屏幕成像原理以及FPS优化Tips

- (void)layoutSubviews
{
    [super layoutSubviews];
    // 正例：
    CGFloat headerWidth = 97.f;
    CGFloat headerHeight = 86.f;
    self.headerImageView.frame = CGRectMake(16.f, 15.f, headerWidth, headerHeight);
    // ...

    // 反例：
    self.priceView.top = 65.f;
    self.priceView.left = 16.f;
}

dispatch_async(dispatch_get_global_queue(0, 0), ^{
  // 尺寸计算
  [@"xxx" boundingRectWithSize:CGSizeMake(CGFLOAT_MAX, CGFLOAT_MAX) options:NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading attributes:@{NSFontAttributeName : [UIFont systemFontOfSize:12], NSForegroundColorAttributeName : HEXCOLOR(0x333333)} context:nil];
});
dispatch_async(dispatch_get_global_queue(0, 0), ^{
  // 文本绘制
  [@"xxx" drawWithRect:CGRectMake(0, 0, SCREEN_WIDTH, 50) options:NSStringDrawingUsesLineFragmentOrigin attributes:nil context:nil];
});

@implementation UIImageView (FPS)
- (void)fps_setImage:(UIImage *)image {
    if (image == nil) {
        return;
    }
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        // 获取CGImage
        CGImageRef cgImage = image.CGImage;

        // alphaInfo
        CGImageAlphaInfo alphaInfo = CGImageGetAlphaInfo(cgImage) & kCGBitmapAlphaInfoMask;
        BOOL hasAlpha = NO;
        if (alphaInfo == kCGImageAlphaPremultipliedLast ||
            alphaInfo == kCGImageAlphaPremultipliedFirst ||
            alphaInfo == kCGImageAlphaLast ||
            alphaInfo == kCGImageAlphaFirst) {
            hasAlpha = YES;
        }

        // bitmapInfo
        CGBitmapInfo bitmapInfo = kCGBitmapByteOrder32Host;
        bitmapInfo |= hasAlpha ? kCGImageAlphaPremultipliedFirst : kCGImageAlphaNoneSkipFirst;

        // size
        size_t width = CGImageGetWidth(cgImage);
        size_t height = CGImageGetHeight(cgImage);

        // context
        CGContextRef context = CGBitmapContextCreate(NULL, width, height, 8, 0, CGColorSpaceCreateDeviceRGB(), bitmapInfo);

        // draw 把cgImage绘制到上下文会触发解码
        CGContextDrawImage(context, CGRectMake(0, 0, width, height), cgImage);

        // get CGImage
        cgImage = CGBitmapContextCreateImage(context);

        // into UIImage
        UIImage *newImage = [UIImage imageWithCGImage:cgImage];

        // release
        CGContextRelease(context);
        CGImageRelease(cgImage);

        // back to the main thread
        dispatch_async(dispatch_get_main_queue(), ^{
            self.image = newImage;
        });
    });
}
@end

static const CGSize kMinReleaseImageOnBackgroundSize = {20.0, 20.0};

// ASDK这样做的：
void ASPerformBlockOnDeallocationQueue(void (^block)()) { 
      static dispatch_queue_t queue;   
      static dispatch_once_t onceToken;
      dispatch_once(&onceToken, ^{ 
            queue = dispatch_queue_create("org.AsyncDisplayKit.deallocationQueue", DISPATCH_QUEUE_SERIAL); 
      }); 
      dispatch_async(queue, block); 
}

// 如何使用:
- (void)_clearImage
{
  // Destruction of bigger images on the main thread can be expensive
  // and can take some time, so we dispatch onto a bg queue to
  // actually dealloc.
  __block UIImage *image = self.image;
  CGSize imageSize = image.size;
  BOOL shouldReleaseImageOnBackgroundThread = imageSize.width > kMinReleaseImageOnBackgroundSize.width ||
                                              imageSize.height > kMinReleaseImageOnBackgroundSize.height;
  if (shouldReleaseImageOnBackgroundThread) {
    ASPerformBlockOnDeallocationQueue(^{
      image = nil;
    });
  }
///TODO
///

 

- (void)display {
    [self _displayAsync:_displaysAsynchronously];
}

- (void)_displayAsync:(BOOL)async {
    // 向delegate也就是YYLabel获取更新任务，newAsyncDisplayTask会返回一个新的绘制的任务
    __strong id<YYAsyncLayerDelegate> delegate = (id)self.delegate;
    YYAsyncLayerDisplayTask *task = [delegate newAsyncDisplayTask];
    if (!task.display) {
        if (task.willDisplay) task.willDisplay(self);
        self.contents = nil;
        if (task.didDisplay) task.didDisplay(self, YES);
        return;
    }

        if (task.willDisplay) task.willDisplay(self);
        YYSentinel *sentinel = _sentinel;
        int32_t value = sentinel.value;
       // 判断是否该取消异步绘制任务
        BOOL (^isCancelled)() = ^BOOL() {
            return value != sentinel.value;
        };
        CGSize size = self.bounds.size;
        BOOL opaque = self.opaque;
        CGFloat scale = self.contentsScale;
        CGColorRef backgroundColor = (opaque && self.backgroundColor) ? CGColorRetain(self.backgroundColor) : NULL;
        if (size.width < 1 || size.height < 1) {
            CGImageRef image = (__bridge_retained CGImageRef)(self.contents);
            self.contents = nil;
            if (image) {
                dispatch_async(YYAsyncLayerGetReleaseQueue(), ^{
                    CFRelease(image);
                });
            }
            if (task.didDisplay) task.didDisplay(self, YES);
            CGColorRelease(backgroundColor);
            return;
        }
        // 异步绘制
        dispatch_async(YYAsyncLayerGetDisplayQueue(), ^{
            if (isCancelled()) {
                CGColorRelease(backgroundColor);
                return;
            }
            // 开启图片上下文
            UIGraphicsBeginImageContextWithOptions(size, opaque, scale);
            CGContextRef context = UIGraphicsGetCurrentContext();
            if (opaque && context) {
                CGContextSaveGState(context);
                {
                    if (!backgroundColor || CGColorGetAlpha(backgroundColor) < 1) {
                        CGContextSetFillColorWithColor(context, [UIColor whiteColor].CGColor);
                        CGContextAddRect(context, CGRectMake(0, 0, size.width * scale, size.height * scale));
                        CGContextFillPath(context);
                    }
                    if (backgroundColor) {
                        CGContextSetFillColorWithColor(context, backgroundColor);
                        CGContextAddRect(context, CGRectMake(0, 0, size.width * scale, size.height * scale));
                        CGContextFillPath(context);
                    }
                }
                CGContextRestoreGState(context);
                CGColorRelease(backgroundColor);
            }
            task.display(context, size, isCancelled);
            if (isCancelled()) {
                UIGraphicsEndImageContext();
                dispatch_async(dispatch_get_main_queue(), ^{
                    if (task.didDisplay) task.didDisplay(self, NO);
                });
                return;
            }
            // 从当前上下文获取图片
            UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
            UIGraphicsEndImageContext();
            if (isCancelled()) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    if (task.didDisplay) task.didDisplay(self, NO);
                });
                return;
            }
            dispatch_async(dispatch_get_main_queue(), ^{
                if (isCancelled()) {
                    if (task.didDisplay) task.didDisplay(self, NO);
                } else {
                    self.contents = (__bridge id)(image.CGImage);
                    if (task.didDisplay) task.didDisplay(self, YES);
                }
            });
        });
    }