iOS基础：iOS图片压缩<1>图片压缩<2>图片裁剪，单独使用<3>网上搜到了个比较详细的介绍

<1>图片压缩

这个三个方法环环紧扣，使用二分法迅速压缩图片大小，试了下，iPad的话400KB差不多图片不虚，小于200就会虚的很

#pragma mark - 图片压缩

+ (NSData *)resetSizeOfImageData:(UIImage *)sourceImage maxSize:(NSInteger)maxSize {

    //先判断当前质量是否满足要求，不满足再进行压缩

    __block NSData *finallImageData = UIImageJPEGRepresentation(sourceImage,1.0);

    NSUInteger sizeOrigin   = finallImageData.length;

    NSUInteger sizeOriginKB = sizeOrigin / 1000;

    if (sizeOriginKB <= maxSize) {

        return finallImageData;

    }

    //获取原图片宽高比

    CGFloat sourceImageAspectRatio = sourceImage.size.width/sourceImage.size.height;

    //先调整分辨率

    CGSize defaultSize = CGSizeMake(1024, 1024/sourceImageAspectRatio);

    UIImage *newImage = [self newSizeImage:defaultSize image:sourceImage];

    finallImageData = UIImageJPEGRepresentation(newImage,1.0);

    //保存压缩系数

    NSMutableArray *compressionQualityArr = [NSMutableArray array];

    CGFloat avg   = 1.0/250;

    CGFloat value = avg;

    for (int i = 250; i >= 1; i--) {

        value = i*avg;

        [compressionQualityArr addObject:@(value)];

    }

    //思路：使用二分法搜索

    finallImageData = [self halfFuntion:compressionQualityArr image:newImage sourceData:finallImageData maxSize:maxSize];

    //如果还是未能压缩到指定大小，则进行降分辨率

    while (finallImageData.length == 0) {

        //每次降100分辨率

        CGFloat reduceWidth = 100.0;

        CGFloat reduceHeight = 100.0/sourceImageAspectRatio;

        if (defaultSize.width-reduceWidth <= 0 || defaultSize.height-reduceHeight <= 0) {

            break;

        }

        defaultSize = CGSizeMake(defaultSize.width-reduceWidth, defaultSize.height-reduceHeight);

        UIImage *image = [self newSizeImage:defaultSize

                                      image:[UIImage imageWithData:UIImageJPEGRepresentation(newImage,[[compressionQualityArr lastObject] floatValue])]];

        finallImageData = [self halfFuntion:compressionQualityArr image:image sourceData:UIImageJPEGRepresentation(image,1.0) maxSize:maxSize];

    }

    return finallImageData;

}

#pragma mark 调整图片分辨率/尺寸（等比例缩放）

+ (UIImage *)newSizeImage:(CGSize)size image:(UIImage *)sourceImage {

    CGSize newSize = CGSizeMake(sourceImage.size.width, sourceImage.size.height);

    CGFloat tempHeight = newSize.height / size.height;

    CGFloat tempWidth = newSize.width / size.width;

    if (tempWidth > 1.0 && tempWidth > tempHeight) {

        newSize = CGSizeMake(sourceImage.size.width / tempWidth, sourceImage.size.height / tempWidth);

    } else if (tempHeight > 1.0 && tempWidth < tempHeight) {

        newSize = CGSizeMake(sourceImage.size.width / tempHeight, sourceImage.size.height / tempHeight);

    }

    UIGraphicsBeginImageContext(newSize);

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

    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return newImage;

}

#pragma mark 二分法

+ (NSData *)halfFuntion:(NSArray *)arr image:(UIImage *)image sourceData:(NSData *)finallImageData maxSize:(NSInteger)maxSize {

    NSData *tempData = [NSData data];

    NSUInteger start = 0;

    NSUInteger end = arr.count - 1;

    NSUInteger index = 0;

    NSUInteger difference = NSIntegerMax;

    while(start <= end) {

        index = start + (end - start)/2;

        finallImageData = UIImageJPEGRepresentation(image,[arr[index] floatValue]);

        NSUInteger sizeOrigin = finallImageData.length;

        NSUInteger sizeOriginKB = sizeOrigin / 1024;

        NSLog(@"当前降到的质量：%ld", (unsigned long)sizeOriginKB);

        NSLog(@"\nstart：%zd\nend：%zd\nindex：%zd\n压缩系数：%lf", start, end, (unsigned long)index, [arr[index] floatValue]);

        if (sizeOriginKB > maxSize) {

            start = index + 1;

        } else if (sizeOriginKB < maxSize) {

            if (maxSize-sizeOriginKB < difference) {

                difference = maxSize-sizeOriginKB;

                tempData = finallImageData;

            }

            if (index<=0) {

                break;

            }

            end = index - 1;

        } else {

            break;

        }

    }

    return tempData;

}

<2>图片裁剪，单独使用

#pragma ---图片裁剪

//裁剪image大小

-(UIImage*)OriginImage:(UIImage *)image scaleToSize:(CGSize)size{

    UIGraphicsBeginImageContext(size);  //size 为CGSize类型，即你所需要的图片尺寸

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

    UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return scaledImage;   //返回的就是已经改变的图片

}

<3>网上搜到了个比较详细的介绍

https://www.jianshu.com/p/99c3e6a6c033