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as it retains the object’s memory from the garbage collection.
one object can have many retaining paths.
objects with at least one retaining path are
called reachable objects.
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example
the following example illustrates the concepts:
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shallow size vs retained size
shallow size includes only the size of the object
and its references, while retained size also includes
the size of the retained objects.
the retained size of the root object includes
all reachable dart objects.
in the following example, the size of myHugeInstance
isn’t part of the parent’s or child’s shallow sizes,
but is part of their retained sizes:
in DevTools calculations, if an object has more
than one retaining path, its size is assigned as
retained only to the members of the shortest retaining path.
in this example the object x has two retaining paths:
only members of the shortest path (d and e) will include
x into their retaining size.
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memory leaks happen in dart?
garbage collector cannot prevent all types of memory leaks, and developers
still need to watch objects to have leak-free lifecycle.
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why can’t the garbage collector prevent all leaks?
while the garbage collector takes care of all
unreachable objects, it’s the responsibility
of the application to ensure that unneeded objects
are no longer reachable (referenced from the root).
so, if non-needed objects are left referenced
(in a global or static variable,
or as a field of a long-living object),
the garbage collector can’t recognize them,
the memory allocation grows progressively,
and the app eventually crashes with an out-of-memory error.
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why closures require extra attention
one hard-to-catch leak pattern relates to using closures.
in the following code, a reference to the
designed-to-be short-living myHugeObject is implicitly
stored in the closure context and passed to setHandler.
as a result, myHugeObject won’t be garbage collected
as long as handler is reachable.
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why BuildContext requires extra attention
an example of a large, short-living object that
might squeeze into a long-living area and thus cause leaks,
is the context parameter passed to flutter’s
build method.
the following code is leak prone,
as useHandler might store the handler
in a long-living area:
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how to fix leak prone code?
the following code is not leak prone,
because:
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general rule for BuildContext
in general, use the following rule for a
BuildContext: if the closure doesn’t outlive
the widget, it’s ok to pass the context to the closure.
stateful widgets require extra attention.
they consist of two classes: the widget and the
widget state,
where the widget is short living,
and the state is long living. the build context,
owned by the widget, should never be referenced
from the state’s fields, as the state won’t be garbage
collected together with the widget, and can significantly outlive it.
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memory leak vs memory bloat
in a memory leak, an application progressively uses memory,
for example, by repeatedly creating a listener,
but not disposing it.
memory bloat uses more memory than is necessary for
optimal performance, for example, by using overly large
images or keeping streams open through their lifetime.
both leaks and bloats, when large,
cause an application to crash with an out-of-memory error.
however, leaks are more likely to cause memory issues,
because even a small leak,
if repeated many times, leads to a crash.
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<topic_start>
memory view guide
the DevTools memory view helps you investigate
memory allocations (both in the heap and external),