What’s a reliable way to depend on another plugin?

Question

Consider plugin A:

$A_message;

add_action('plugins_loaded', function() {
    do_action('plugin-A:init');
});

// Hooks allow B to access A (both during initialization of A and elsewhere in WordPress) without having to check if A exists.

add_filter('plugin-A:set', function($msg) {
    $A_message = $msg;
}, PHP_INT_MAX);

add_filter('plugin-A:get', function($msg) {
    return $A_message;
}, PHP_INT_MIN);

Now, consider plugin B:

add_action('plugin-A:init', function() {
    // We're 100% sure that A has loaded (if it exists)
    // Store what Cheech says in A.  (Using a filter allows us to not have to check if A exists).
    do_action('plugin-A:set', "Brent! Open the door!");
});

add_filter('the_title', function($title) {
    // Get something from A (and we don't have to check if it exists).
    // If A exists, return what Cheech says; if A does not exist, return what Chong says.
    $title = apply_filters('plugin-A:get', "Dave's not here, man!");
    return $title;
});

Most of this code sounds like it is nothing new to you. When Foo is loaded, it initializes the Bar plugin by way of bar_on_plugins_loaded() and foo_load_bar(). What is new here is that Foo does not need to do any fancy checks to see if Bar exists or not. This is because foo_load_bar() executes a hook that is defined by Bar instead of a property of Bar itself. (Pretty cool, huh?)

Later on in your code when a title is requested (like in post list tables) foo_get_bar_message() and bar_get_message() will return Bar’s value that was set during the initialization of Bar by way of bar_set_message(). Again, this is all done without the Foo plugin having to check for Bar’s existence. And, in the event that Bar does not exist, the Foo default will be returned. (Special thanks to Cheech and Chong for the inspiration.)

Edit: In the above example, B depends more on A than the other way around. But, you asked for A depending on B and the same concept holds true here. Consider this addition to plugin A:

// This function is used somewhere in plugin-A ...
function a_func() {
    // Not sure if B exists, but we can make B do something if it does.
    do_actions('plugin-B:some_func', '*knock knock knock*', 'Who Is It?');
}

And this addition to plugin B:

add_action('plugin-B:some_func', function($cheech, $chong) {
    echo '<p>'. $cheech .'<br>'. $chong .'</p>';
}

In addition to B (if it exists) turning all the titles into either Dave or Brent’s message, the beginning of Cheech and Chong’s skit will output by plugin A when it calls its a_func() somewhere later on in its own code. As per your desire, A need not do anything to check if plugin B exists.

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