_wait(_framesLock, DISPATCH_TIME_FOREVER);
    _frames = frames;
    dispatch_semaphore_signal(_framesLock);
}

3.各个帧数据进行解码和显示，获取imageRef的步骤如3.1所示

  //解码，从GPU中拿到imageRef
        CGImageRef imageRef = [self _newUnblendedImageAtIndex:index extendToCanvas:extendToCanvas decoded:&decoded];
        if (!imageRef) return nil;
        if (decodeForDisplay && !decoded) {
            CGImageRef imageRefDecoded = YYCGImageCreateDecodedCopy(imageRef, YES);
            if (imageRefDecoded) {
                CFRelease(imageRef);
                imageRef = imageRefDecoded;
                decoded = YES;
            }
        }
　　　　//获取当前帧
        UIImage *image = [UIImage imageWithCGImage:imageRef scale:_scale orientation:_orientation];
        CFRelease(imageRef);
        if (!image) return nil;
        image.isDecodedForDisplay = decoded;
        frame.image = image;
        return frame;

3.1，获取imageRef

 //开始解码
        CGImageRef imageRef = CGImageSourceCreateImageAtIndex(_source, index, (CFDictionaryRef)@{(id)kCGImageSourceShouldCache:@(YES)});
        if (imageRef && extendToCanvas) {
            size_t width = CGImageGetWidth(imageRef);
            size_t height = CGImageGetHeight(imageRef);
            if (width == _width && height == _height) {
                CGImageRef imageRefExtended = YYCGImageCreateDecodedCopy(imageRef, YES);
                if (imageRefExtended) {
                    CFRelease(imageRef);
                    imageRef = imageRefExtended;
                    if (decoded) *decoded = YES;
                }
            } else {
                //没有就绘制
                CGContextRef context = CGBitmapContextCreate(NULL, _width, _height, 8, 0, YYCGColorSpaceGetDeviceRGB(), kCGBitmapByteOrder32Host | kCGImageAlphaPremultipliedFirst);
                if (context) {
                    //获取像素信息
                    CGContextDrawImage(context, CGRectMake(0, _height - height, width, height), imageRef);
                    CGImageRef imageRefExtended = CGBitmapContextCreateImage(context);
                    CFRelease(context);
                    if (imageRefExtended) {
                        CFRelease(imageRef);
                        imageRef = imageRefExtended;
                        if (decoded) *decoded = YES;
                    }
                }
            }
        }
        return imageRef;

YYImage处理图片流程

1.用imageWithData:(NSData *)data scale:(CGFloat)scale获取图片源数据。

2.解析图片数据，分为两步，判断图片格式（如果是动图，就开启loop），以及返回当前帧的信息yy_png_info_create(_data.bytes, (uint32_t)_data.length)。

3.根据帧信息开始解码CGImageSourceCreateImageAtIndex，并展示出来。