This commit removes the `NullAstVisitor` and `visitAstChildren` exported
from `packages/compiler/src/expression_parser/ast.ts` because they
contain duplicate and buggy implementation, and their use cases could be
sufficiently covered by `RecursiveAstVisitor` if the latter implements the
`visit` method. This use case is only needed in the language service.
With this change, any visitor that extends `RecursiveAstVisitor` could
just define their own `visit` function and the parent class will behave
correctly.
A bit of historical context:
In language service, we need a way to tranverse the expression AST in a
selective manner based on where the user's cursor is. This means we need a
"filtering" function to decide which node to visit and which node to not
visit. Instead of refactoring `RecursiveAstVisitor` to support this,
`visitAstChildren` was created. `visitAstChildren` duplicates the
implementation of `RecursiveAstVisitor`, but introduced some bugs along
the way. For example, in `visitKeyedWrite`, it visits
```
obj -> key -> obj
```
instead of
```
obj -> key -> value
```
Moreover, because of the following line
```
visitor.visit && visitor.visit(ast, context) || ast.visit(visitor, context);
```
`visitAstChildren` visits every node *twice*.
PR Close#35619
Prior to this commit, the absolute spans (relative to template source
file rather than the start of an expression) of expressions in a
template attribute like `*ngIf` were generated incorrectly, equating to
the relative spans.
This fixes the bug by passing an `absoluteOffset` parameter when parsing
template bindings.
Through some levels of indirection, this is required for the Language
Service to support text replacement in
https://github.com/angular/angular/pull/33091.
PR Close#33189
Currently, the spans of expressions are recorded only relative to the
template node that they reside in, not their source file.
Introduce a `sourceSpan` property on expression ASTs that records the
location of an expression relative to the entire source code file that
it is in. This may allow for reducing duplication of effort in
ngtsc/typecheck/src/diagnostics later on as well.
Child of #31898
PR Close#31897
Keen Yee Liau
ayazhafiz