ios 根据frame截取图片

//创建UIImage Category 
-(UIImage *)clipImage:(CGSize)size {
    UIImage *aImage = [self fixOrientation] ;
    //被切图片宽比例比高比例小 或者相等，以图片宽进行放大
    if (aImage.size.width*size.height <= aImage.size.height*size.width) {
        //以被剪裁图片的宽度为基准，得到剪切范围的大小
        CGFloat width  = aImage.size.width;
        CGFloat height = aImage.size.width * size.height / size.width;
        // 调用剪切方法
        // 这里是以中心位置剪切，也可以通过改变rect的x、y值调整剪切位置
        return [aImage imageFromRect:CGRectMake(0, (aImage.size.height -height)/2, width, height)];
    } else {
        //被切图片宽比例比高比例大，以图片高进行剪裁
        // 以被剪切图片的高度为基准，得到剪切范围的大小
        CGFloat width  = aImage.size.height * size.width / size.height;
        CGFloat height = aImage.size.height;
        // 调用剪切方法
        // 这里是以中心位置剪切，也可以通过改变rect的x、y值调整剪切位置
        return [aImage imageFromRect:CGRectMake((aImage.size.width -width)/2, 0, width, height)];
    }
}

/**
 *从图片中按指定的位置大小截取图片的一部分
 * UIImage image 原始的图片
 * CGRect rect 要截取的区域
 */
- (UIImage *)imageFromRect:(CGRect)rect{
    //将UIImage转换成CGImageRef
    CGImageRef sourceImageRef = [self CGImage];
    //按照给定的矩形区域进行剪裁
    CGImageRef newImageRef = CGImageCreateWithImageInRect(sourceImageRef, rect);
    //将CGImageRef转换成UIImage
    UIImage *newImage = [UIImage imageWithCGImage:newImageRef];
    //返回剪裁后的图片
    return newImage;
}
- (UIImage *)fixOrientation{
    // No-op if the orientation is already correct
    UIImage *aImage = self ;
    if (aImage.imageOrientation == UIImageOrientationUp)
        return aImage;
    // 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 (aImage.imageOrientation) {
        case UIImageOrientationDown:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, aImage.size.width, aImage.size.height);
            transform = CGAffineTransformRotate(transform, M_PI);
            break;
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
            transform = CGAffineTransformTranslate(transform, aImage.size.width,0);
            transform = CGAffineTransformRotate(transform, M_PI_2);
            break;
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, 0, aImage.size.height);
            transform = CGAffineTransformRotate(transform, -M_PI_2);
            break;
        default:
            break;
    }
    switch (aImage.imageOrientation) {
        case UIImageOrientationUpMirrored:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, aImage.size.width,0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, aImage.size.height,0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
        default:
            break;
    }
    // Now we draw the underlying CGImage into a new context, applying the transform
    // calculated above.
    CGContextRef ctx =CGBitmapContextCreate(NULL, aImage.size.width, aImage.size.height,
                                            CGImageGetBitsPerComponent(aImage.CGImage),0,
                                            CGImageGetColorSpace(aImage.CGImage),
                                            CGImageGetBitmapInfo(aImage.CGImage));
    CGContextConcatCTM(ctx, transform);
    switch (aImage.imageOrientation) {
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            // Grr...
            CGContextDrawImage(ctx,CGRectMake(0,0,aImage.size.height,aImage.size.width), aImage.CGImage);
            break;
        default:
            CGContextDrawImage(ctx,CGRectMake(0,0,aImage.size.width,aImage.size.height), aImage.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;
}