[FLEX-33311] Null pointer fatal in ConstraintLayout when executing Transition on DropDownList

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Apache Flex 4.8 (parity release)
• Fix Version/s: Apache Flex 4.16.0
• Component/s: Spark: DropDownList, Spark: Layout, Transitions
• Assignee: Mihai Chira
• Reporter: Maxime Cowez
• Created: 17/Dec/12 16:36
• Updated: 18/Aug/16 13:46
• Resolved: 18/Aug/16 13:44

Description

In some very specific cases ConstraintLayout will throw a null pointer error when a component inside it is playing a transition. I tried a few things and found out that:

• parseConstraints() creates a new rowBaselines Vector (or empties it if it exists)
• some other process calls clearConstraintCache() in the middle of the execution of parseConstraints(), setting rowBaselines back to null
• parseConstraints() calls parseElementConstraints() which tries to access elements in rowBaselines and throws the error

I know very little about how Effects and Transitions work, so the "some other process" part is a bit of a mistery to me.

Steps to reproduce:

1. Compile and run Main.mxml
2. Select the first item in the list
3. Click on the list and, before the resize transition finishes, click on it again.

Workaround: subclass ConstraintLayout (or FormItemLayout), override `measure()`, put a try/cacth block around it and use this custom layout. Not exactly pretty but it works without apparent side effects.
Quick fix (but probably not the ideal solution): just test whether rowBaselines exists before trying to access it in ConstraintLayout.parseElementConstraints().