Easy way to check which method to use for current page

Ah, you mean like "find__node__" for router path "node/%" ?
You can always implement find(), and then dpm($item->route). If this says "node/%", you can then implement "find__node__".

But yes, could be useful to have this kind of debug message as a feature.

I hope you noticed the 2.x branch?

A quick question for the API redesign:
For a router path of "node/%", would it help if the method was $plugin->findParent__node_x(), instead of $plugin->findParent__node__() ?
The only "risk" would be if some router path does actually use the letter x as an url fragment. I don't believe this is realistic.
For that rare case we could introduce an escape sequence, such as: "/%/" becomes "_x_", but "/x" becomes "__x_", but maybe it is better to just not start with that paranoia.

Views API ? Where do they use double underscore?
We need to think about the following characters that are not allowed in method names, but common in router paths:
'-', '_', '%', '/', and in rare cases '.'.

Of course we could invent a really safe notatation, with '_' as the escape character.
So, '-' would become '_d' (d = dash), '_' would become '_u' or '__', '%' would become '_x', '/' would become '_s', and '.' would become '_t' (dot).
The result would be a pain to read,
'node/%/edit' would become 'node_s_x_sedit'. Ouch.

To improve this a little bit, we need to have shortcut for '/%/' and for '/':
'/%/' becomes '_x_'
'/%' becomes '_x', if at the end of the path.
'/' becomes '_slash_' (no, I really don't like this one)
'_' becomes '__'.
'-' becomes '_dash_'
'.' becomes '_dot_'.

Another attempt. Replacements ordered by priority.
'/%/' becomes '_x_'
'/%' becomes '_x', if at the end of the path.
'/x/' becomes '__x_'.
'/x' becomes '__x_', if at the end of the path.
'/' becomes '_', if not followed by '%'.
'_' becomes '__u_' (ugly)
'-' becomes '__dash_' (ugly)
'.' becomes '__dot_' (ugly)

Thus,
'_x_' becomes '__u_x__u_'
'_x' becomes '__u_x', if at the end of the path.
'__' becomes '__u___u_'

If this pattern is now completely reversible, I dunno. But it is horribly ugly for '-', '_' and '.'.
I wish method names were not that restricted ..
Yes I know you can always do if/else magic, but then you don't need these special method names anyway.

The point is not that we want to have method names with '__x_', but simply to avoid false positives for exotic paths. For instance, findParent__node_x() should only match 'node/%', and not 'node/-' or 'node/.' or 'node-_' etc.

I decided to change this mechanic.
Here is the new implementation (on my local version), in crumbs.plugin_engine.inc.

<?php
  protected function _buildMethodSuffix($router_path) {
    $method_suffix = strtolower($router_path);
    $method_suffix = preg_replace('#[^a-z0-9\%]#', '_', $method_suffix);
    $method_suffix = strtr($method_suffix, array('%' => 'x'));
    $reverse = strtr($method_suffix, array('_' => '/'));
    $reverse = preg_replace(array('#/x/#', '#/x$#'), array('/%/', '/%'), $reverse);
    // we need to do this two time to catch things like "/x/x/x/x".
    $reverse = strtr($reverse, array('/x/' => '/%/'));
    if ($reverse === $router_path) {
      return $method_suffix;
    }
    return FALSE;
  }
?>

If this thing returns FALSE for a given router path, then this router path can not be represented by a method name.
In such cases you need to use the basic method (in 2.x this is findParent() and findTitle()), without a suffix.
You can use switch ($item['path']) or if ($item['path'] === '...') to filter for specific router paths.

For instance, router paths that contain a dash "-" or an underscore, an upper case character, etc, do no longer their own method names.

I think this restriction is much better than having to worry about possible ambiguities, or having awkward-looking method names with things like "__dash_" in them. After all, that method suffix trick is just a shortcut, not a technical necessity.

The new syntax is now
findParent__node_x_edit
findParent__node_x
etc.
Where '/%/' is replaced by '_x_'.
Dashes and underscores are simply not allowed in router paths. Or, if you have such a router path, you simply can't use it for a method suffix. You need to deal with that path in the suffix-less method.

<?php
class MyPlugin {
  function findParent__node_x($path, $item) {
    // now we know absolutely that the router path is "node/%".
    ...
  }
  
  function findParent($path, $item) {
    switch ($item['route']) {
      case 'path/with/%/ugly/under_score':
        ...
        break;
      case 'path-with-dashes':
        ...
        break;
    }
  }
}
?>

"needs review" = "probably fixed in 2.x"

I think the new scheme is quite obvious. In case of doubt, run dpm(menu_get_item()) to have a look at the router path.
Btw, there is a new debug tool in the 2.x branch, where you can check how the parent path is determined for a given path.

Ah yes.
We also use double underscore, to separate the method base name from the method suffix.
So, what does it mean for crumbs?
I think the new pattern is quite ok, isn't it?

A lot of this discussion is outdated now, and will only confuse people.
It is no longer findParent__node__(), but findParent__node_x(). Etc.

If there is still a need for documentation on the method suffix, let's discuss this in a new issue focused on the Drupal 7 version.

See #1814556: Method suffix for paths with dashes