Alex Rickabaugh 7d954dffd0 feat(ivy): detect cycles and use remote scoping of components if needed (#28169)
By its nature, Ivy alters the import graph of a TS program, adding imports
where template dependencies exist. For example, if ComponentA uses PipeB
in its template, Ivy will insert an import of PipeB into the file in which
ComponentA is declared.

Any insertion of an import into a program has the potential to introduce a
cycle into the import graph. If for some reason the file in which PipeB is
declared imports the file in which ComponentA is declared (maybe it makes
use of a service or utility function that happens to be in the same file as
ComponentA) then this could create an import cycle. This turns out to
happen quite regularly in larger Angular codebases.

TypeScript and the Ivy runtime have no issues with such cycles. However,
other tools are not so accepting. In particular the Closure Compiler is
very anti-cycle.

To mitigate this problem, it's necessary to detect when the insertion of
an import would create a cycle. ngtsc can then use a different strategy,
known as "remote scoping", instead of directly writing a reference from
one component to another. Under remote scoping, a function
'setComponentScope' is called after the declaration of the component's
module, which does not require the addition of new imports.

FW-647 #resolve

PR Close #28169
2019-01-28 12:10:25 -08:00

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HTML

<!doctype html>
<html>
<head>
<title>Angular Cyclic Import Example</title>
</head>
<body>
<!-- The Angular application will be bootstrapped into this element. -->
<trigger></trigger>
<!--
Script tag which bootstraps the application. Use `?debug` in URL to select
the debug version of the script.
There are two scripts sources: `bundle.min.js` and `bundle.min_debug.js` You can
switch between which bundle the browser loads to experiment with the application.
- `bundle.min.js`: Is what the site would serve to their users. It has gone
through rollup, build-optimizer, and uglify with tree shaking.
- `bundle.min_debug.js`: Is what the developer would like to see when debugging
the application. It has also done through full pipeline of rollup, build-optimizer,
and uglify, however special flags were passed to uglify to prevent inlining and
property renaming.
-->
<script>
document.write('<script src="' +
(document.location.search.endsWith('debug') ? '/bundle.min_debug.js' : '/bundle.min.js') +
'"></' + 'script>');
</script>
</body>
</html>