** Changed in: mahara
    Milestone: 17.04.0 => 17.10.0

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https://bugs.launchpad.net/bugs/1564117

Title:
  Refactor blocktype JS methods

Status in Mahara:
  Confirmed

Bug description:
  Currently the PluginBlocktype abstract class has three methods for
  loading Javascript libraries, and invoking Javascript code to run when
  a block is loaded:

  1. PluginBlocktype::get_instance_javascript(BlockInstance $instance):
  Called when a block instance is displayed (either on the page in view
  mode, or the page in edit mode). Expects to return an array of sub-
  arrays where each sub-array has a "file" item, which is the relative
  path to a JS file (relative to the blocktype's root), and an optional
  "init" item, which is a snippet of code that gets executed once the
  "file" library has loaded. (It currently uses jQuery.getScript() to do
  this).

  2. PluginBlocktype::get_instance_inline_javascript(BlockInstance
  $instance): Called when a block instance is displayed (either on the
  page in view mode, or the page in edit mode). The code here is
  inserted into a <script> tag and placed after all the files loaded via
  get_instance_javascript

  3. PluginBlocktype::get_instance_config_javascript(BlockInstance
  $instance): Called when a block instance's config modal is displayed.
  Seems to behave the same as PluginBlocktipe::get_instance_javascript()

  The "init" sub-item in PluginBlocktype::get_instance_javascript() was
  added a while back to resolve a problem with the older
  PluginBlocktype::get_instance_inline_javascript() method. Sometimes,
  when a script was being loaded, the inline javascript would execute
  before the script finished loading, and that would cause errors.
  Putting the init code inside of jQuery.getScript()'s success callback
  resolves that problem.

  But now we have a couple of new problems. The return format of
  get_instance_javascript() is now rather confusing (containing an array
  of sub-arrays as it does). We have this obsolete
  "get_instance_inline_javascript()" method, which none of the blocks
  use anymore. And, when you're on the page editor screen, adding and
  removing blocks dynamically, any JS library files for the block get
  called every time you add a block of that type. This is not only
  redundant, but also causes bugs when library files aren't namespaced
  properly and contain init code that runs each time they're loaded.

  What we should probably switch to, is something like this:

  1. PluginBlocktype::get_javascript_libraries(): Returns a list of
  Javascript library files to load on any page a blocktype of this sort
  is displayed on. When viewing a page in view mode, we can load the
  libraries for the blocks on the page; when viewing a page in edit
  mode, we can load the libraries for all blocktypes pre-emptively.

  2. PluginBlocktype::get_javascript_config_libraries(): Likewise, but
  for libraries needed in the config modal

  3. PluginBlocktype::get_instance_javascript(BlockInstance $instance):
  Returns a snippet of JS code to run when a block instance is loaded.
  (Should be tied into the JS libraries method, so that it only runs
  after *all* libraries for this blocktype are loaded.)

  4. PluginBlocktype::get_instance_config_javascript(BlockInstance
  $instance): Likewise, but for the config modal

  And to make sure that all JS libraries needed by a blocktype are
  loaded before the block instance is displayed, we could use the HeadJS
  library's "Resource Loading" module, which allows for a JS equivalent
  of PHP's "require_once()", callbacks on file loadings, testing whether
  a file has been loaded, etc.
  http://headjs.com/site/api/v1.00.html#load

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