Undo/Redo in a Cocoa Application (Example to Undo/Redo addition & deletion of rows in a table view)

mac developers

Undo/Redo functionality in a Cocoa application – NSUndoManager

This article will give you an overview of adding undo and redo functionality to a cocoa application by an example that undo/redo addition and deletion of rows in a TableView.

Lets start with How Undo Manager works?
As per Apple’s Reference Guide “You register an undo operation by specifying the object that’s changing (or the owner of that object), along with a method to invoke to revert its state, and the arguments for that method. When performing undo an NSUndoManager saves the operations reverted so that you can redo the undos.”

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Memory management in Objective C

How memory management is done in objective C?

The latest OS X and IOS versions (IOS 5, OSX 10.7 and above) supports Automatic Reference Counting (ARC), a memory management enhancement in Objective C. Before that programmers have to follow the Manual Retain Release procedure. Go through the slides below to understand Memory Management in Objective C with reference to IOS and Cocoa. Also learn about the introduction of ARC and how it works.

Topics Covered:

– About Memory Management and IOS, OSX versions

– Reference Counting

– Taking ownership of an Object

– Relinquishing ownership of an Object

– Memory Leak and Dangling pointer

– Strong, Retain and Copy

– Autorelease method

– Autorelease pools

– Dealloc

– Automatic Reference Counting

– Zeroing Weak References

– Garbage Collector

Written By: Neha Gupta

Access/control scriptable application using scripting bridge and Cocoa

mac developers

ScriptingBridge framework in Cocoa can be used to access features from any scriptable application. In this article I will discuss how to access the features of iTunes such as current playing track, duration of the track, playlists etc by using ScriptingBridge.framework in a Cocoa Application. However you can follow the same steps to allow your application to interact with any other scriptable application.

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Upload files using POCO C++ Libraries

I have recently worked on uploading files using POCO and it gave me a hard time to get it work properly. I dint get much working tutorials on client side programming to upload a binary file to a upload link using POCO. So I thought of writing one by myself to help others who are facing the same problem.

Below is the code to upload any file as multipart form data by reading the entire data of file as binary :

Note that this may not work for large files as reading the entire data (1GB or more) in a variable will take a lot of memory and will slow down the system. You must consider chunked upload for this case by reading the file data in a fixed size buffer and uploading in chunks.

void uploadFile(const char* uploadLink, const char* filePath, const char* filename)
      //Prepare request
      Poco::URI uri(uploadLink);

      Poco::Net::HTTPClientSession session(uri.getHost(), uri.getPort());


      // prepare path
      std::string path(uri.getPathAndQuery());
      if (path.empty())
         path = "/";

      Poco::Net::HTTPRequest req(Poco::Net::HTTPRequest::HTTP_POST, path, Poco::Net::HTTPMessage::HTTP_1_1);

      // Set headers
      // Set Multipart type and some boundary value 
      req.setContentType("multipart/form-data; boundary=-------------------------87142694621188");

     // Append boundary, content type and disposition to file data

      std::string boundary = "-------------------------87142694621188";
      std::string data1("---------------------------87142694621188\r\nContent-Disposition: form-data; name=\"data\"; filename=\"");
      std::string data2(filename);
      std::string data3("\";\r\nContent-Type: application/octet-stream\r\n\r\n");

      std::string data4("\r\n---------------------------87142694621188--\r\n");
    // Read File Data in binary
      std::ifstream file (filePath,std::ios::binary);
      std::ostringstream ostrm;
      ostrm << file.rdbuf();

      std::string reqBody;


      // sends request, returns open stream
      std::ostream& myOStream = session.sendRequest(req);
      // sends the body
      myOStream << reqBody;

      Poco::Net::HTTPResponse res;

      std::istream& rs = session.receiveResponse(res);

      //Get status code
      int statusCode = (int)res.getStatus();

      //Get status
      std::string status = res.getReason();

      std::string response;
      std::cout<<"\nStatusCode: "<<statusCode<<"\nStatus: "<<status<<"\nResponse: "<<response;
  catch(Poco::Exception& exception)
       //Set Response for Exception
       std::cout<<"\nException occurred while uploading: "<<exception.displayText();

The above method accepts the file name, upload link and complete file path (path along with file name) as parameters. Then we read the entire file data as binary and send the HTTP POST request to the server with the complete file data and boundaries appended. This uploads the file to server and we get the response received and the Http Status code.

Written By: Neha Gupta

QFileSystemWatcher: Monitor directories and get Rename, Delete, Add, Modify notifications with the name and path of the modified file/dir.

QFileSystemWatcher gives notification if the watched directory is modified. But it does not give any information about the type of change. So here I am going to explain that how you can find the change (rename, add, delete, modify) and the name of the file/dir (inside the watched dir) that is modified.

First of all add a class to your project. Lets name it “MyFileSystemWatcher”.

Now add a class variable of type QFileSystemWatcher to this class and a function to add paths to watch.

We will add two slots directoryUpdated(QString path) and fileUpdated(QString path) that will be connected to corresponding signals that are emitted when some change occurs in the watched dir/file.

Also we need a variable to maintain the list of current contents in a directory, and that can be compared to the directory contents after a change is notified to find out the name of file changed and the type of change. We’ll use a QMap having a  QString as key and QStringList as its value for this purpose. The key will be the path of some directory and the value will be the list of names of all files/directories it contains.

Hence moving on, your header file  MyFileSystemWatcher.h will look like:



#include <QObject>
#include <QStringList>
#include <QMap>
#include <QFileSystemWatcher>

class MyFileSystemWatcher : public QObject


      static MyFileSystemWatcher* _instance; //singleton class instance

      QMap<QString, QStringList> _currContents; //maintain list of current contents of each watched directory
      QFileSystemWatcher* _sysWatcher;  //QFileSystemWatcher variable
      static void addWatchPath(QString path);

public slots:

      void directoryUpdated(const QString & path);  //called when a watched directory is updated. "path" is the path of watched directory in which change occured
      void fileUpdated(const QString & path);   //called when a watched file is modified."path" is the path of watched file which is modified

In the implementation file, we will allocate memory to the singleton instance of class and the QFileSystemWatcher variable “_sysWatcher”, also we will maintain the currents contents of each directory path that is watched(added to watcher) and compare the contents in the directoryUpdated() method called when a watched directory is modified.

So here is what the implementation file MyFileSystemWatcher.cpp will look like:

#include "MyFileSystemWatcher.h"

MyFileSystemWatcher* MyFileSystemWatcher::_instance = 0;

// Add a file or directory path to be watched

void MyFileSystemWatcher::addWatchPath(QString path)
   qDebug()<<"Add to watch: "<<path;
// Allocate memory to instance if not already allocated 

   if(_instance == 0)
        _instance = new MyFileSystemWatcher;
        _instance->_sysWatcher = new QFileSystemWatcher();

// Connect the directoryChanged and fileChanged signals of QFileSystemWatcher to corresponding slots

        connect(_instance->_sysWatcher,SIGNAL(directoryChanged( QString )),_instance,SLOT(directoryUpdated(QString)));
        connect(_instance->_sysWatcher,SIGNAL(fileChanged( QString )),_instance,SLOT(fileUpdated(QString)));
   _instance->_sysWatcher->addPath(path);  //add path to watch

// Save the list of current contents if the added path is a directory
    QFileInfo f(path);

         const QDir dirw(path);
         _instance->_currContents[path] = dirw.entryList(QDir::NoDotAndDotDot | QDir::AllDirs | QDir::Files, QDir::DirsFirst);


// Slot invoked whenever any of the watched directory is updated (some file in the watched dir is added, deleted or renamed)

void MyFileSystemWatcher::directoryUpdated(const QString & path)
    qDebug()<<"Directory updated: "<<path;

// Compare the latest contents to saved contents for the dir updated to find out the difference(change) 
    QStringList currEntryList = _instance->_currContents[path];
    const QDir dir(path);

    QStringList newEntryList = dir.entryList(QDir::NoDotAndDotDot  | QDir::AllDirs | QDir::Files, QDir::DirsFirst);

    QSet<QString> newDirSet = QSet<QString>::fromList( newEntryList );
    QSet<QString> currentDirSet = QSet<QString>::fromList( currEntryList );

   // Files that have been added
    QSet<QString> newFiles = newDirSet - currentDirSet;
    QStringList newFile = newFiles.toList();

   // Files that have been removed
    QSet<QString> deletedFiles = currentDirSet - newDirSet;
    QStringList deleteFile = deletedFiles.toList();

   // Update the current set
    _instance->_currContents[path] = newEntryList;

   if(!newFile.isEmpty() && !deleteFile.isEmpty())
       // File/Dir is renamed

       if(newFile.count() == 1 && deleteFile.count() == 1)
           qDebug()<<"File Renamed from "<<newFile.first()<<" to "<<deleteFile.first();

        // New File/Dir Added to Dir

             qDebug()<<"New Files/Dirs added: "<<newFile;

             foreach(QString file, newFile)
                     //Handle Operation on new files.....

        // File/Dir is deleted from Dir

            qDebug()<<"Files/Dirs deleted: "<<deleteFile;
            foreach(QString file, deleteFile)
                // Handle operation of each deleted file....



// Slot invoked whenever the watched file is modified

void MyFileSystemWatcher::fileUpdated(const QString & path)
    QFileInfo file(path);

    QString path1 = file.absolutePath();

    QString name = file.fileName();

    qDebug()<<"The file "<<name<<" at path "<<path1<<" is updated";

This way you may add any no. of paths to the watcher by calling the static method “addWatchPath:” and the path can be either a file or directory.
If you want to watch a directory as well as all its sub directories, you need to add the path of parent directory and path of all its sub directories recursively to the watcher.

For Example:

If you want to watch a directory “ABC” in the C drive, just call the method


This is all about QFileSystemWatcher. However there is a known bug in using it for recursive monitoring on Windows. To know more about this check out my other post.

Written By: Neha Gupta