Issue #654 working on adding feature fs.watchFile()

src/filesystem/interface.js

@@ -206,6 +206,59 @@ function FileSystem(options, callback) {
     return watcher;
   };
 
+  //Object that uses filenames as keys
+  const statWatchers = new Map();
+
+  this.watchFile = function (filename, options, listener) {
+    let prevStat, currStat;
+
+    if (Path.isNull(filename)) {
+      throw new Error('Path must be a string without null bytes.');
+    }
+    // Checks to see if there were options passed in and if not, the callback function will be set here
+    if (typeof options === 'function') {
+      listener = options;
+      options = {};
+    }
+    // default 5007ms interval, persistent is not used this project
+    const interval = options.interval || 5007;
+    listener = listener || nop;
+
+    let intervalValue = statWatchers.get(filename);
+
+    // Checks to see if there's a pre-existing watcher on the file
+    if (intervalValue === undefined) {
+      // Stores initial prev value to compare 
+      fs.stat(filename, function (err, stats) {
+        var value = setInterval(function () {
+          prevStat = currStat;
+
+          //Conditional check for first run to set initial state for prevStat
+          if(prevStat === undefined) { 
+            prevStat = stats;
+          } 
+
+          currStat = stats;
+
+          if (err) {
+            clearInterval(value);
+            console.warn('[Filer Error] fs.watchFile encountered an error: ' + err);
+          }
+          if (JSON.stringify(prevStat) !== JSON.stringify(currStat)) {
+            listener(prevStat, currStat);
+          }
+          // Set a new prevStat based on previous
+          prevStat = currStat;
+        },
+        interval
+        );
+
+        // Stores interval return values
+        statWatchers.set(filename, value);
+      });
+    }
+  };
+
   // Deal with various approaches to node ID creation
   function wrappedGuidFn(context) {
     return function (callback) {

tests/index.js

@@ -41,6 +41,7 @@ require('./spec/trailing-slashes.spec');
 require('./spec/times.spec');
 require('./spec/time-flags.spec');
 require('./spec/fs.watch.spec');
+require('./spec/fs.watchFile.spec');
 require('./spec/fs.unwatchFile.spec');
 require('./spec/errors.spec');
 require('./spec/fs.shell.spec');

tests/spec/fs.watchFile.spec.js

@@ -0,0 +1,34 @@
+'use strict';
+
+var util = require('../lib/test-utils.js');
+var expect = require('chai').expect;
+
+describe('fs.watchFile', function() {
+  beforeEach(util.setup);
+  afterEach(util.cleanup);
+
+  it('should be a function', function() {
+    const fs = util.fs();
+    expect(typeof fs.watchFile).to.equal('function');
+  });
+
+  it('should throw an error if a file path is not defined', function() { 
+    const fs = util.fs();
+
+    const fn = () => fs.watchFile(undefined);
+    expect(fn).to.throw();
+  });
+
+  it('prev and curr should be populated', function() {
+    const fs = util.fs(); 
+
+    fs.writeFile('/myfile', 'data', function(error) {
+      if(error) throw error;
+    }); 
+
+    fs.watchFile('/myfile', function(prev, curr) {
+      expect(prev).to.exist;
+      expect(curr).to.exist;
+    });
+  });
+});