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    <p>Rob is right about <code>renderInContext:</code> being the right method to use here. Render in context does actually render the layer's pixel data into a context. Here's a sample application that will draw 10,000 layers on a background thread...</p> <p>The application does the following:</p> <ol> <li>Create's a UIView</li> <li>Adds 10,000 layers to that view's layer</li> <li>Rendering begins when you touch the screen (i.e. it's an iOS sample app)</li> <li>Creates a background thread</li> <li>Renders the UIView's layer into a context (which in turn renders its sublayers)</li> <li>Creates a UIImage with the content of the render context</li> <li>Adds the new image to the screen in a UIImageView</li> <li>It does all this while running a timer on the <strong>main</strong> thread, showing that the background thread actually doesn't block the main thread</li> </ol> <p>Here is the code...</p> <p>First, create a subview with a lot of layers:</p> <pre><code>@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 &lt; 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; } </code></pre> <p>Second, create a method that will start a timer and then triggers the render...</p> <pre><code>-(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); } </code></pre> <p>Third, create a render loop with a callback method that puts an image on the screen after rendering is completed.</p> <pre><code>-(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]; }); }); } </code></pre> <p>NOTE: if your layers aren't all in the same sublayer, you can easily call a <code>for</code> loop that translates the context to and from the origin of each CALayer and draw each layer individually into the context itself</p> <p>When I run this I get the following output:</p> <pre><code>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 </code></pre>
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