How do intellisense and autocompletions work?

Question

I've been wondering for a while: how do autocompletions work?

Example: In PhpStorm whenever I use a class and type -> it shows me all properties and methods of that class. It also auto completes namespaces and even functions inside libraries such as jQuery.

Does it run some sort of regex on the files, or does it parse them somehow?

Answer 1

PhpStorm developer here. I'd like to go through some basics in case it may be helpful for those who want to implement their own plugin.

First of all, a code has to be broken into tokens using a lexer. Then AST (an abstract syntax tree) and PSI (a program structure interface) are built using a parser. PhpStorm has its own implementations of the lexer and the parser. This is how a PSI tree looks like for a simple class.

When you type in an editor or explicitly invoke a completion action (Ctrl+Space) a number of completion contributors are invoked. They're intended to return a list of suggestions based on a cursor's position.

Let's consider a case when completion is invoked inside a field reference.

PhpStorm knows that at the current position all class members can be suggested. It starts obtaining a class reference (the $class variable in our case) and determining its type. If a variable resolves to a class its type is a class' FQN (fully qualified name).

To obtain methods and fields of a class its PSI element is needed. A special index is used to map an FQN to an appropriate PhpClass tree element. Indices are initially built when a project is opened for the first time and updated for each modified file.

PhpStorm collects all members from the PSI element (including parent's ones), then from its traits. They're filtered depending on a current context (e.g. access scope) and an already typed name's part (f).

Suggestions are shown in a list which is sorted by how good element's name matches, its type, position and so on. The list rearranges when you type.

When you press Enter to insert an element PhpStorm invokes one more handler. It knows how to properly insert the element into a code. For instance, it can add parentheses for a method or import a class reference. In our case, it's enough to put brackets and place a cursor just after them because the method has no parameters.

That's basically it. It worth mention that the IntelliJ IDEA platform allows a plugin to provide an implementation for each step described above. Thus completion can be improved or extended for some particular framework or language.