iOS - 绘图效率（滑块更改更新）

Question

我正在 iOS 上为图像着色。我正在使用滑块来选择颜色。

当我将滑块上的“更新事件”设置为连续时，被调用的函数会被大量调用（滑块从 0 变为 1535），因此用户界面的响应速度不是很快。

有没有办法让下面的代码更有效率？我意识到每次调用函数时我都会开始一个新的绘图上下文 - 我可以“保存”这个上下文并重新使用它吗？

提前致谢。

- (IBAction)bodyColourChanged:(UISlider *)sender {
// get the UIColor from self.colourArray
UIColor *color = [self.colourArray objectAtIndex:sender.value];

UIImage *myImage = [UIImage imageNamed:@"body.png"];

// Begin a new image context to draw the coloured image onto
UIGraphicsBeginImageContext(self.bodyView.image.size);

// Get a reference to the context we created
CGContextRef context = UIGraphicsGetCurrentContext();

// Set the fill colour
//[[UIColor colorWithRed:color.CGColor green:green blue:blue alpha:1.0] setFill];
[color setFill];

// translate/flip the graphics context (for transforming from CG* coords to UI* coords
CGContextTranslateCTM(context, 0, self.bodyView.image.size.height);
CGContextScaleCTM(context, 1.0, -1.0);

// set the blend mode and the original image
CGContextSetBlendMode(context, kCGBlendModeOverlay);
CGRect rect = CGRectMake(0, 0, self.bodyView.image.size.width, self.bodyView.image.size.height);
CGContextDrawImage(context, rect, myImage.CGImage);

// Set a mask that matches the shape of the image, then draw (colour burn) a coloured rectangle
CGContextClipToMask(context, rect, self.bodyView.image.CGImage);
CGContextAddRect(context, rect);
CGContextDrawPath(context, kCGPathFill);

// Generate a new UIImage from the graphics context we drew onto
UIImage *colouredImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

self.bodyView.image = colouredImage;
}

编辑：我正在着色的图像非常大。它是 1541 x 2000 像素，因为我希望能够在不损失质量的情况下放大。也许这就是问题所在。我会继续修补，看看我能找到什么。

Answer 1

我不确定您的着色方法，但我通过在一小段延迟（即用户已暂停滑动）后仅调用性能密集型方法来解决与滑块类似的性能问题。

创建一些类变量/属性来保存与时间相关的对象：

@property (nonatomic, strong) NSDate *sliderValueChangedDate;
@property (nonatomic, strong) NSTimer *sliderValueChangedTimer;

在您连接到UISlider 事件的方法中：

- (IBAction)sliderValueChanged:(id)sender {
    // Save the time the slider was changed.
    self.sliderValueChangedDate = [NSDate date];
    // Start a timer if it's not already running.
    if (!self.sliderValueChangedTimer) {
        self.sliderValueChangedTimer = [NSTimer scheduledTimerWithTimeInterval:0.3 target:self selector:@selector(checkIfImageShouldBeColoured:) userInfo:nil repeats:YES];
    }
}

然后在您的 checkIfImageShouldBeColoured: 方法中，您可以查看值是否在此期间发生了变化：

- (void)checkIfImageShouldBeColoured:(NSTimer *)timer {
    // Get how long has been elapsed since the slider was last changed.
    NSTimeInterval elapsed = -[self.sliderValueChangedDate timeIntervalSinceNow];
    // If this is over our threshold, then perform the intensive method.
    if (elapsed > 0.3) {
        [self.sliderValueChangedTimer invalidate];
        self.sliderValueChangedTimer = nil;
        [self changeBodyColour];
    }
}