IOS應用開發－圖檔處理（拉伸圖檔 建立縮略圖 解決圖檔旋轉的問題 圖檔編碼及上傳 将圖檔寫入磁盤）

在做項目時我們經常要對圖檔進行一些處理，以達到性能優化或滿足需求。常見的情形有以下幾種

拉伸圖檔

項目中使用的圖檔素材如果能通過拉伸獲得就盡量這樣去做。這樣做有兩個顯而易見的好處，一是能夠減少App安裝包的大小，另外一個則是減少App運作時占據的記憶體空間大小。畢竟App的UI基本上來說是建立在大量的精緻的圖檔上，如果這些圖檔都一概使用螢幕等大小的圖檔，那麼對App的性能及安裝量都是有一定的負面影響的。

對于拉伸圖檔，适配ios 5及之後可以使用

1	- (UIImage *)resizableImageWithCapInsets:(UIEdgeInsets)capInsets

這個方法隻接收一個UIEdgeInsets類型的參數，可以通過設定UIEdgeInsets的top、left、bottom、right來分别指定上端蓋高度、左端蓋寬度、下端蓋高度、右端蓋寬度。這個端蓋的距離值是用機關pt（點，point）衡量的，在普通顯示屏中，1pt = 1pix；在retina顯示屏中，1pt ＝ 2pix。還有一個需要注意的地方，如果端蓋距離值不是整數的話，拉伸後的圖檔會有白條細線。

建立縮略圖

如果有一張大圖，我們隻想要顯示它的指定大小的縮略圖内容，可以這樣做:在UIImage的類别中實作如下方法，調用方法建立縮略圖

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- (UIImage *)imageByScalingAndCroppingForSize:(CGSize)targetSize { UIImage *sourceImage = self; UIImage *newImage = nil; CGSize imageSize = sourceImage.size; CGFloat width = imageSize.width; CGFloat height = imageSize.height; CGFloat targetWidth = targetSize.width; CGFloat targetHeight = targetSize.height; CGFloat scaleFactor = 0.0 ; CGFloat scaledWidth = targetWidth; CGFloat scaledHeight = targetHeight; CGPoint thumbnailPoint = CGPointMake( 0.0 , 0.0 ); if (CGSizeEqualToSize(imageSize, targetSize) == NO) { CGFloat widthFactor = targetWidth / width; CGFloat heightFactor = targetHeight / height; if (widthFactor > heightFactor) scaleFactor = widthFactor; // scale to fit height else scaleFactor = heightFactor; // scale to fit width scaledWidth  = width * scaleFactor; scaledHeight = height * scaleFactor; // center the image if (widthFactor > heightFactor) { thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5 ; } else if (widthFactor < heightFactor) { thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5 ; } } UIGraphicsBeginImageContext(targetSize); // this will crop CGRect thumbnailRect = CGRectZero; thumbnailRect.origin = thumbnailPoint; thumbnailRect.size.width  = scaledWidth; thumbnailRect.size.height = scaledHeight; [sourceImage drawInRect:thumbnailRect]; newImage = UIGraphicsGetImageFromCurrentImageContext(); if (newImage == nil) NSLog(@ "could not scale image" ); //pop the context to get back to the default UIGraphicsEndImageContext(); return newImage; }

解決圖檔旋轉的問題

ios程式中使用系統相機拍照和從相冊選取圖檔，直接上傳後在非mac系統下看到的圖檔會發生旋轉的現象，那是因為我們沒有通過圖檔的旋轉屬性修改圖檔轉向。可以用下面的方法解決這個問題:

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@interface UIImage (fixOrientation) - (UIImage *)fixOrientation; @end @implementation UIImage (fixOrientation) - (UIImage *)fixOrientation { // No-op if the orientation is already correct if (self.imageOrientation == UIImageOrientationUp) return self; // We need to calculate the proper transformation to make the image upright. // We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored. CGAffineTransform transform = CGAffineTransformIdentity; switch (self.imageOrientation) { case UIImageOrientationDown: case UIImageOrientationDownMirrored: transform = CGAffineTransformTranslate(transform, self.size.width, self.size.height); transform = CGAffineTransformRotate(transform, M_PI); break ; case UIImageOrientationLeft: case UIImageOrientationLeftMirrored: transform = CGAffineTransformTranslate(transform, self.size.width, ); transform = CGAffineTransformRotate(transform, M_PI_2); break ; case UIImageOrientationRight: case UIImageOrientationRightMirrored: transform = CGAffineTransformTranslate(transform, , self.size.height); transform = CGAffineTransformRotate(transform, -M_PI_2); break ; } switch (self.imageOrientation) { case UIImageOrientationUpMirrored: case UIImageOrientationDownMirrored: transform = CGAffineTransformTranslate(transform, self.size.width, ); transform = CGAffineTransformScale(transform, - 1 , 1 ); break ; case UIImageOrientationLeftMirrored: case UIImageOrientationRightMirrored: transform = CGAffineTransformTranslate(transform, self.size.height, ); transform = CGAffineTransformScale(transform, - 1 , 1 ); break ; } // Now we draw the underlying CGImage into a new context, applying the transform // calculated above. CGContextRef ctx = CGBitmapContextCreate(NULL, self.size.width, self.size.height, CGImageGetBitsPerComponent(self.CGImage), , CGImageGetColorSpace(self.CGImage), CGImageGetBitmapInfo(self.CGImage)); CGContextConcatCTM(ctx, transform); switch (self.imageOrientation) { case UIImageOrientationLeft: case UIImageOrientationLeftMirrored: case UIImageOrientationRight: case UIImageOrientationRightMirrored: // Grr... CGContextDrawImage(ctx, CGRectMake( , ,self.size.height,self.size.width), self.CGImage); break ; default : CGContextDrawImage(ctx, CGRectMake( , ,self.size.width,self.size.height), self.CGImage); break ; } // And now we just create a new UIImage from the drawing context CGImageRef cgimg = CGBitmapContextCreateImage(ctx); UIImage *img = [UIImage imageWithCGImage:cgimg]; CGContextRelease(ctx); CGImageRelease(cgimg); return img; } @end

圖檔編碼及上傳

有時候我們會需要将圖檔資料以字元串的形式上傳到伺服器。在将UIImage對象轉化為NSData再轉化為NSString的時候，NSString對象中會出現有亂碼的情況，這個時候再将NSData轉化為NSString之前要編碼NSData對象

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# import "UIImage+Ext.h" # import "GTMBase64.h" @interface UIImage (Ext) - (NSString *)convertToString; @end @implementation UIImage (Ext) - (NSString *)convertToString { if (!self) { return nil; } NSData *imgData = UIImageJPEGRepresentation(self, 0.5 ); NSData *encode = [GTMBase64 encodeData:imgData]; // base64編碼NSData(解決亂碼問題) NSString *imgStr = [[NSString alloc] initWithData:encode encoding:NSUTF8StringEncoding]; return imgStr; } @end

将圖檔寫入磁盤

要将圖檔存儲到本地磁盤中，需要先把圖檔對象轉化為NSData對象，然後調用writeToFile:接口寫入

1	- (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile;