異步繪製專題

1 圖片處理

1.1 編輯圖片的幾個方法

第一種

先用UIImage對象加載一張圖片

然後轉化成CGImageRef放到CGContext中去編輯

第二種

用CGImageCreate函數創建CGImageRef

然後把CGImageRef放到CGContext中去編輯

第三種

用CGImageCreateCopy或者CGImageCreateCopyWithColorSpace

函數拷貝

CGImageRef CGImageCreate (

size_t width, //圖片的寬度

size_t height, //圖片的高度

size_t bitsPerComponent, //圖片每個顏色的bits，比如rgb顏色空間，有可能是5或者 8 ==

size_t bitsPerPixel, //每一個像素佔用的buts，15位24位 32位等等

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

CGColorSpaceRef colorspace, //顏色空間，比如rgb

CGBitmapInfo bitmapInfo, //layout，像素中bit的佈局，是rgba還是 argb，＝＝

CGDataProviderRefprovider, //數據源提供者，url或者內存＝＝

const CGFloat decode[], //一個解碼數組

bool shouldInterpolate, //抗鋸齒參數

CGColorRenderingIntent intent

//圖片渲染相關參數

);

1.2 示例代碼

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);

通過這個方法，我們可以創建出一個CGImageRef類型的對象，下面分別對參數進行解釋：

sizt_t是定義的一個可移植性的單位，在64位機器中爲8字節，32位位4字節。

width：圖片寬度像素

height：圖片高度像素

bitsPerComponent：每個顏色的比特數，例如在rgba-32模式下爲8

bitsPerPixel：每個像素的總比特數

bytesPerRow：每一行佔用的字節數，注意這裏的單位是字節

space：顏色空間模式，例如constCFStringRef kCGColorSpaceGenericRGB這個函數可以返回一個顏色空間對象。

bitmapInfo：位圖像素佈局，枚舉如下：

typedef CF_OPTIONS(uint32_t, CGBitmapInfo) {

kCGBitmapAlphaInfoMask = 0x1F,

kCGBitmapFloatComponents = (1 << 8),

kCGBitmapByteOrderMask = 0x7000,

kCGBitmapByteOrderDefault = (0 << 12),

kCGBitmapByteOrder16Little = (1 << 12),

kCGBitmapByteOrder32Little = (2 << 12),

kCGBitmapByteOrder16Big = (3 << 12),

kCGBitmapByteOrder32Big = (4 << 12)

}

provider：數據源提供者

decode[]：解碼渲染數組

shouldInterpolate：是否抗鋸齒

intent：圖片相關參數

CGImageRef CGImageMaskCreate(size_t width,size_t height,

size_t bitsPerComponent,size_t bitsPerPixel,size_t bytesPerRow,

CGDataProviderRef provider,constCGFloat decode[],bool shouldInterpolate)

這個方法用於創建mask圖片圖層，可以設置其顯示部分與不顯示部分達到特殊的效果，參數意義同上。

CGImageRef CGImageCreateCopy(CGImageRef image)

這個方法可以複製一個CGImageRef對象

CGImageRef CGImageCreateWithJPEGDataProvider(CGDataProviderRef

source, constCGFloat decode[],bool shouldInterpolate,

CGColorRenderingIntent intent)

通過JPEG數據源獲取圖像

CGImageRef CGImageCreateWithPNGDataProvider(CGDataProviderRef source,

constCGFloat decode[],bool shouldInterpolate,

CGColorRenderingIntent intent)

通過PNG數據源獲取圖像

CGImageRef CGImageCreateWithImageInRect(CGImageRef image,

CGRect rect)

截取圖像的一個區域重繪圖像

CGImageRef CGImageCreateWithMask(CGImageRef image,CGImageRef mask)

截取mask圖像的某一區域重繪

CGImageRef CGImageCreateWithMaskingColors(CGImageRef image,

constCGFloat components[])

通過顏色分量數組創建位圖

CGImageRef CGImageCreateCopyWithColorSpace(CGImageRef image,

CGColorSpaceRef space)

通過顏色空間模式複製位圖

CGImageRef CGImageRetain(CGImageRef image)

引用+1

void CGImageRelease(CGImageRef image)

引用-1

bool CGImageIsMask(CGImageRef image)

返回是否爲Mask圖層

size_t CGImageGetWidth(CGImageRef image)

獲取寬度像素

size_t CGImageGetHeight(CGImageRef image)

獲取高度像素

下面這些方法分別獲取相應屬性

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與JPEG優劣比較

存儲速度：JPG更快

壓縮比：JPG更大；

圖片質量：JPG更好

JPG不支持透明效果；

UIImageJPEGRepresentation方法在耗時上比較少而UIImagePNGRepresentation耗時操作時間比較長

UIImageJPEGRepresentation函數需要兩個參數:圖片的引用和壓縮係數.而UIImagePNGRepresentation只需要圖片引用作爲參數.通過在實際使用過程中,比較發現:UIImagePNGRepresentation(UIImage* image) 要比UIImageJPEGRepresentation(UIImage*image, 1.0) 返回的圖片數據量大很多.譬如,同樣是讀取攝像頭拍攝的同樣景色的照片, UIImagePNGRepresentation()返回的數據量大小爲199K ,而 UIImageJPEGRepresentation(UIImage* image, 1.0)返回的數據量大小隻爲140KB,比前者少了50多KB.如果對圖片的清晰度要求不高,還可以通過設置 UIImageJPEGRepresentation函數的第二個參數,大幅度降低圖片數據量.譬如,剛纔拍攝的圖片, 通過調用UIImageJPEGRepresentation(UIImage*image, 1.0)讀取數據時,返回的數據大小爲140KB,但更改壓縮係數後,通過調用UIImageJPEGRepresentation(UIImage* image, 0.5)讀取數據時,返回的數據大小隻有11KB多,大大壓縮了圖片的數據量 ,而且從視角角度看,圖片的質量並沒有明顯的降低.因此,在讀取圖片數據內容時,建議優先使用UIImageJPEGRepresentation,並可根據自己的實際使用場景,設置壓縮係數,進一步降低圖片數據量大小.

1.4 圖片縮放

圖片縮放的三個函數

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

程序中一個界面用到了好多張大圖，內存報警告了，所以做了一下圖片縮放，在網上找了別人寫的代碼

//把圖片做等比縮放，生成一個新圖片

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 }

//把圖片按照新大小進行裁剪，生成一個新圖片

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;//這個是設置轉換後圖片的尺寸大小

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 //裁剪圖片

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 }

實際應用簡化

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

{

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

//裁剪圖片

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

UIGraphicsBeginImageContext(rect.size);

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

//如果不裁剪圖片可以直接畫

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

UIImage *thumbnail=UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

return thumbnail;

}

附：

UIImage類並沒有提供縮放圖片需要用到的API，是不是覺得很吃驚？沒關係，我們自己來添加一個。

定義縮放圖片的Category

// UIImage+Scale.h

@interface UIImage (scale)

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

@end

實現這個Category的定義

// UIImage+Scale.m

#import "UIImage+Scale.h"

@implementation UIImage (scale)

-(UIImage*)scaleToSize:(CGSize)size

{// 創建一個bitmap的context//並把它設置成爲當前正在使用的context

UIGraphicsBeginImageContext(size);

// 繪製改變大小的圖片

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

// 從當前context中創建一個改變大小後的圖片

UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();

// 使當前的context出堆棧

UIGraphicsEndImageContext();

// 返回新的改變大小後的圖片

return scaledImage;

}

@end

如何使用

// 創建圖片

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

// 更改圖片大小

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

1.5 參考鏈接

IOS-圖片操作集合

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

UIImagePNGRepresentation 存在緩慢問題

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

<iOS>UIImage變爲NSData並進行壓縮

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無

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畫出的按鈕，使用透明的png圖片，爲什麼會出現白邊

http://segmentfault.com/q/1010000000095447

JPG、PNG和GIF圖片的基本原理及優化方法

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

JPEG 原理詳細

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

IOS開發中圖片資源使用png還是jpg格式

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

(good)ios開發圖片格式的選擇:png和jpg

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

IOS開發之保存圖片到Documents目錄及PNG，JPEG格式相互轉換

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

iOS過濾png圖片透明部分點擊事件

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

JPEG壓縮原理

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

png壓縮原理

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

iOS開發，圖片使用png好還是jpg好？

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

2 繪製文本

2.1 NSMutableAttributedString繪製

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 參考資料

IOS開發（78）之繪製文本

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

iOS 界面上繪製不同字體顏色大小的字符串

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 字符屬性NSAttributedString描述

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

NSAttributedString 詳解

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

3 異步繪製

3.1 異步繪製示例

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之後注意用hitTest:withEvent:方法處理事件接收

//用戶觸摸時第一時間加載內容

- (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 參考鏈接

[iOS Animation]-CALayer 繪圖效率-異步繪製

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

CGDataProviderCreateWithData對內存數據的釋放

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

IOS中使用像素位圖(CGImageRef)對圖片進行處理

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

4 Asyncdisplaykit

4.1 參考鏈接

Asyncdisplaykit 指南（一）

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

AsyncDisplayKit 教程：達到 60FPS 的滾動幀率

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

http://asyncdisplaykit.org/guide/

AsyncDisplayKit入門指南

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

5 開發技巧

5.1 常見問題

5.1.1 CGBitmapContextCreateImage繪製後內存泄露導致內存告警

1、 CGBitmapContextCreateImage繪製的圖片會造成內存無法釋放，應該換用CGDataProviderCreateWithCFData。

5.1.1.1 方案一：修改源代碼，入緩存前壓縮

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

SDWebImage有一個SDWebImageDownloaderOperation類來執行下載操作的。裏面有個下載完成的方法：

- (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];就是將data轉換成image。

再看看sd_imageWithData:這個方法：

+ (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;

}

這個方法在UIImage+MultiFormat裏面，是UIImage的一個類別處理。這句話很重要image =[[UIImage alloc] initWithData:data]; SDWebImage把下載下來的data直接轉成image，然後沒做等比縮放直接存起來使用。所以，我們只需要在這邊做處理即可：

UIImage+MultiFormat添加一個方法：

+(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;

// 創建一個bitmap的context

// 並把它設置成爲當前正在使用的context

UIGraphicsBeginImageContext(CGSizeMake(width,height));

if (widthScale> heightScale) {

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

}

else {

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

}

// 從當前context中創建一個改變大小後的圖片

UIImage*newImage = UIGraphicsGetImageFromCurrentImageContext();

// 使當前的context出堆棧

UIGraphicsEndImageContext();

returnnewImage;

}

然後在：image =[[UIImage alloc] initWithData:data];下面調用以下：

if(data.length/1024 > 1024) {

image = [selfcompressImageWith:image];

}

當data大於1M的時候做壓縮處理。革命尚未成功，還需要一步處理。在SDWebImageDownloaderOperation的connectionDidFinishLoading方法裏面的：

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

//將等比壓縮過的image在賦在轉成data賦給self.imageData

NSData *data =UIImageJPEGRepresentation(image, 1);

self.imageData = [NSMutableDatadataWithData:data];

5.1.1.2 方案二：設置全局緩存大小

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

1、首先在appdelegate方法 didFinishLaunchingWithOptions

SDImageCache.sharedImageCache().maxCacheSize=1024*1024*8設置一下最大的緩存大小。

2、在appdelegate applicationDidReceiveMemoryWarning里加入

SDImageCache.sharedImageCache().clearMemory()

SDWebImageManager.sharedManager().cancelAll()

5.1.1.3 方案三：定時清理內存緩存

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

經過嘗試，發現了一個最簡單的完美解決該問題的方法

在使用SDWebImage加載較多圖片造成內存警告時，定期調用

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

5.1.1.4 方案四（不推薦）：修復SD庫代碼，不做解壓，直接返回壓縮的原圖

5.1.1.5 方案五（推薦）：使用CGDataProviderRef進行圖形解壓重繪

iOS開發中界面展示大圖片時UIImage的性能有關問題

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;

}

//僅僅作爲臨時應付方案

// return image;

UIImage*decompressedImage;

@autoreleasepool {

//核心代碼，可以解決內存未釋放問題

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 繪製時底部出現高度不定的細微黑線

問題原因：

將Text做寬高計算時，高度值容易得出小數數值，而頁面繪製均是基於整數像素點繪製，對於小數點部分，系統會做捨去處理（即便有縮放），固留下高度不定的未繪製區域（爲黑色）。

解決方案：

將計算出來的高度值做向下取整處理即可。

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