IOS开发系列——异步绘制专题

江中散人_Jun

发布于 2022-03-08 14:24:16

1.4K0

发布于 2022-03-08 14:24:16

文章被收录于专栏：云原生布道专栏

异步绘制专题

1 图片处理

1.1 编辑图片的几个方法

第一种

先用UIImage对象加载一张图片

然后转化成CGImageRef放到CGContext中去编辑

第二种

用CGImageCreate函数创建CGImageRef

然后把CGImageRef放到CGContext中去编辑

第三种

用CGImageCreateCopy或者CGImageCreateCopyWithColorSpace

函数拷贝

CGImageRef CGImageCreate (

size_t width, //图片的宽度

size_t height, //图片的高度

size_t bitsPerComponent, //图片每个颜色的bits，比如rgb颜色空间，有可能是5或者 8 ==

size_t bitsPerPixel, //每一个像素占用的buts，15位24位 32位等等

size_t bytesPerRow, //每一行占用多少bytes注意是bytes不是bits 1byte＝ 8bit

CGColorSpaceRef colorspace, //颜色空间，比如rgb

CGBitmapInfo bitmapInfo, //layout，像素中bit的布局，是rgba还是 argb，＝＝

CGDataProviderRefprovider, //数据源提供者，url或者内存＝＝

const CGFloat decode[], //一个解码数组

bool shouldInterpolate, //抗锯齿参数

CGColorRenderingIntent intent

//图片渲染相关参数

);

1.2 示例代码

CGImageRef CGImageCreate(size_t width,size_t height,

size_tbitsPerComponent,size_t bitsPerPixel,size_t bytesPerRow,

CGColorSpaceRef space, CGBitmapInfo bitmapInfo, CGDataProviderRefprovider,

constCGFloat decode[],bool shouldInterpolate,

CGColorRenderingIntent intent);

通过这个方法，我们可以创建出一个CGImageRef类型的对象，下面分别对参数进行解释：

sizt_t是定义的一个可移植性的单位，在64位机器中为8字节，32位位4字节。

width：图片宽度像素

height：图片高度像素

bitsPerComponent：每个颜色的比特数，例如在rgba-32模式下为8

bitsPerPixel：每个像素的总比特数

bytesPerRow：每一行占用的字节数，注意这里的单位是字节

space：颜色空间模式，例如constCFStringRef kCGColorSpaceGenericRGB这个函数可以返回一个颜色空间对象。

bitmapInfo：位图像素布局，枚举如下：

provider：数据源提供者

decode[]：解码渲染数组

shouldInterpolate：是否抗锯齿

intent：图片相关参数

CGImageRef CGImageMaskCreate(size_t width,size_t height,

size_t bitsPerComponent,size_t bitsPerPixel,size_t bytesPerRow,

CGDataProviderRef provider,constCGFloat decode[],bool shouldInterpolate)

这个方法用于创建mask图片图层，可以设置其显示部分与不显示部分达到特殊的效果，参数意义同上。

CGImageRef CGImageCreateCopy(CGImageRef image)

这个方法可以复制一个CGImageRef对象

CGImageRef CGImageCreateWithJPEGDataProvider(CGDataProviderRef

source, constCGFloat decode[],bool shouldInterpolate,

CGColorRenderingIntent intent)

通过JPEG数据源获取图像

CGImageRef CGImageCreateWithPNGDataProvider(CGDataProviderRef source,

constCGFloat decode[],bool shouldInterpolate,

CGColorRenderingIntent intent)

通过PNG数据源获取图像

CGImageRef CGImageCreateWithImageInRect(CGImageRef image,

CGRect rect)

截取图像的一个区域重绘图像

CGImageRef CGImageCreateWithMask(CGImageRef image,CGImageRef mask)

截取mask图像的某一区域重绘

CGImageRef CGImageCreateWithMaskingColors(CGImageRef image,

constCGFloat components[])

通过颜色分量数组创建位图

CGImageRef CGImageCreateCopyWithColorSpace(CGImageRef image,

CGColorSpaceRef space)

通过颜色空间模式复制位图

CGImageRef CGImageRetain(CGImageRef image)

引用+1

void CGImageRelease(CGImageRef image)

引用-1

bool CGImageIsMask(CGImageRef image)

返回是否为Mask图层

size_t CGImageGetWidth(CGImageRef image)

获取宽度像素

size_t CGImageGetHeight(CGImageRef image)

获取高度像素

下面这些方法分别获取相应属性

size_t CGImageGetBitsPerComponent(CGImageRef image)

size_t CGImageGetBitsPerPixel(CGImageRef image)

size_t CGImageGetBytesPerRow(CGImageRef image)

CGColorSpaceRef CGImageGetColorSpace(CGImageRef image)CG_EXTERNCGImageAlphaInfo CGImageGetAlphaInfo(CGImageRef image)

CGDataProviderRef CGImageGetDataProvider(CGImageRef image)

constCGFloat *CGImageGetDecode(CGImageRef image)

bool CGImageGetShouldInterpolate(CGImageRef image)

CGColorRenderingIntent CGImageGetRenderingIntent(CGImageRef image)

CGBitmapInfo CGImageGetBitmapInfo(CGImageRef image)

1.3 PNG与JPEG优劣比较

存储速度：JPG更快

压缩比：JPG更大；

图片质量：JPG更好

JPG不支持透明效果；

UIImageJPEGRepresentation方法在耗时上比较少而UIImagePNGRepresentation耗时操作时间比较长

UIImageJPEGRepresentation函数需要两个参数:图片的引用和压缩系数.而UIImagePNGRepresentation只需要图片引用作为参数.通过在实际使用过程中,比较发现:UIImagePNGRepresentation(UIImage* image) 要比UIImageJPEGRepresentation(UIImage*image, 1.0) 返回的图片数据量大很多.譬如,同样是读取摄像头拍摄的同样景色的照片, UIImagePNGRepresentation()返回的数据量大小为199K ,而 UIImageJPEGRepresentation(UIImage* image, 1.0)返回的数据量大小只为140KB,比前者少了50多KB.如果对图片的清晰度要求不高,还可以通过设置 UIImageJPEGRepresentation函数的第二个参数,大幅度降低图片数据量.譬如,刚才拍摄的图片, 通过调用UIImageJPEGRepresentation(UIImage*image, 1.0)读取数据时,返回的数据大小为140KB,但更改压缩系数后,通过调用UIImageJPEGRepresentation(UIImage* image, 0.5)读取数据时,返回的数据大小只有11KB多,大大压缩了图片的数据量 ,而且从视角角度看,图片的质量并没有明显的降低.因此,在读取图片数据内容时,建议优先使用UIImageJPEGRepresentation,并可根据自己的实际使用场景,设置压缩系数,进一步降低图片数据量大小.

1.4 图片缩放

图片缩放的三个函数

http://www.cnblogs.com/pengyingh/articles/2355052.html

程序中一个界面用到了好多张大图，内存报警告了，所以做了一下图片缩放，在网上找了别人写的代码

//把图片做等比缩放，生成一个新图片

1 - (UIImage*) imageByScalingProportionallyToSize:(CGSize)targetSize sourceImage:(UIImage*)sourceImage {

3 UIGraphicsBeginImageContext(targetSize);

4 [sourceImage drawInRect:CGRectMake(0,0,targetSize.width, targetSize.height)];

5 UIImage* scaledImage =UIGraphicsGetImageFromCurrentImageContext();

6 UIGraphicsEndImageContext();

7 returnscaledImage;

9 UIImage *newImage = nil;

10 CGSize imageSize = sourceImage.size;

11 CGFloat width = imageSize.width;

12 CGFloat height = imageSize.height;

13 CGFloat targetWidth = targetSize.width;

14 CGFloat targetHeight = targetSize.height;

15 CGFloat scaleFactor = 0.0;

16 CGFloat scaledWidth = targetWidth;

17 CGFloat scaledHeight = targetHeight;

18 CGPoint thumbnailPoint = CGPointMake(0.0,0.0);

20 UIGraphicsBeginImageContext(targetSize);// this will crop

22 CGRect thumbnailRect = CGRectZero;

23 thumbnailRect.origin = thumbnailPoint;

24 thumbnailRect.size.width = scaledWidth;

25 thumbnailRect.size.height = scaledHeight;

27 [sourceImage drawInRect:thumbnailRect];

29 newImage =UIGraphicsGetImageFromCurrentImageContext();

30 if(newImage== nil)

31 NSLog(@"couldnot scale image");

33 //pop thecontext to get back to the default

34 UIGraphicsEndImageContext();

35 returnnewImage;

36 }

//把图片按照新大小进行裁剪，生成一个新图片

1 - (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize image:(UIImage *)sourceImage

2 {

3 // UIImage *sourceImage = self;

4 UIImage *newImage = nil;

5 CGSize imageSize = sourceImage.size;

6 CGFloat width = imageSize.width;

7 CGFloat height = imageSize.height;

8 CGFloat targetWidth = targetSize.width;

9 CGFloat targetHeight = targetSize.height;

10 CGFloat scaleFactor = 0.0;

11 CGFloat scaledWidth = targetWidth;

12 CGFloat scaledHeight = targetHeight;

13 CGPoint thumbnailPoint = CGPointMake(0.0,0.0);

15 if(CGSizeEqualToSize(imageSize, targetSize) == NO)

16 {

17 CGFloat widthFactor = targetWidth /width;

18 CGFloat heightFactor = targetHeight / height;

20 if(widthFactor > heightFactor)

21 scaleFactor = widthFactor;// scale to fit height

22 else

23 scaleFactor = heightFactor;// scale to fit width

24 scaledWidth = width * scaleFactor;

25 scaledHeight = height * scaleFactor;

27 // centerthe image

28 if(widthFactor > heightFactor)

29 {

30 thumbnailPoint.y = (targetHeight -scaledHeight) *0.5;

31 }

32 else

33 if(widthFactor < heightFactor)

34 {

35 thumbnailPoint.x =(targetWidth - scaledWidth) *0.5;

36 }

37 }

39 UIGraphicsBeginImageContext(targetSize);// this will crop

41 CGRect thumbnailRect = CGRectZero;

42 thumbnailRect.origin = thumbnailPoint;

43 thumbnailRect.size.width = scaledWidth;

44 thumbnailRect.size.height = scaledHeight;

46 [sourceImage drawInRect:thumbnailRect];

48 newImage =UIGraphicsGetImageFromCurrentImageContext();

49 if(newImage== nil)

50 NSLog(@"couldnot scale image");

52 //pop thecontext to get back to the default

53 UIGraphicsEndImageContext();

54 returnnewImage;

55 }

1 - (UIImage*)generatePhotoThumbnail:(UIImage *)image

2 {

3 // Create a thumbnail version of the imagefor the event object.

4 CGSize size = image.size;

5 CGSize croppedSize;

6 CGFloat ratio = 64.0;//这个是设置转换后图片的尺寸大小

7 CGFloat offsetX = 0.0;

8 CGFloat offsetY = 0.0;

10 // check thesize of the image, we want to make it

11 // a square with sides the size of the smallest dimension

12 if(size.width > size.height) {

13 offsetX = (size.height - size.width) /2;

14 croppedSize = CGSizeMake(size.height,size.height);

15 } else {

16 offsetY = (size.width - size.height) /2;

17 croppedSize = CGSizeMake(size.width,size.width);

18 }

20 // Crop theimage before resize

21 CGRect clippedRect = CGRectMake(offsetX *-1, offsetY * -1,croppedSize.width, croppedSize.height);

22 //裁剪图片

CGImageRef imageRef =CGImageCreateWithImageInRect([image CGImage], clippedRect);

23 // Donecropping

25 // Resize the image

26 CGRect rect = CGRectMake(0.0,0.0,ratio, ratio);

28 UIGraphicsBeginImageContext(rect.size);

29 [[UIImage imageWithCGImage:imageRef]drawInRect:rect];

30 UIImage *thumbnail =UIGraphicsGetImageFromCurrentImageContext();

31 UIGraphicsEndImageContext();

32 // DoneResizing

34 returnthumbnail;

35 }

实际应用简化

- (UIImage *)generatePhotoThumbnail:(UIImage*)image

{

CGRect rect=CGRectMake(0, 0, 60, 78);

//裁剪图片

CGImageRef imageRef=CGImageCreateWithImageInRect([image CGImage],CGRectMake(0,0,140,182));

UIGraphicsBeginImageContext(rect.size);

[[UIImage imageWithCGImage:imageRef]drawInRect:rect];

//如果不裁剪图片可以直接画

//[image drawInRect:CGRectMake(0, 0, theSize.width, theSize.height)];

UIImage *thumbnail=UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

return thumbnail;

}

附：

UIImage类并没有提供缩放图片需要用到的API，是不是觉得很吃惊？没关系，我们自己来添加一个。

定义缩放图片的Category

// UIImage+Scale.h

@interface UIImage (scale)

-(UIImage*)scaleToSize:(CGSize)size;

@end

实现这个Category的定义

// UIImage+Scale.m

#import "UIImage+Scale.h"

@implementation UIImage (scale)

-(UIImage*)scaleToSize:(CGSize)size

{// 创建一个bitmap的context//并把它设置成为当前正在使用的context

UIGraphicsBeginImageContext(size);

// 绘制改变大小的图片

[self drawInRect:CGRectMake(0,0,size.width, size.height)];

// 从当前context中创建一个改变大小后的图片

UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();

// 使当前的context出堆栈

UIGraphicsEndImageContext();

// 返回新的改变大小后的图片

return scaledImage;

}

@end

如何使用

// 创建图片

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

// 更改图片大小

UIImage *scaledImage =[image scaleToSize:CGSizeMake(25.0f,35.0f)]

1.5 参考链接

IOS-图片操作集合

http://blog.csdn.net/ch_soft/article/details/7685753

UIImagePNGRepresentation 存在缓慢问题

http://blog.sina.com.cn/s/blog_95a3991f010162ws.html

<iOS>UIImage变为NSData并进行压缩

http://www.cnblogs.com/robinkey/archive/2013/01/21/2869930.html

UIImageJPEGRepresentation和UIImagePNGRepresentation

http://blog.csdn.net/mideveloper/article/details/11473627

png有透明通道，JPEG无

http://blog.163.com/chentong1115@126/blog/static/45314732200972323921819/

透明PNG圖片有黑邊的解決方法

http://www.minroad.com/?p=9

用UIImage和UIButton画出的按钮，使用透明的png图片，为什么会出现白边

http://segmentfault.com/q/1010000000095447

JPG、PNG和GIF图片的基本原理及优化方法

http://www.mahaixiang.cn/Photoshop/400.html

JPEG 原理详细

http://blog.chinaunix.net/uid-27002868-id-3220554.html

IOS开发中图片资源使用png还是jpg格式

http://www.cnblogs.com/wengzilin/p/3485298.html

(good)ios开发图片格式的选择:png和jpg

http://m.blog.csdn.net/blog/awaylin113/22712317

IOS开发之保存图片到Documents目录及PNG，JPEG格式相互转换

http://blog.csdn.net/sanpintian/article/details/7418755

iOS过滤png图片透明部分点击事件

http://www.cocoachina.com/industry/20121127/5192.html

JPEG压缩原理

http://blog.csdn.net/xfortius/article/details/8904012

png压缩原理

http://blog.csdn.net/zykun/article/details/1825086

iOS开发，图片使用png好还是jpg好？

http://www.cocoachina.com/bbs/read.php?tid=110115

2 绘制文本

2.1 NSMutableAttributedString绘制

CGRect textViewRect = CGRectMake(ICON_SPACE*1-2,_imageHeight +ICON_SPACE,_postContentTextView.frame.size.width,_labelSize);//ceilf(_labelSize)

// [_postContentTextViewdrawRect:textViewRect];

NSMutableAttributedString *str = [[NSMutableAttributedStringalloc]initWithString:_vm.contentText];

[str addAttribute:NSForegroundColorAttributeNamevalue:kContentTextColorrange:NSMakeRange(0,[_vm.contentTextlength])];

[str addAttribute:NSFontAttributeNamevalue:kContentTextFontrange:NSMakeRange(0,[_vm.contentTextlength])];

[str addAttribute:NSBackgroundColorDocumentAttributevalue:[UIColorwhiteColor]range:NSMakeRange(0,[_vm.contentTextlength])];

// NSLog(@"ContentText:%@, Frame:%@",_vm.contentText, NSStringFromCGRect(_postContentTextView.frame));

// NSLog(@"TextViewRect:%@",NSStringFromCGRect(textViewRect));

[str drawInRect:textViewRect];

2.2 参考资料

IOS开发（78）之绘制文本

http://www.2cto.com/kf/201305/212045.html

iOS 界面上绘制不同字体颜色大小的字符串

http://blog.csdn.net/wsk_123_123/article/details/23277457

初探NSAttributedString和NSMutableAttributedString的使用 -LiuWJ

http://www.tuicool.com/articles/Fvqia2

iOS 字符属性NSAttributedString描述

http://my.oschina.net/lanrenbar/blog/395909

NSAttributedString 详解

http://www.cnblogs.com/zhw511006/archive/2012/09/21/2696700.html

3 异步绘制

3.1 异步绘制示例

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{

CGRect drawRect = _bgImageView.frame;

UIGraphicsBeginImageContextWithOptions(drawRect.size,YES,0);

CGContextRef context = UIGraphicsGetCurrentContext();

if (!context) {

return;

}

[[UIColor whiteColor] set];

CGContextFillRect(context, drawRect);

CGRect imgRect = CGRectZero;

if ([_vm.contentImgPathlength] >0) {

imgRect = CGRectMake(0,0,BODY_HEIGTH,_imageHeight);//_postContentImageView.frame;

[_vm.contentImagedrawInRect:_postContentImageView.frameblendMode:kCGBlendModeNormalalpha:1];

}

CGRect textViewRect = CGRectZero;

if ([_vm.contentTextlength] >0) {

NSMutableAttributedString*str;

if (!_isContentDisplayCompletly) {

if (_vm.digestText) {

str = [[NSMutableAttributedStringalloc]initWithString:_vm.digestTextattributes:_postContentTextView.typingAttributes];

} else

str = [[NSMutableAttributedStringalloc]initWithString:_vm.contentTextattributes:_postContentTextView.typingAttributes];

} else

str = [[NSMutableAttributedStringalloc]initWithString:_vm.contentTextattributes:_postContentTextView.typingAttributes];

[str drawInRect:_postContentTextView.frame];

}

if (_subjectTitleHeight>0) {

CGRectsubjectIconFrame =CGRectMake(_subjectButton.frame.origin.x,_subjectButton.frame.origin.y,10,12);//kContentToSubjectSpace

UIImage*iconImg = [UIImageimageNamed:@"PostChannelFlageIcon"];

subjectIconFrame.size =iconImg.size;

[iconImg drawInRect:subjectIconFrameblendMode:kCGBlendModeNormalalpha:1];

CGRectsubjectTitleFrame =CGRectMake(subjectIconFrame.origin.x +subjectIconFrame.size.width +2,subjectIconFrame.origin.y-1,200,_subjectTitleHeight);

[_subjectButton.titleLabel.attributedTextdrawInRect:subjectTitleFrame];

// [_subjectButtondrawRect:_subjectButton.frame];

}

UIImage*temp = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

dispatch_async(dispatch_get_main_queue(),^{

_bgImageView.image =nil;

_bgImageView.image =temp;

[self setHidden:NO];

});

3.2 DrawRect之后注意用hitTest:withEvent:方法处理事件接收

//用户触摸时第一时间加载内容

- (UIView*)hitTest:(CGPoint)point withEvent:(UIEvent*)event{

UIView*result = [superhitTest:pointwithEvent:event];

CGPointbuttonPoint = [_subjectButtonconvertPoint:pointfromView:self];

if ([_subjectButtonpointInside:buttonPointwithEvent:event]){

return _subjectButton;

}

returnresult;

}

3.3 参考链接

[iOS Animation]-CALayer 绘图效率-异步绘制

http://my.oschina.net/u/2438875/blog/507545?fromerr=R4LnEaJ5

CGDataProviderCreateWithData对内存数据的释放

http://www.taofengping.com/2012/11/04/cgdataprovidercreatewithdata_memory_release/#.VnJQ6jaitZF

IOS中使用像素位图(CGImageRef)对图片进行处理

http://my.oschina.net/u/2340880/blog/406437?p={{currentPage-1}}

4 Asyncdisplaykit

4.1 参考链接

Asyncdisplaykit 指南（一）

http://www.th7.cn/Program/IOS/201410/302413.shtml

AsyncDisplayKit 教程：达到 60FPS 的滚动帧率

http://www.cocoachina.com/swift/20141124/10298.html

http://asyncdisplaykit.org/guide/

AsyncDisplayKit入门指南

http://www.cocoachina.com/ios/20141020/9975.html

5 开发技巧

5.1 常见问题

5.1.1 CGBitmapContextCreateImage绘制后内存泄露导致内存告警

1、 CGBitmapContextCreateImage绘制的图片会造成内存无法释放，应该换用CGDataProviderCreateWithCFData。

5.1.1.1 方案一：修改源代码，入缓存前压缩

http://my.oschina.net/u/1244672/blog/510379

SDWebImage有一个SDWebImageDownloaderOperation类来执行下载操作的。里面有个下载完成的方法：

- (void)connectionDidFinishLoading:(NSURLConnection*)aConnection {

SDWebImageDownloaderCompletedBlockcompletionBlock = self.completedBlock;

@synchronized(self) {

CFRunLoopStop(CFRunLoopGetCurrent());

self.thread= nil;

self.connection= nil;

[[NSNotificationCenterdefaultCenter] postNotificationName:SDWebImageDownloadStopNotificationobject:nil];

}

if(![[NSURLCache sharedURLCache] cachedResponseForRequest:_request]) {

responseFromCached= NO;

}

if(completionBlock)

{

if(self.options & SDWebImageDownloaderIgnoreCachedResponse &&responseFromCached) {

completionBlock(nil,nil, nil, YES);

}

else {

UIImage *image= [UIImage sd_imageWithData:self.imageData];

NSString *key= [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];

image = [selfscaledImageForKey:key image:image];

// Do notforce decoding animated GIFs

if(!image.images) {

image = [UIImagedecodedImageWithImage:image];

}

if(CGSizeEqualToSize(image.size, CGSizeZero)) {

completionBlock(nil,nil, [NSError errorWithDomain:@"SDWebImageErrorDomain" code:0userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0pixels"}], YES);

}

else {

completionBlock(image,self.imageData, nil, YES);

}

self.completionBlock= nil;

[self done];

}

其中，UIImage*image = [UIImage sd_imageWithData:self.imageData];就是将data转换成image。

再看看sd_imageWithData:这个方法：

+ (UIImage*)sd_imageWithData:(NSData *)data {

UIImage*image;

NSString*imageContentType = [NSData sd_contentTypeForImageData:data];

if([imageContentType isEqualToString:@"image/gif"]) {

image =[UIImage sd_animatedGIFWithData:data];

}

#ifdef SD_WEBP

else if([imageContentType isEqualToString:@"image/webp"])

{

image =[UIImage sd_imageWithWebPData:data];

}

#endif

else {

image =[[UIImage alloc] initWithData:data];

UIImageOrientationorientation = [self sd_imageOrientationFromImageData:data];

if(orientation != UIImageOrientationUp) {

image =[UIImage imageWithCGImage:image.CGImage

scale:image.scale

orientation:orientation];

}

return image;

}

这个方法在UIImage+MultiFormat里面，是UIImage的一个类别处理。这句话很重要image =[[UIImage alloc] initWithData:data]; SDWebImage把下载下来的data直接转成image，然后没做等比缩放直接存起来使用。所以，我们只需要在这边做处理即可：

UIImage+MultiFormat添加一个方法：

+(UIImage*)compressImageWith:(UIImage *)image

{

float imageWidth= image.size.width;

float imageHeight = image.size.height;

float width =640;

float height =image.size.height/(image.size.width/width);

float widthScale = imageWidth /width;

float heightScale = imageHeight /height;

// 创建一个bitmap的context

// 并把它设置成为当前正在使用的context

UIGraphicsBeginImageContext(CGSizeMake(width,height));

if (widthScale> heightScale) {

[imagedrawInRect:CGRectMake(0, 0, imageWidth /heightScale , height)];

}

else {

[imagedrawInRect:CGRectMake(0, 0, width , imageHeight /widthScale)];

}

// 从当前context中创建一个改变大小后的图片

UIImage*newImage = UIGraphicsGetImageFromCurrentImageContext();

// 使当前的context出堆栈

UIGraphicsEndImageContext();

returnnewImage;

}

然后在：image =[[UIImage alloc] initWithData:data];下面调用以下：

if(data.length/1024 > 1024) {

image = [selfcompressImageWith:image];

}

当data大于1M的时候做压缩处理。革命尚未成功，还需要一步处理。在SDWebImageDownloaderOperation的connectionDidFinishLoading方法里面的：

UIImage *image= [UIImage sd_imageWithData:self.imageData];

//将等比压缩过的image在赋在转成data赋给self.imageData

NSData *data =UIImageJPEGRepresentation(image, 1);

self.imageData = [NSMutableDatadataWithData:data];

5.1.1.2 方案二：设置全局缓存大小

http://www.myexception.cn/swift/2033029.html

1、首先在appdelegate方法 didFinishLaunchingWithOptions

SDImageCache.sharedImageCache().maxCacheSize=1024*1024*8设置一下最大的缓存大小。

2、在appdelegate applicationDidReceiveMemoryWarning里加入

SDImageCache.sharedImageCache().clearMemory()

SDWebImageManager.sharedManager().cancelAll()

5.1.1.3 方案三：定时清理内存缓存

http://www.bubuko.com/infodetail-956863.html

经过尝试，发现了一个最简单的完美解决该问题的方法

在使用SDWebImage加载较多图片造成内存警告时，定期调用

[[SDImageCache sharedImageCache] setValue:nilforKey:@"memCache"];

5.1.1.4 方案四（不推荐）：修复SD库代码，不做解压，直接返回压缩的原图

5.1.1.5 方案五（推荐）：使用CGDataProviderRef进行图形解压重绘

iOS开发中界面展示大图片时UIImage的性能有关问题

http://www.myexception.cn/operating-system/578931.html

#import "SDWebImageDecoder.h"

@implementation UIImage (ForceDecode)

+ (UIImage*)decodedImageWithImage:(UIImage *)image {

if (image.images) {

// Do notdecode animated images

return image;

}

//仅仅作为临时应付方案

// return image;

UIImage*decompressedImage;

@autoreleasepool {

//核心代码，可以解决内存未释放问题

NSData *data=UIImageJPEGRepresentation(image,1);

CGDataProviderRef dataProvider =CGDataProviderCreateWithCFData((__bridgeCFDataRef)data);

CGImageRefimageRef =CGImageCreateWithPNGDataProvider(dataProvider,

NULL,NO,

kCGRenderingIntentDefault);

// CGImageRef imageRef = image.CGImage;

CGSizeimageSize = CGSizeMake(CGImageGetWidth(imageRef),CGImageGetHeight(imageRef));

CGRectimageRect = (CGRect){.origin =CGPointZero, .size =imageSize};

CGColorSpaceRefcolorSpace =CGColorSpaceCreateDeviceRGB();

CGBitmapInfobitmapInfo = CGImageGetBitmapInfo(imageRef);

intinfoMask = (bitmapInfo &kCGBitmapAlphaInfoMask);

BOOLanyNonAlpha = (infoMask ==kCGImageAlphaNone ||

infoMask == kCGImageAlphaNoneSkipFirst ||

infoMask == kCGImageAlphaNoneSkipLast);

// CGBitmapContextCreatedoesn't support kCGImageAlphaNone with RGB.

// https://developer.apple.com/library/mac/#qa/qa1037/_index.html

if(infoMask == kCGImageAlphaNone&& CGColorSpaceGetNumberOfComponents(colorSpace)>1) {

// Unset the old alpha info.

bitmapInfo &= ~kCGBitmapAlphaInfoMask;

// Set noneSkipFirst.

bitmapInfo |= kCGImageAlphaNoneSkipFirst;

}

// Some PNGs tell us theyhave alpha but only 3 components. Odd.

else if(!anyNonAlpha && CGColorSpaceGetNumberOfComponents(colorSpace)==3) {

// Unset the old alphainfo.

bitmapInfo &= ~kCGBitmapAlphaInfoMask;

bitmapInfo |= kCGImageAlphaPremultipliedFirst;

}

// It calculates thebytes-per-row based on the bitsPerComponent and width arguments.

CGContextRefcontext = CGBitmapContextCreate(NULL,

imageSize.width,

imageSize.height,

CGImageGetBitsPerComponent(imageRef),

colorSpace,

bitmapInfo);

CGColorSpaceRelease(colorSpace);

// If failed, returnundecompressed image

if(!context) return image;

CGContextDrawImage(context,imageRect, imageRef);

CGImageRefdecompressedImageRef =CGBitmapContextCreateImage(context);

CGContextRelease(context);

decompressedImage = [UIImageimageWithCGImage:decompressedImageRefscale:image.scaleorientation:image.imageOrientation];

CGImageRelease(decompressedImageRef);

}

// CVPixelBufferRef pixelBuffer;

// CreateCGImageFromCVPixelBuffer(pixelBuffer,&decompressedImageRef);

// CGImage *cgImage =CGBitmapContextCreateImage(context);

// CFDataRef dataRef =CGDataProviderCopyData(CGImageGetDataProvider(cgImage));

// CGImageRelease(cgImage);

// image->imageRef = dataRef;

// image->image =CFDataGetBytePtr(dataRef);

returndecompressedImage;

}

5.1.2 绘制时底部出现高度不定的细微黑线

问题原因：

将Text做宽高计算时，高度值容易得出小数数值，而页面绘制均是基于整数像素点绘制，对于小数点部分，系统会做舍去处理（即便有缩放），固留下高度不定的未绘制区域（为黑色）。

解决方案：

将计算出来的高度值做向下取整处理即可。

CGRect labelFrame = [content boundingRectWithSize:size options: NSStringDrawingTruncatesLastVisibleLine | NSStringDrawingUsesLineFragmentOrigin |NSStringDrawingUsesFontLeadingattributes:_postContentTextView.typingAttributescontext:nil];

labelSize = labelFrame.size;

labelSize.height = ceilf(labelSize.height);