以前使用drawInRect改变图片大小，小图片没有问题，大图片在老机型上可能会有内存问题。
前几天看了微信团队的分享，换一种方式，记录一下。

+(UIImage *)scaleALAsset:(ALAsset *)asset withSize:(CGSize)size{
    
    ALAssetRepresentation *representation = [asset defaultRepresentation];
    uint8_t *buffer = (Byte*)malloc(representation.size);
    NSUInteger length = [representation getBytes:buffer fromOffset: 0.0  length:representation.size error:nil];
    
    if (length)  {
        
        NSData *data = [[NSData alloc] initWithBytesNoCopy:buffer length:representation.size freeWhenDone:YES];
        return [self scaledImageData:data withSize:size orientation:(UIImageOrientation)representation.orientation];
    }
    
    return nil;
}

+(UIImage *)scalePHAsset:(PHAsset *)asset withSize:(CGSize)size{
    
    PHImageRequestOptions *options = [PHImageRequestOptions new];
    options.version = PHImageRequestOptionsVersionCurrent;
    options.resizeMode = PHImageRequestOptionsResizeModeNone;
    options.synchronous = YES;
    
    __block NSData            *imageData;
    __block UIImageOrientation orientation;
    
    [[PHImageManager defaultManager] requestImageDataForAsset:asset 
               options: options 
               resultHandler:^( NSData * _Nullable imageData1, 
                               NSString * _Nullable dataUTI, 
                               UIImageOrientation orientation1, 
                                NSDictionary * _Nullable info) {
        
        imageData = imageData1;
        orientation = orientation1;
    }];
    
    return [self scaledImageData:imageData withSize:size orientation:orientation];
    
}


+(UIImage *)scaledImageData:(NSData *)data withSize:(CGSize)size orientation:(UIImageOrientation)orientation{
    
    CGImageSourceRef sourceRef = CGImageSourceCreateWithData((__bridge CFDataRef) data,  nil);
    CGFloat makePixelSize = MAX(size.width, size.height);
    NSDictionary *options = @{
                              (__bridge id)kCGImageSourceCreateThumbnailFromImageAlways:(__bridge id)kCFBooleanTrue,
                              (__bridge id)kCGImageSourceThumbnailMaxPixelSize:[NSNumber numberWithFloat:makePixelSize]};
    CGImageRef imageFef = CGImageSourceCreateThumbnailAtIndex(sourceRef,
                                                              0,
                                                              (__bridge CFDictionaryRef)options);
    UIImage *resultImage = [UIImage imageWithCGImage:imageFef
                                               scale:UIScreen.mainScreen.scale
                                         orientation:orientation];
    
    CGImageRelease(imageFef);
    CFRelease(sourceRef);
    return resultImage;
}