Make positionInlineChildren assert much clearer

[BrainLUX]

fixes flutter/flutter#182090

The assertion message should clearly state the actual requirement of the code to help developers understand and fix the error.

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Code Review

This pull request improves the clarity of an assertion message in positionInlineChildren. The change is correct and makes the error message much easier to understand. I've suggested a small improvement to make the assertion condition more explicit, which further enhances readability.

[victorsanni]

I'm assuming the fact that we are in this if-block means there is an additional box not accounted for.

[BrainLUX]

In fact, yes, that's true.
But explicitly specifying the condition will make debugging easier for developers.

[LongCatIsLooong]

I think I initially wrote the assert under the assumption that it wasn't going to be an app developer facing error message (so the error message was only useful for framework developers), but it looks like this is not the case. Was it because you didn't push placeholders in the custom InlineSpan in InlineSpan.build? In any case it might be helpful to expand the error message a little more to include the most probable cause (e.g. did you forget to push placeholders if you are using a custom InlineSpan subclass?)

[LongCatIsLooong]

LGTM but I think this may need a test, since you already have an implementation that would trigger the assert.

[justinmc]

@BrainLUX Would you be able to add a test here that catches the logical change of when the error occurs?

[BrainLUX]

@BrainLUX Would you be able to add a test here that catches the logical change of when the error occurs?

Yes, of course

[LongCatIsLooong]

still LGTM, but maybe this should be expanded to a more detailed message, like the one over here:

flutter/packages/flutter/lib/src/rendering/object.dart

Lines 2224 to 2241 in 1e97fd3

	if (activeLayoutRoot == null) {
	throw FlutterError.fromParts(<DiagnosticsNode>[
	ErrorSummary('A $runtimeType was mutated in $culpritFullMethodName.'),
	ErrorDescription(
	'The RenderObject was mutated when none of its ancestors is actively performing layout.',
	),
	DiagnosticsProperty<RenderObject>(
	'The RenderObject being mutated was',
	this,
	style: DiagnosticsTreeStyle.errorProperty,
	),
	DiagnosticsProperty<RenderObject>(
	'The RenderObject that was mutating the said $runtimeType was',
	debugActiveLayout,
	style: DiagnosticsTreeStyle.errorProperty,
	),
	]);
	}

so people ran into the same problem don't have to figure out what could have caused the assert to fire.

[LongCatIsLooong]

Still LGTM, thank you for adding the detailed error message!

[LongCatIsLooong]

nit: I feel this is no longer needed since there is now a detailed message.

[BrainLUX]

Agreed.