DownloadManager.h

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

@interface DownloadManager : NSObject
+ (void)DownloadVideoWithURL:(NSString *)url completionHandler:(void (^)(NSURL *filePath, NSError *error))completionHandler;
@end

DownloadManager.m

@implementation DownloadManager
+ (void)DownloadVideoWithURL:(NSString *)url completionHandler:(void (^)(NSURL *filePath, NSError *error))completionHandler {
    NSError *err;
    NSString *documentsDirectoryURL = [NSString stringWithFormat:@"%@/%@", NSHomeDirectory(), @"Documents"];
    NSURL *fileURL = [NSURL URLWithString:url];
    NSString *fileName = fileURL.lastPathComponent;
    NSString *downloadPath = [documentsDirectoryURL stringByAppendingPathComponent:fileName];
    NSURL *downloadPathURL = [NSURL fileURLWithPath:downloadPath];
    
    NSData *dataFile = [NSData dataWithContentsOfURL:fileURL];
    if (dataFile) {
        [dataFile writeToFile:downloadPath options:nil error:&err];
    }
    
    completionHandler(downloadPathURL, err);
}
@end

Example.m

@implementation ViewController
- (void)viewDidLoad {
  [super viewDidLoad];
  [DownloadManager DownloadVideoWithURL:@"https://ff.com/ff.mp4" completionHandler:^(NSURL *filePath, NSError *error) {
        if (error) {
            // catch error and handle it
        } else {
            // no error
        }
    }];
}
@end