我有许多 CALayer，它们是在我的应用程序运行时动态创建的，我需要能够生成这些 CALayer 的单个位图，稍后将被屏蔽。

当我需要创建 mask 时，CALayer 已经被绘制到背景中(也使用 shouldRasterize = YES)，并且使用 renderInContext 我能够获得位图。但是随着CAlayer数量的增加，renderInContext造成的停顿时间越来越长。有没有我可以用来 renderInContext 的替代方法，或者我可以用它来停止我的应用程序暂时卡住的替代方法？

理想情况是不使用 OpenGL 直接从内存/缓冲区/缓存访问已经绘制的像素数据，但我不确定这是否可以通过 CoreAnimation 实现。

谢谢，任何额外的信息都会非常有用!

最佳答案

Rob 认为 renderInContext: 是在这里使用的正确方法是正确的。在上下文中渲染实际上是将层的像素数据渲染到上下文中。这是一个示例应用程序，它将在后台线程上绘制 10,000 个图层...

该应用程序执行以下操作:

1. 创建一个 UIView
2. 向该 View 的图层添加 10,000 个图层
3. 渲染在您触摸屏幕时开始(即它是一个 iOS 示例应用)
4. 创建后台线程
5. 将 UIView 的层渲染到上下文中(进而渲染其子层)
6. 使用渲染上下文的内容创建一个 UIImage
7. 将新图像添加到 UIImageView 中的屏幕
8. 它在主 线程上运行计时器时执行所有这些操作，表明后台线程实际上并没有阻塞主线程

这是代码...

首先，创建一个有很多层的 subview :

@implementation C4WorkSpace {
    UIView *v;
    dispatch_queue_t backgroundRenderQueue;
    CFTimeInterval beginTime;
    NSTimer *timer;
    NSInteger timerCallCount;
}

-(void)setup {
    //create a view to hold a bunch of CALayers
    v = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 400, 400)];
    v.center = CGPointMake(384,512);

    //create a buch of CALayers and add them to a view
    for(int i = 0; i < 10000; i++) {
        CALayer *l = [[CALayer alloc] init];
        l.frame = CGRectMake([self random:390],[self random:390],10,10);
        l.backgroundColor = [UIColor blueColor].CGColor;
        l.borderColor = [UIColor orangeColor].CGColor;
        l.borderWidth = 2.0f;
        [v.layer addSublayer:l];
    }
    //add the view to the application's main view
    [self.view addSubview:v];
}

-(NSInteger)random:(NSInteger)value {
    srandomdev();
    return ((NSInteger)random())%value;
}

其次，创建一个将启动计时器然后触发渲染的方法...

-(void)touchesBegan {
    timer = [NSTimer scheduledTimerWithTimeInterval:0.03f target:self selector:@selector(printTime) userInfo:nil repeats:YES];
    [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];
    [self render];
}

-(void)printTime {
    NSLog(@"%d (main thread running)",++timerCallCount);
}

第三，创建一个带有回调方法的渲染循环，在渲染完成后将图像放在屏幕上。

-(void)render {
    NSLog(@"render was called");
    //create the queue
    backgroundRenderQueue = dispatch_queue_create("backgroundRenderQueue",DISPATCH_QUEUE_CONCURRENT);    
    //create a async call on the background queue
    dispatch_async(backgroundRenderQueue, ^{
        //create a cgcontext
        NSUInteger width = (NSUInteger)v.frame.size.width;
        NSUInteger height = (NSUInteger)v.frame.size.height;
        CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
        NSUInteger bytesPerPixel = 4;
        NSUInteger bytesPerRow = bytesPerPixel * width;
        unsigned char *rawData = malloc(height * bytesPerRow);
        NSUInteger bitsPerComponent = 8;
        CGContextRef context = CGBitmapContextCreate(rawData, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);

        //render the layer and its subviews
        [v.layer renderInContext:context];

        //create a callback async on the main queue when rendering is complete
        dispatch_async(dispatch_get_main_queue(), ^{
            //create an image from the context
            UIImage *m = [UIImage imageWithCGImage:CGBitmapContextCreateImage(context)];
            UIImageView *uiiv = [[UIImageView alloc] initWithImage:m];
            //add the image view to the main view
            [self.view addSubview:uiiv];
            CGColorSpaceRelease(colorSpace);
            CGContextRelease(context);
            NSLog(@"rendering complete");
            [timer invalidate];
        });
    });
}

注意:如果您的图层不在同一个子图层中，您可以轻松调用 for 循环，将上下文与每个 CALayer 的原点进行转换，并将每个图层单独绘制到上下文本身

当我运行它时，我得到以下输出:

2013-03-18 07:14:28.617 C4iOS[21086:907] render was called
2013-03-18 07:14:28.648 C4iOS[21086:907] 1 (main thread running)
2013-03-18 07:14:28.680 C4iOS[21086:907] 2 (main thread running)
2013-03-18 07:14:28.709 C4iOS[21086:907] 3 (main thread running)
2013-03-18 07:14:28.737 C4iOS[21086:907] 4 (main thread running)
2013-03-18 07:14:28.767 C4iOS[21086:907] 5 (main thread running)
2013-03-18 07:14:28.798 C4iOS[21086:907] 6 (main thread running)
2013-03-18 07:14:28.828 C4iOS[21086:907] 7 (main thread running)
2013-03-18 07:14:28.859 C4iOS[21086:907] 8 (main thread running)
2013-03-18 07:14:28.887 C4iOS[21086:907] 9 (main thread running)
2013-03-18 07:14:28.917 C4iOS[21086:907] 10 (main thread running)
2013-03-18 07:14:28.948 C4iOS[21086:907] 11 (main thread running)
2013-03-18 07:14:28.978 C4iOS[21086:907] 12 (main thread running)
2013-03-18 07:14:29.010 C4iOS[21086:907] 13 (main thread running)
2013-03-18 07:14:29.037 C4iOS[21086:907] 14 (main thread running)
2013-03-18 07:14:29.069 C4iOS[21086:907] 15 (main thread running)
2013-03-18 07:14:29.097 C4iOS[21086:907] 16 (main thread running)
2013-03-18 07:14:29.130 C4iOS[21086:907] 17 (main thread running)
2013-03-18 07:14:29.159 C4iOS[21086:907] 18 (main thread running)
2013-03-18 07:14:29.189 C4iOS[21086:907] 19 (main thread running)
2013-03-18 07:14:29.217 C4iOS[21086:907] 20 (main thread running)
2013-03-18 07:14:29.248 C4iOS[21086:907] 21 (main thread running)
2013-03-18 07:14:29.280 C4iOS[21086:907] 22 (main thread running)
2013-03-18 07:14:29.309 C4iOS[21086:907] 23 (main thread running)
2013-03-18 07:14:29.337 C4iOS[21086:907] 24 (main thread running)
2013-03-18 07:14:29.369 C4iOS[21086:907] 25 (main thread running)
2013-03-18 07:14:29.397 C4iOS[21086:907] 26 (main thread running)
2013-03-18 07:14:29.405 C4iOS[21086:907] rendering complete

关于iphone - 从 CALayers 获取位图，renderInContext 的任何替代方法，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7216547/