Examples of File Operations in Apache Cordova

To be able to do any operation with directory/file in Apache Cordova, we need to access and get hold of the directory/file entry first. Then only after that we will be able to do any directory/file operation. This could be a very different approach from other frameworks that we have used before in dealing with file and file system.


Creating and Writing File
Let start with an example to create and write a file:

window.resolveLocalFileSystemURL(cordova.file.dataDirectory, function (dirEntry) {
 console.log("got directory entry", dirEntry);
 dirEntry.getFile("myFilename.txt", { create: true }, 
  // getFile() success
  function (fileEntry) {
   console.log("got file entry", fileEntry);
   fileEntry.createWriter(
    // createWriter() success
    function (fileWriter) {
     fileWriter.onwriteend = function (e) {
      console.log('write is successful');
      . . .
     };

     fileWriter.onerror = function (e) {
      console.log('write is failed', e);
      . . .
     };

     var blob = new Blob(['. . . some text . . .'], { type: 'text/plain' });
     fileWriter.write(blob);
    },
    // createWriter() error
    function (error) {
     . . .
    }
   );
  },
  // getFile() error
  function (error) {
   . . .
  }
 );
});

In the example above, we use resolveLocalFileSystemURL() function to get a directory or file entry from a directory/file system path (i.e. cordova.file.dataDirectory). resolveLocalFileSystemURL() has two parameters:
- the first parameter is a directory or file path
- the second one is the callback function to be executed once it succeed. A directory or file reference entry object is passed to the callback function.

Once we have hold of the file or directory entry object, we can do a number of operations such as creating, writing, reading, copying, moving, renaming or deleting the file. In this case, we want to create a new file and write into it. To do this, we need to call getFile() function first. The function has four parameters:
- the filename or file path
- function options (optional parameter). It has two properties with boolean values, namely; create and exclusive. create: true will create a file if it does not exist. To make the function throws an error if the file exists, set exclusive: true.
- on success callback function, passing a file reference entry object
- on error callback function (optional parameter), an error object is passed to the callback
Then after we get the file entry object, we call createWriter() function that will create a file writer object once successful. Next we call its write() function. The file writer object also has onwriteend() and onerror() callback functions.


Reading File

window.resolveLocalFileSystemURL(myFilePath, function (dirEntry) {
 fileEntry.file(
  // success
  function(file) {
     var reader = new FileReader();

     reader.onloadend = function(e) {
       console.log('file is read');
       . . .
     };

     reader.readAsText(file);
  }, 
  // error
  function (error) {
    . . .
  }
 );
});

In the example, we see that after we get a file reference entry object, we use file() method in order to create a file object of the intended file to be passed to FileReader readAsText() function.


Deleting File

window.resolveLocalFileSystemURL(myFilePath, 
 function (fileEntry) { 
  fileEntry.remove(
   // success
   function () { 
    console.log('file is removed'); 
    . . .
   }, 
   // error
   function (error) {
    . . .
   }
  ); 
 } 
); 

In this last example, we call file entry object's remove() function to delete a file.