为什么更新 CALayer 会导致 CPU 使用率（其他进程）增加 60-70%？

Question

当界面由许多 CALayers 呈现时，我正在使用 IOS 应用程序。一旦我注意到 CPU 在图形更新期间被其他进程（实际上不是应用程序）加载。我开始禁用界面部件的更新，直到只有一个 CALayer 被更新（50-60 Hz），但所有其他层（数百个）都是静态的也显示出来了。因此，仅更新这一层会消耗 60-70% 的 CPU 其他进程的负载。当禁用此唯一层的更新时，CPU 不会加载。

谁能说说这是怎么回事？

[CATransaction begin];
[CATransaction setValue:(id)kCFBooleanTrue forKey:kCATransactionDisableActions];

// update layer which is sublayer of self.view.layer
self.headingCircularScaleLayer.transform = CATransform3DMakeRotation(DegToRad(-newHeadingAngle_deg), 0, 0, 1);

[self.view.layer setNeedsLayout];

[CATransaction commit];

这里 headingCircularScaleLayer 是 CALayer，其内容设置为某个 Image

NOTE_1：（通过添加许多层并更新它们来测试 CPU 负载）

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
}

-(void) viewDidAppear:(BOOL)animated {

    [self createAndSetupLayers];

    CADisplayLink *displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(updateLayers)];
    [displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];

    self.startTime = [[NSDate date] timeIntervalSince1970];
}


- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}


-(void) createAndSetupLayers {

    NSMutableArray<CALayer*>* newLayersArray = [[NSMutableArray alloc] init];

    long numOfRows = 28;
    long numOfCols = 21;
    long numOfLayers = numOfRows * numOfCols;

    CGFloat cellWidth = self.view.bounds.size.width / numOfCols;
    CGFloat cellHeight = self.view.bounds.size.height / numOfRows;

    CGFloat layerWidth = cellWidth * 0.9;
    CGFloat layerHeight = cellHeight * 0.9;

    long currRow = 0;
    long currCol = 0;

    for (long i = 0; i < numOfLayers; i++)
    {
        currRow = i / numOfCols;
        currCol = i % numOfCols;

        CALayer* newLayer = [[CALayer alloc] init];

        newLayer.bounds = CGRectMake(0.0, 0.0, layerWidth, layerHeight);
        newLayer.anchorPoint = CGPointMake(0.5, 0.5);
        newLayer.position = CGPointMake((currCol + 0.5) * cellWidth, (currRow + 0.5) * cellHeight);
        newLayer.backgroundColor = [UIColor greenColor].CGColor;
        //newLayer.opacity = 0.5;

        //NSDictionary *newActions = @{ @"transform": [NSNull null] };
        //newLayer.actions = newActions;
        newLayer.actions = @{ @"contents": [NSNull null], @"position": [NSNull null],
                           @"frame": [NSNull null], @"opacity": [NSNull null],
                           @"bounds": [NSNull null], @"affineTransform": [NSNull null],
                           @"sublayerTransform": [NSNull null], @"transform": [NSNull null],
                           @"zPosition": [NSNull null], @"anchorPoint": [NSNull null],
                           @"cornerRadius": [NSNull null], @"sublayers": [NSNull null],
                           @"onLayout": [NSNull null], };


        CALayer* newColorLayer = [[CALayer alloc] init];
        newColorLayer.bounds = CGRectMake(0.0, 0.0, layerWidth, layerHeight);
        newColorLayer.anchorPoint = CGPointMake(0.5, 0.5);
        newColorLayer.position = CGPointMake(0.5 * layerWidth, 0.5 * layerHeight);
        newColorLayer.opacity = 0.5;
        newColorLayer.opaque = YES;
        newColorLayer.backgroundColor = [UIColor redColor].CGColor;

        //[newLayer addSublayer:newColorLayer];
        [self.view.layer addSublayer:newLayer];
        [newLayersArray addObject:newLayer];
    }

    self.layersArray = newLayersArray;
}

-(void) updateLayers {
    double currTime = [[NSDate date] timeIntervalSince1970] - self.startTime;

    double currLayerAngle = 0.5 * currTime;

    NSLog(@"%.2f  %.2f", currTime, currLayerAngle);

    //[CATransaction begin];
    //[CATransaction setValue:(id)kCFBooleanTrue forKey:kCATransactionDisableActions];
    //[CATransaction setAnimationDuration:0.0];

    for (CALayer *currLayer in self.layersArray)
    {
        currLayer.transform = CATransform3DMakeRotation(currLayerAngle, 0, 0, 1);
    }

    //[CATransaction commit];
}

@end

Answer 1

如上面cmets所说，设置kCATransactionDisableActions只会影响当前事务。您可能在其他地方有启动隐式动画的代码。尝试将图层操作显式设置为空。您只需在创建图层时执行此操作一次。在这种情况下，您不需要进行显式事务。

layer.actions = @{ @"contents": [NSNull null], @"position": [NSNull null],
                   @"frame": [NSNull null], @"opacity": [NSNull null],
                   @"bounds": [NSNull null], @"affineTransform": [NSNull null],
                   @"sublayerTransform": [NSNull null], @"transform": [NSNull null],
                   @"zPosition": [NSNull null], @"anchorPoint": [NSNull null],
                   @"cornerRadius": [NSNull null], @"sublayers": [NSNull null],
                   @"onLayout": [NSNull null], };