


Agent and Exception Customization in PHP5 OOP Programming_PHP Tutorial
1. DBQuery object
Now, our DBQuery object simply emulates a stored procedure - once executed, it returns a result resource that must be saved; and if you want to use functions on that result set (such as num_rows() or fetch_row()), You have to pass the MySqlDB object. So, what is the effect if the DBQuery object implements the functions implemented by the MySqlDB object (which is designed to operate on the results of an executed query)? Let's continue using the code from the previous example; and let's assume that our result resources are now managed by a DBQuery object. The source code of the DBQuery class is shown in Listing 1.
Listing 1. Using the DBQuery class.
require mysql_db.php;
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What we are most interested in in the modified code above are the catch statement and execute statement.
· The execute statement no longer returns a result resource, it now returns the DBQuery object itself.
· The DBQuery object now implements the num_rows() function—which we are already familiar with from the DB interface.
· If the query execution fails, it throws an exception of type QueryException. When converted to a string, it returns details of the error that occurred.
To do this, you need to use a proxy. In fact, you already use proxies in our DBQuery object, but now you'll use it in more depth to tie it tightly to the MySqlDB object. The DBQuery object has been initialized with an object that implements the DB interface, and it already contains a member function execute—which calls the query() method of the DB object to execute the query. The DBQuery object itself does not actually query the database, it leaves this task to the DB object. This is a proxy, which is a process by which an object can implement a specific behavior by sending messages to another object that implements the same or similar behavior.
To do this, you need to modify the DBQuery object to include all functions that operate on a result resource from the DB object. You need to use the stored results when executing a query to call the corresponding function of the DB object and return its results. The following functions will be added:
Listing 2: Extending the DBQuery class using proxies.
class DBQuery
..... public function fetch_array() { if (! is_resource($this->result)) { Throw new Exception(Query not executed.); } return $this->db->fetch_array($this->result);
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public function __construct(DB $db) { $this->db = $db; } |
When using type hints, you can specify not only object types, but also abstract classes and interfaces.
3. Throw exception
You may have noticed from the above code that you are catching an exception called QueryException (we will implement this object later). An exception is similar to an error, but more general. The best way to describe an exception is to use emergency. Although an emergency may not be "fatal," it must still be dealt with. When an exception is thrown in PHP, the current scope of execution is quickly terminated, whether it is a function, try..catch block or the script itself. The exception then traverses the call stack—terminating each execution scope—until it is either caught in a try..catch block or it reaches the top of the call stack—at which point it generates a fatal error.
Exception handling is another new feature in PHP 5. When used in conjunction with OOP, it can achieve good control over error handling and reporting. A try..catch block is an important mechanism for handling exceptions. Once caught, script execution will continue from the next line of code where the exception was caught and handled.
If the query fails, you need to change your execute function to throw an exception. You will throw a custom exception object called QueryException - the DBQuery object that caused the error is passed to it.
Listing 3. Throw an exception.
/** *Execute current query * * Execute the current query—replacing any dot markers with the provided arguments * . * * @parameters: mixed $queryParams,... query parameters * @return: Resource A—reference describing the resource on which the query is executed. */ public function execute($queryParams = ) { //For example: SELECT * FROM table WHERE name=:1S AND type=:2I AND level=:3N $args = func_get_args(); if ($this->stored_procedure) { /*Call the compile function to get the query*/ $query = call_user_func_array(array($this, compile), $args); } else { /*A stored procedure has not been initialized, therefore, it is executed as a standard query*/ $query = $queryParams; } $result = $this->db->query($query); if (! $result) { throw new QueryException($this); |

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