Field::viewable() method 
Is this field viewable?
- To maximize efficiency check that 
$field->useRolesis true before calling this. - If you have already verified that the page is viewable, omit or specify null for $page argument.
 - Please note: this does not check that the provided $page itself is viewable. If you want that check, then use 
$page->viewable($field)instead. 
Usage
// basic usage
$bool = $field->viewable();
// usage with all arguments
$bool = $field->viewable($page = null, $user = null);Arguments
| Name | Type(s) | Description | 
|---|---|---|
$page (optional) | Page null | Optionally specify a Page for context (i.e. Is field viewable on $page?)  | 
$user (optional) | User null | Optionally specify a different user for context Default:current user  | 
Return value
boolTrue if viewable, false if not
Hooking $field→viewable(…)
You can add your own hook events that are executed either before or after the $field->viewable(…) method is executed. Examples of both are included below. A good place for hook code such as this is in your /site/ready.php file.
Hooking before
The 'before' hooks are called immediately before each $field->viewable(…) method call is executed. This type of hook is especially useful for modifying arguments before they are sent to the method.
$this->addHookBefore('Field::viewable', function(HookEvent $event) {
  // Get the object the event occurred on, if needed
  $Field = $event->object;
  // Get values of arguments sent to hook (and optionally modify them)
  $page = $event->arguments(0);
  $user = $event->arguments(1);
  /* Your code here, perhaps modifying arguments */
  // Populate back arguments (if you have modified them)
  $event->arguments(0, $page);
  $event->arguments(1, $user);
}); Hooking after
The 'after' hooks are called immediately after each $field->viewable(…) method call is executed. This type of hook is especially useful for modifying the value that was returned by the method call.
$this->addHookAfter('Field::viewable', function(HookEvent $event) {
  // Get the object the event occurred on, if needed
  $Field = $event->object;
  // An 'after' hook can retrieve and/or modify the return value
  $return = $event->return;
  // Get values of arguments sent to hook (if needed)
  $page = $event->arguments(0);
  $user = $event->arguments(1);
  /* Your code here, perhaps modifying the return value */
  // Populate back return value, if you have modified it
  $event->return = $return;
}); API reference based on ProcessWire core version 3.0.252