如何优化包含带有 CATiledLayer 层的子视图的 UIView 的绘制

Question

我在 UIScrollView 中有一个 UIView，其中包含许多 UIView 子视图。每个子视图都有一个 CATiledLayer 层。此外，我还有一个放大镜功能，可以在其上下文中绘制容器 UIView（以及所有子视图）。相关代码：

这是放大镜的drawRect方法：

- (void)drawRect:(CGRect)rect 
{
CGContextRef context = UIGraphicsGetCurrentContext();

CGContextClipToMask( context , loupeRect, self.maskImage);
CGContextSetFillColorWithColor(context, [[UIColor whiteColor] CGColor]);
CGContextFillRect(context, loupeRect);

CGContextSaveGState( context );
CGContextScaleCTM(context, gridScale,  gridScale);
CGContextTranslateCTM(context, offset.x, offset.y);

CGRect rectToDraw = CGRectMake(-offset.x, -offset.y, 512, 512);
[appDelegate.gridViewController.objectContainerView drawInContext:context forRect:rectToDraw];

CGContextRestoreGState( context );

[overlayImage drawAtPoint:CGPointZero]; 
}

这是绘制子视图的容器UIView的drawInContext:forRect方法：

- (void)drawInContext:(CGContextRef)ctx forRect:(CGRect)rect {

CGRect newrect = CGRectMake(rect.origin.x-1024, rect.origin.y-1024, 2048, 2048);    

for (UIView* v in [self subviews]) {
    float viewscale = v.transform.a;        
    if (CGRectIntersectsRect(newrect,v.frame)) {
        CGContextSaveGState(ctx);
        CGContextScaleCTM(ctx, viewscale, viewscale);
        CGContextTranslateCTM(ctx, v.frame.origin.x/viewscale,  v.frame.origin.y/viewscale);
        [v drawLayer:v.layer inContext:ctx];
        CGContextRestoreGState(ctx);
    }
}   

[super drawLayer:self.layer inContext:ctx];
}

最后，这是带有CATiledLayer的子视图的drawRect方法：

- (void)drawRect:(CGRect)rect {
    CGContextRef context = UIGraphicsGetCurrentContext();

    CGFloat scale = CGContextGetCTM(context).a;
    scale = (scale <= .125) ? .125 : (scale <= .250 ? .250 : (scale <= .5 ? .5 : 1));

    CATiledLayer *tiledLayer = (CATiledLayer *)[self layer];
    CGSize tileSize = tiledLayer.tileSize;

    tileSize.width /= scale;
    tileSize.height /= scale;

    int firstCol = floorf(CGRectGetMinX(rect) / tileSize.width);
    int lastCol = floorf((CGRectGetMaxX(rect)-1) / tileSize.width);
    int firstRow = floorf(CGRectGetMinY(rect) / tileSize.height);
    int lastRow = floorf((CGRectGetMaxY(rect)-1) / tileSize.height);


    for (int row = firstRow; row <= lastRow; row++) {
        for (int col = firstCol; col <= lastCol; col++) {           
            UIImage *tile = [self tileForScale:scale row:row col:col];
            CGRect tileRect = CGRectMake(tileSize.width * col, tileSize.height * row,
                                         tileSize.width, tileSize.height);

        tileRect = CGRectIntersection(self.bounds, tileRect);
            [tile drawInRect:tileRect];
        }
    }
}

现在，一切都是这样工作的然而，当放大镜打开并移动时，应用程序的速度会显着减慢。问题是，每次移动放大视图时，都会调用其drawRect方法（因此它可以更新放大的内容），该方法随后调用容器UIView的drawInContext方法等等......导致所有CATiledLayers更新其图像每次移动放大镜时都会平铺。

正如您所看到的，我尝试在放大镜的上下文中绘制容器视图的较大部分，但这就是我陷入困境的地方。我看不出如何“缓冲”此容器视图的较大部分，因此当移动放大镜时，仅当要重绘的矩形超出“缓冲”矩形时，才会重绘子视图。

抱歉，如果代码很草率/新手 - 我正在其中寻求一些帮助。

谢谢！

Answer 1

您可以对放大镜/放大镜视图将显示的任何内容进行一次性预渲染吗？我不确定您尝试放大的显示是相对静态的还是不断变化的。

在我的应用程序中，用户点击屏幕来放置其他视图。当他们触摸&移动手指，放大镜视图会准确显示该项目的放置位置。此外，当他们积极使用放大镜时，背景图像不会改变（他们可能会在其他时间平移/缩放/移动视图，只是在放大镜恰好打开时除外）。

由于我放大的显示在放大过程中是静态的，因此我能够实现一个不会覆盖“绘制”方法的放大镜。下面是整个实现；它由 ViewController/touch 处理程序实例化并保存。

下面的技巧是截取整个窗口的屏幕截图并将其提供给放大镜的layer.contents。然后使用layer.contentsRect进行裁剪、缩放和缩放。将屏幕截图放置在触摸点处。

• 从调用者的 touchesBegin 调用 [_magnifier magnify...
• 并从 touchesMoved 调用 [_magnifier touchPointMovedTo...
• 最后，touchesEnded调用[_magnifier removeFromSuperview];

MagnifierView.h

#import <UIKit/UIKit.h>
@interface MagnifierView : UIView {
    float _xscale;
    float _yscale;
    float _loupewidth;
    float _loupeheight;
    float _xzoom;
    float _yzoom;
}

- (void) magnify:(CGPoint) touchPoint;
- (void) touchPointMovedTo:(CGPoint) touchPoint;
@end

MagnifierView.m

#import "MagnifierView.h"
#import <QuartzCore/QuartzCore.h>

#define kMagnifierDiameter 120

@implementation MagnifierView

- (id)initWithFrame:(CGRect)frame {
    self = [super initWithFrame:CGRectMake(0, 0, kMagnifierDiameter, kMagnifierDiameter)];

    if (self) {
        self.layer.borderColor = [[UIColor darkGrayColor] CGColor];
        self.layer.borderWidth = 3;
        self.layer.cornerRadius = kMagnifierDiameter / 2;
        self.layer.masksToBounds = YES;


        UIImageView *crosshair = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"crosshairs.png"]];
        [crosshair setCenter:self.center];
        [self addSubview:crosshair];
        [crosshair release];
    }
    return self;
}

- (void) magnify:(CGPoint)touchPoint
{
    UIView *windowview = [[UIApplication sharedApplication] keyWindow];

    /// grab a screenshot of the window (sloppy; goal is to get a CGImageRef to put into this view's layer)
    UIGraphicsBeginImageContext(windowview.bounds.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    [windowview.layer renderInContext:context];
    UIImage *screenshot = UIGraphicsGetImageFromCurrentImageContext();  /// autoreleased
    UIGraphicsEndImageContext();


    /// while we have the image size update our positioning and zooming numbers
    _xscale = 1 / screenshot.size.width;    /// layer units are 0.0 - 1.0 so scale the numbers to that range
    _yscale = 1 / screenshot.size.height;
    _loupewidth = _xscale * CGRectGetWidth(self.frame);     /// scaled size of this view
    _loupeheight = _yscale * CGRectGetHeight(self.frame);
    _xzoom = (_xscale * 18);    /// arbitrary 16; make sure it's in scaled units (negative to zoom out)
    _yzoom = (_yscale * 18);


    /// set our layer contents to the imageRef of the screenshot above (again, sloppy; can we skip UIImage altogether?)
    CGImageRef imageRef = CGImageCreateWithImageInRect([screenshot CGImage], CGRectMake(0, 0, screenshot.size.width, screenshot.size.height));
    self.layer.contents = (id)imageRef;
    CGImageRelease(imageRef);

    /// add us to the window view and move us
    [windowview addSubview:self];
    [self touchPointMovedTo:touchPoint];
}

- (void) touchPointMovedTo:(CGPoint)touchPoint
{
    self.center = CGPointMake(touchPoint.x, touchPoint.y - 80); /// arbitrary 80 so we float 'above' your fat finger

    /// touchPoint is the center; figure out the x,y (top,left)
    float xcenter = (_xscale * touchPoint.x);
    float x = xcenter - (_loupewidth / 2);

    float ycenter = (_yscale * touchPoint.y);
    float y = ycenter - (_loupeheight / 2);

    /// with no additional adjustments this rect will just 'see through' to the screenshot
    CGRect seethrough = CGRectMake(x , y, _loupewidth, _loupeheight);

    /// RectInset with the zoom factor to scale up the contents
    self.layer.contentsRect = CGRectInset(seethrough, _xzoom, _yzoom);

}


- (void)dealloc {
    [super dealloc];
}


@end