Created Filtering (markdown)

Filtering.md

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+Flight allows you to filter methods before and after they are called. There are no predefined hooks
+you need to memorize. You can filter any of the default framework methods as well as any custom methods that
+you've mapped.
+
+A filter function looks like this:
+
+    function(&$params, &$output) {
+        // Filter code
+    }
+
+Using the passed in variables you can manipulate the input parameters and/or the output.
+
+You can have a filter run before a method by doing:
+
+    Flight::before('start', function(&$params, &$output){
+        // Do something
+    });
+
+You can have a filter run after a method by doing:
+
+    Flight::after('start', function(&$params, &$output){
+        // Do something
+    });
+
+You can add as many filters as you want to any method. They will be called in the order
+that they are declared.
+
+Here's an example of the filtering process:
+
+    // Map a custom method
+    Flight::map('hello', function($name){
+        return "Hello, $name!";
+    });
+
+    // Add a before filter
+    Flight::before('hello', function(&$params, &$output){
+        // Manipulate the parameter
+        $params[0] = 'Fred';
+    });
+
+    // Add an after filter
+    Flight::after('hello', function(&$params, &$output){
+        // Manipulate the output
+        $output .= " Have a nice day!";
+    }
+
+    // Invoke the custom method
+    echo Flight::hello('Bob');
+
+This should display:
+
+    Hello Fred! Have a nice day! 
+
+If you have defined multiple filters, you can break the chain by returning `false` in any of your filter functions:
+
+    Flight::before('start', function(&$params, &$output){
+        echo 'one';
+    });
+
+    Flight::before('start', function(&$params, &$output){
+        echo 'two';
+
+        // This will end the chain
+        return false;
+    });
+
+    // This will not get called
+    Flight::before('start', function(&$params, &$output){
+        echo 'three';
+    });
+
+Note, core methods such as `map` and `register` cannot be filtered because they are called
+directly and not invoked dynamically.