CALayer setCornerRadius 在 iPad 的 UITableView 中滞后？

Question

我正在为 iPhone/iPad 制作一个通用应用程序，似乎下面的代码有很多滞后：

CALayer *imageLayer = [cell.myImageView layer];
[imageLayer setMasksToBounds:YES];
[imageLayer setCornerRadius:6.0];

奇怪的是，iPhone 版本没有滞后，而 iPad 版本有一些滞后。我对两种设备之间的代码没有区别，但是如果我注释掉上面的代码，那么这两种设备似乎都很好。但是，我仍然想要 UIImageView 上的圆角，“myImageView”。

我只在第一个单元格初始化期间在 cellForRowAtIndexPath 中执行那段代码，因为我的所有单元格都是相同的。

有人知道为什么会这样吗？

谢谢！

Answer 1

CALayer 操作使用大量 cpu 功率来绘制。

因此，最好的解决方案是使用核心图形绘制图像。 Loren Brichter 是第一个向我们展示如何：

这是他的 ABTableViewCell 标头 (.h) 文件的代码

// Copyright (c) 2008 Loren Brichter
// 
// Permission is hereby granted, free of charge, to any person
// obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without
// restriction, including without limitation the rights to use,
// copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following
// conditions:
// 
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.
//
//  ABTableViewCell.h
//
//  Created by Loren Brichter
//  Copyright 2008 Loren Brichter. All rights reserved.
//

#import <UIKit/UIKit.h>

// to use: subclass ABTableViewCell and implement -drawContentView:

@interface ABTableViewCell : UITableViewCell
{
    UIView *contentView;
}

- (void)drawContentView:(CGRect)r; // subclasses should implement

@end

这里是 impelementation (.m) 文件的代码：

// Copyright (c) 2008 Loren Brichter
// 
// Permission is hereby granted, free of charge, to any person
// obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without
// restriction, including without limitation the rights to use,
// copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following
// conditions:
// 
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.
//
//  ABTableViewCell.m
//
//  Created by Loren Brichter
//  Copyright 2008 Loren Brichter. All rights reserved.
// 

#import "ABTableViewCell.h"

@interface ABTableViewCellView : UIView
@end

@implementation ABTableViewCellView

- (void)drawRect:(CGRect)r
{
    [(ABTableViewCell *)[self superview] drawContentView:r];
}

@end



@implementation ABTableViewCell

- (id)initWithStyle:(UITableViewCellStyle)style reuseIdentifier:(NSString *)reuseIdentifier
{
    if ((self = [super initWithStyle:style reuseIdentifier:reuseIdentifier])) {
        contentView = [[ABTableViewCellView alloc]initWithFrame:CGRectZero];
        contentView.opaque = YES;
        [self addSubview:contentView];
    }
    return self;
}


- (void)setFrame:(CGRect)f
{
    [super setFrame:f];
    CGRect b = [self bounds];
    b.size.height -= 1; // leave room for the seperator line
    [contentView setFrame:b];
}

- (void)setNeedsDisplay
{
    [super setNeedsDisplay];
    [contentView setNeedsDisplay];
}

- (void)drawContentView:(CGRect)r
{
    // subclasses should implement this
}

@end

现在您需要做的是使用 Core Graphics 在 drawContentView 方法中绘制您的单元格。

这是我在正在开发的应用程序中使用的部分代码：

    CALayer *cellLayer = [[CALayer alloc] init];
    CGRect cellFrame = self.bounds;
    CGRect layerFrame = CGRectInset(cellFrame, 3, 3);

    cellLayer.frame = cellFrame;
    cellLayer.contentsScale = [[UIScreen mainScreen] scale];

     //round corners in Core Graphics (much faster than using the CALayer cornerRadius)
     UIGraphicsBeginImageContextWithOptions(self.bounds.size, NO, [[UIScreen mainScreen] scale]);

     CGContextRef imgContext = UIGraphicsGetCurrentContext();

    CGContextSaveGState(imgContext);  
    //create the rounded rectangle to draw the image in
    CGPathRef clippingPath = [UIBezierPath bezierPathWithRoundedRect:layerFrame cornerRadius:6.0f].CGPath;
    CGContextAddPath(imgContext, clippingPath);
    CGContextClip(imgContext);

CGDataProviderRef imgDataProvider = CGDataProviderCreateWithCFData((__bridge CFDataRef)[NSData dataWithContentsOfFile:ImageName]);
            CGImageRef imageRef = CGImageCreateWithJPEGDataProvider(imgDataProvider, NULL, true, kCGRenderingIntentDefault);
            CGDataProviderRelease(imgDataProvider);
            CGContextDrawImage (imgContext, layerFrame, imageRef);
            CGImageRelease(imageRef);

CGContextRestoreGState(imgContext);

        //draw white outline
        CGPathRef path = [UIBezierPath bezierPathWithRoundedRect:CGRectInset(cellFrame, 3, 3) cornerRadius:6.0f].CGPath;
        CGContextAddPath(imgContext, path);
        CGContextSetLineWidth(imgContext, 2.0);
        CGContextSetRGBStrokeColor(imgContext,1,1,1,1);
        CGContextStrokePath(imgContext);

        // Get the image from the context
        UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();


        cellLayer.contents = (__bridge id)img.CGImage;

要使用自定义单元格，您需要以这种方式在 tableView CellForRowAtIndexPath 数据源方法中对其进行初始化：

    static NSString *CellIdentifier = @"TableCell";

    ABTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

    if (cell == nil) {        
        cell = [[ABTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
        cell.selectionStyle = UITableViewCellSelectionStyleNone;
}

还请记住，由于您正在进行自定义绘图，因此当您使用以下方法修改自己的内容时，您需要始终重新调整单元格视图：

[cell setNeedsDisplay];