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#import <UIKit/UIKit.h>

@interface UIImage (Resize)

- (UIImage*)scaleBy:(float)scale;

@end

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- (void)presentImagePicker:(UIView *)sender {
  UIImagePickerController *imagePicker = [[UIImagePickerController alloc] init];
  imagePicker.modalPresentationStyle = UIModalPresentationPopover;
  imagePicker.popoverPresentationController.sourceView = sender;
  imagePicker.popoverPresentationController.permittedArrowDirections = UIPopoverArrowDirectionAny;
  [self presentViewController:imagePicker animated:YES completion:nil];
}

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// import AVFoundation

- (UIImage*)thumbnailFromVideo:(NSURL *)url atTime:(float)time {
    AVAsset *asset = [AVAsset assetWithURL:url];
    
    // get 3 decimal places
    // ex: 3.39235350 --> 3.392
    float nearest = roundf(time * 1000) / 1000;
    // make CMTime from float
    // because of 3 decimal places, we use 10000 (10^4)

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kxmovie on github
First: cd /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/developer/SDKs/ to find out what SDKs are existed in your OS
Second: open Rake file and modify SDK_VERSION='9.2' -> your SDK version

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#!/bin/bash

###########################################################################
#  Choose your ffmpeg version and your currently-installed iOS SDK version:
#
VERSION="2.0.2"
SDKVERSION="7.0"
ARCHS="armv7 armv7s i386"
#
#

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Here are steps:
1. Build FFmpeg for iOS
  There some scripts to build FFmpeg for iOS
  ex: https://github.com/kewlbear/FFmpeg-iOS-build-script
2. After build complete, copy all *.a to your iOS project folder. 
  They will be automatically add to Linked Frameworks and Libraries (in Project settings -> General)
  ex: libavutil.a, libavformat.a, libavcodec.a,...
3. Add depedencies to Linked Frameworks and Libraries
  Add libz.dylib (or libz.tbd), libiconv.dylib
4. Go to Project settings -> Build Settings, add the path which contains all *.a files in your iOS project folder to Library Search Paths

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    NSMutableAttributedString *string = [[NSMutableAttributedString alloc] initWithString:@"Hello, world!"];
    [string addAttribute:NSForegroundColorAttributeName value:[UIColor redColor] range:NSMakeRange(0, 3)];
    [string addAttribute:NSFontAttributeName value:[UIFont systemFontOfSize:24] range:NSMakeRange(3, 3)];
    
    NSData *data = [string dataFromRange:NSMakeRange(0, string.length) 
                            documentAttributes:@{NSDocumentTypeDocumentAttribute: NSRTFDTextDocumentType, 
                            NSCharacterEncodingDocumentAttribute: [NSNumber numberWithInt:NSUTF8StringEncoding]} 
                            error:nil];
    
    

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Sometimes, for some reason, you have to use AVAssetWriter to record videos from camera instead of using movile file output.
I had experienced some issue by that recording method due to lacking of tutorial and SO questions :))

Noted for everyone who met the issue: black first frame

if (CMSampleBufferDataIsReady(sampleBuffer)) {
  if (videoWriter.status != AVAssetWriterStatusWriting) {
    CMTime startTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
    [videoWriter startWriting];
    [videoWriter startSessionAtSourceTime:startTime];

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#include <arpa/inet.h>
- (NSString *)getStringFromAddressData:(NSData *)dataIn {
    struct sockaddr_in  *socketAddress = nil;
    NSString            *ipString = nil;
    
    socketAddress = (struct sockaddr_in *)[dataIn bytes];
    ipString = [NSString stringWithFormat: @"%s",
                inet_ntoa(socketAddress->sin_addr)];  ///problem here
    return ipString;
}

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NSURL *videoURL = [NSURL URLWithString:@"https://clips.vorwaerts-gmbh.de/big_buck_bunny.mp4"];
AVPlayer *player = [AVPlayer playerWithURL:videoURL];
AVPlayerViewController *playerViewController = [AVPlayerViewController new];
playerViewController.player = player;
[self presentViewController:playerViewController animated:YES completion:nil];

NSURL *videoURL = [NSURL URLWithString:@"https://clips.vorwaerts-gmbh.de/big_buck_bunny.mp4"];
AVPlayer *player = [AVPlayer playerWithURL:videoURL];
AVPlayerLayer *playerLayer = [AVPlayerLayer playerLayerWithPlayer:player];
playerLayer.frame = self.view.bounds;