我正在为 iPhone/iPad 制作一个通用应用程序，以下代码似乎有很多滞后：
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，那么这两种设备上的一切似乎都很好。但是，我仍然想要我的 UIImageView 上的圆角，“myImageView”。
我只在第一个单元初始化期间在 cellForRowAtIndexPath 中执行那段代码，因为我的所有单元都是相同的。
有谁知道为什么会这样？
谢谢！
CALayer 操作正在使用大量的 cpu 功率来绘制。
所以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];