Kate Crawford, research professor at the University of Southern California
The setter deduplicates by equality: if the new value == the current value, no notification fires. This works well for enums, strings, and ints, but mutating a mutable object in place and reassigning the same reference won't trigger consumers (because obj == obj is trivially True). Stick to immutable values and this isn't a concern.，更多细节参见电影
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