Reading Files with Cordova File Plugin
Introduction
Cordova File Plugin provides an interface for accessing and managing files within a mobile application’s file system. This plugin is crucial for handling local storage, downloading files, and managing user data on a device. This article will guide you through the process of reading files using the Cordova File Plugin.
Prerequisites
Before getting started, ensure you have:
* **Cordova CLI installed:** If you haven’t already, install the Cordova CLI using: `npm install -g cordova`
* **Cordova Project:** Create a new Cordova project using `cordova create myApp com.example.myapp My App`.
* **File Plugin installed:** Add the File plugin to your project: `cordova plugin add cordova-plugin-file`
Getting Started
1. **Initialization:** Begin by creating a JavaScript file (e.g., `fileReader.js`) in your `www` directory and include it in your HTML file.
“`html
“`
2. **File System Access:** First, you’ll need to obtain a reference to the file system using the `resolveLocalFileSystemURL` method.
“`javascript
function getFile() {
window.resolveLocalFileSystemURL(filePath, function(fileEntry) {
// Success: fileEntry is a FileEntry object
readFile(fileEntry);
}, function(error) {
// Error: Handle file system access errors
console.error(“Error accessing file system: ” + error.code);
});
}
“`
**Explanation:**
* **`resolveLocalFileSystemURL(filePath, successCallback, errorCallback)`:** This function is used to retrieve a `FileEntry` object representing the file or directory located at the specified path.
* **`successCallback(fileEntry)`:** This callback function receives the `FileEntry` object, which allows you to access file information and perform operations on it.
* **`errorCallback(error)`:** This callback function handles errors that occur during file system access.
3. **File Reading:** Once you have the `FileEntry` object, you can use the `fileEntry.file(successCallback, errorCallback)` method to access the underlying `File` object.
“`javascript
function readFile(fileEntry) {
fileEntry.file(function(file) {
var reader = new FileReader();
reader.onloadend = function(e) {
// Success: File content is available in reader.result
console.log(“File content:”, reader.result);
};
reader.onerror = function(e) {
// Error: Handle file reading errors
console.error(“Error reading file: ” + e.target.error.code);
};
reader.readAsText(file);
}, function(error) {
// Error: Handle file access errors
console.error(“Error accessing file: ” + error.code);
});
}
“`
**Explanation:**
* **`fileEntry.file(successCallback, errorCallback)`:** This method retrieves the `File` object associated with the `FileEntry`.
* **`FileReader` object:** Used to read the contents of a file.
* **`reader.readAsText(file)`:** Starts the file reading process.
* **`reader.onloadend(e)`:** This callback is invoked when the file reading is complete. The `reader.result` property contains the file’s contents as a string.
* **`reader.onerror(e)`:** Handles errors that occur during file reading.
Example
Here’s a complete example demonstrating reading a file from the `persistent` file system:
“`html
“`
**Output:**
“`
File content: This is the content of myfile.txt
“`
File System Locations
The Cordova File Plugin supports different file system locations, each offering unique characteristics:
| File System Location | Description |
|—|—|
| `persistent` | Provides persistent storage for your app’s data. Files are saved even after the app is closed and reopened. |
| `temporary` | Provides temporary storage for your app’s data. Files are deleted when the app closes or the device restarts. |
| `cache` | Provides a cache directory for storing downloaded files and other temporary content. |
| `external` | On Android, it refers to the device’s external storage (SD card), while on iOS, it corresponds to the Documents directory. |
| `externalFileSystems` | On Android, it exposes a list of available external storage devices (e.g., SD cards, USB drives). |
| `applicationDirectory` | Represents the root directory of your application package. |
| `applicationStorageDirectory` | Represents a directory within your application’s package that you can use to store data. |
Error Codes
When encountering errors, the Cordova File Plugin provides error codes that can help you identify the issue:
* **`1`:** `NOT_FOUND_ERR`: The file or directory does not exist.
* **`2`:** `SECURITY_ERR`: Access to the file system is forbidden.
* **`3`:** `ABORT_ERR`: The file system operation was aborted.
* **`4`:** `NOT_READABLE_ERR`: The file or directory cannot be read.
* **`5`:** `ENCODING_ERR`: The data provided is not encoded correctly.
* **`6`:** `NO_MODIFICATION_ALLOWED_ERR`: The file or directory cannot be modified.
* **`7`:** `INVALID_STATE_ERR`: The object is in an invalid state.
* **`8`:** `SYNTAX_ERR`: The supplied syntax is incorrect.
* **`9`:** `INVALID_MODIFICATION_ERR`: The modification is not allowed.
* **`10`:** `QUOTA_EXCEEDED_ERR`: The file system quota has been exceeded.
* **`11`:** `TYPE_MISMATCH_ERR`: The provided data type is incorrect.
* **`12`:** `PATH_EXISTS_ERR`: The path already exists.
Conclusion
The Cordova File Plugin empowers you to seamlessly interact with the device’s file system, enabling your hybrid mobile apps to handle file operations effectively. Understanding the basics of file access, error handling, and the available file system locations is crucial for creating robust and feature-rich applications. Remember to utilize error codes and carefully handle file operations for a stable and reliable experience.