Revert "Fix #2492 - Remove all Deoplete support for now"

This reverts commit 975cc7af8f.

rplugin/python3/deoplete/sources/ale.py

@@ -0,0 +1,54 @@
+"""
+A Deoplete source for ALE completion via tsserver and LSP.
+"""
+__author__ = 'Joao Paulo, w0rp'
+
+try:
+    from deoplete.source.base import Base
+except ImportError:
+    # Mock the Base class if deoplete isn't available, as mock isn't available
+    # in the Docker image.
+    class Base(object):
+        def __init__(self, vim):
+            pass
+
+
+# Make sure this code is valid in Python 2, used for running unit tests.
+class Source(Base):
+
+    def __init__(self, vim):
+        super(Source, self).__init__(vim)
+
+        self.name = 'ale'
+        self.mark = '[L]'
+        self.rank = 100
+        self.is_bytepos = True
+        self.min_pattern_length = 1
+
+    # Returns an integer for the start position, as with omnifunc.
+    def get_completion_position(self):
+        return self.vim.call('ale#completion#GetCompletionPosition')
+
+    def gather_candidates(self, context):
+        # Stop early if ALE can't provide completion data for this buffer.
+        if not self.vim.call('ale#completion#CanProvideCompletions'):
+            return None
+
+        if context.get('is_refresh'):
+            context['is_async'] = False
+
+        if context['is_async']:
+            # Result is the same as for omnifunc, or None.
+            result = self.vim.call('ale#completion#GetCompletionResult')
+
+            if result is not None:
+                context['is_async'] = False
+
+                return result
+        else:
+            context['is_async'] = True
+
+            # Request some completion results.
+            self.vim.call('ale#completion#GetCompletions', 'deoplete')
+
+        return []