how to display "There are updates available for your Custom Plugin" in wordpress - wordpress

Question

I have developed a custom wordpress plugin, many users have started using it, but now I have updates available for the plugin and want to display a message to the users who have older versions of the plugin on there site.
How can I modify the code of my plugin so that once I make updates to it, it should trigger a message to the users on the plugin dashboard that there are updates to available to your plugin.
Here is a scenario:
Say a user has version 1.0 of my plugin and the place where I host the plugin has version 1.2, how can I notify the user on his plugins page that my plugin has an updated version??

Answer 1

Although user3042036 answer is great, and very comprehensive, I thought I would entend his / her answer with a open source solution.
This is what you are looking for: WordPress Plugin Update Notifier

Answer 2

First, good practice is to create a constant for your current plugin version, and create an activation and deactivation hook for your plugin. This allows you to check things like version numbers, and do some general initialization.
define ( 'MY_PLUGIN_VERSION', '2.0.0');
register_activation_hook(__FILE__, 'my_plugin_activation'));
register_deactivation_hook(__FILE__, 'my_plugin_deactivation'));
function my_plugin_activation() {
// Initialize some stuff for my_plugin
}
function my_plugin_deactivation() {
// Welp, I've been deactivated - are there some things I should clean up?
}
Here is an example of a typical update function:
function my_plugin_activation() {
$version = get_option( 'my_plugin_version' );
if( version_compare($version, '2.0.0', '<')) {
// Do some special things when we update to 2.0.0.
}
update_option( 'my_plugin_version', MY_PLUGIN_VERSION );
return MY_PLUGIN_VERSION;
}
There is no hook for when your plugin is updated. You, as a plugin
author, have to manually check the plugin version. First, you want to
create a simple function which will tell you if your plugin is up to
date:
function my_plugin_is_current_version(){
$version = get_option( 'my_plugin_version' );
return version_compare($version, MY_PLUGIN_VERSION, '=') ? true : false;
}
Then, test if your plugin is up to date, and call your update function (or in this case we call the same function as we would if the plugin was updated!):
if ( !my_plugin_is_current_version() ) my_plugin_activation();
Testing the update process from one version to the next is not all that complicated, though it is kinda cumbersome. Maybe someone has a better way, if so please tell me!
You can’t really see any errors when you activate a plugin, so the first step is to create a very simple hook to store plugin activation errors. In this case, we store these errors in error_activation.html in the plugin folder
add_action('activated_plugin', 'my_plugin_activation_error');
my_plugin_activation_error() {
file_put_contents( plugin_dir_path(__FILE__) . '/error_activation.html', ob_get_contents());
}