"stop this script" and "return" would previously cause all the procedure
arguments to be re-evaluated and then discarded. In vanilla Scratch the
worst case is a performance penalty, but with our custom reporters it
can cause infinite loops.
Fixes https://github.com/TurboWarp/scratch-vm/issues/201
- Add pointer member to thread. This is the current executing block.
- Add stackFrame member to thread. This is the current frame
information like procedure parameters.
This is a step potentially towards stack-less threads. With further
modifications we could have stack and stackFrames be null if a script
does not call a procedure.
A previous change fixed compatibility with Scratch 2 removing 3's
unintentional scope leaking. This furthers that change so that
procedures with no parameters will also not accidentally use values in
other procedure stacks.
Turn the reporter tree execute walks into a sequence of block
functions. After calling a block function store the reported value in
the arguments object its depending block will use.
- Set the reported value on the thread. There is only ever one "just"
reported value, which is read and stored elsewhere after being
reported.
- Add stackFrame.justReported
Report the block result to a known key `justReported` instead of a
dynamic key on `reported`. Assuming blocks with a promised value are
relatively infrequent the most common recursive input flow immediately
reads the value "just" reported. In the assumed uncommon case of a
promised thread status, empty the already argValues assigned values
onto the currentStackFrame's reported member. In the next execute call
on this stackFrame, values assigned to reported are read back off onto
argValues, and execute will returned to the assumed common case. This
is a safe assumption since a thread in the promise state will not exit
that state until the next frame when javascript has a chance to call
the resolve handle, setting the thread's state back to another
executable state.
Using direct assignment to `justReported` saves building an object
dynamically. Instead of always building `reported` and `argValues` only
`argValues` is built until a promised state is reached. This also gives
a known time when `reported` is used, allowing cleanup of a
stackFrame's reported to only happen when it was used to persist
already reported values.
Thomas Weber