How Not to Crash #2: Mutation Exceptions 可变异常(不要枚举可变的集合)

2023-02-05 07:14:21

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How Not to Crash #2: Mutation Exceptions 可变异常

problem

You get a collection from somewhere and enumerate it — and then you get an error about the collection being mutated as it was being enumerated. (在遍历可变数组的时候,数组被修改了.)The app crashes.

solove

You can avoid this unhappy fate with one simple trick: don’t enumerate mutable collections.(不要枚举可变集合)

Disagree with me

You might hold the reasonable position that the real answer is not to mutate a mutable collection while enumerating it. You should have enough understanding of your app to be able to write code that safely enumerates a mutable collection.

Yes, you should. You absolutely should.

However: writing crash-free code is about removing doubt. It’s about minimizing the chances for errors, and minimizing the chance that future changes (by you or somebody else) introduce a crash.

Mutable collections should not be part of public API(可变集合不应该是公开API的一部分)

It should be extremely rare — or, better, never — that an object has a public property that is a mutable collection. Mutable collections should be internal to the object.(可变集合最好,尽可能的不要作为一对象的公开属性.可变的集合应该作为一个内部的变量.)

(Furthermore, as much as possible, public collections should be read-only. This isn’t always possible, of course.[公开的集合属性应该是只读的,尽可能做到这点])

潜在问题的代码

Now, it’s entirely likely that an object has a public collection that is internally a mutable collection. Think of an object that tracks operations. It might make the following public:

@property (nonatomic, readonly) NSArray *operations;

And internally there’s this:

@property (nonatomic) NSMutableArray *mutableOperations;(NSArray *)operations {

return self.mutableOperations;

}

  • (NSArray *)operations {

    return self.mutableOperations;

That’s perfectly legal code: because mutableOperations is an NSMutableArray, it’s also an NSArray. (I did it this way for years. I thought to myself, “Hey, I’m a grownup. I can handle it.” But what I didn’t realize was that grownup developers write code to make errors less likely.)

Properties specified as immutable should be immutable in fact

In the above example, you’re advertising operations as an array that can be enumerated safely at any time. Another person — or you yourself, looking at this in six months — won’t necessarily realize that really you’re getting back a mutable array that can’t necessarily be safely enumerated.

改进后的代码

Here’s the truth-in-advertising solution:

(NSArray *)operations {

return [self.mutableOperations copy];

}

(It wouldn’t hurt to modify the property declaration also to make it clear that it’s a copy, but I admit that I don’t always do that. It would have the advantage of making it completely clear to the user of the API what’s going on.)

性能问题

You might push back, citing performance or memory use issues or both — but I’ll admit something: I’m a performance junkie, and I spend an inappropriate amount of time in Instruments making sure things are fast and use a non-weird amount of memory. And I’ve never, ever found this to be a problem. If your app has performance or memory use issues, the problem is something else, not these copies. (Though you might consider using @autoreleasepool so that these copies don’t last very long.)

Make the copy.

Bonus points: don’t believe their lies(从Bug中得到的教训)

I recently fixed a mutation error when enumerating NSTextStorage layoutManagers:

@property (readonly, copy) NSArray *layoutManagers;

Obviously it’s safe to enumerate. It’s an NSArray, it says, and it’s a copy. Cool. Enumerate away.

But it’s a lie. In the debugger I found that it’s an NSMutableArray (__NSArrayM) — and that it’s not a copy at all. It’s NSTextStorage’s _layoutManagers instance variable, which is declared as an NSMutableArray.

And some code in my enumeration block did a thing that triggered a mutation in layoutManagers, and the app crashed.

The answer: enumerate a copy of layoutManagers. Problem solved.

There’s a general point: if you’re getting a collection from code that isn’t yours, it doesn’t hurt to be defensive and enumerate a copy.

原文和参考译文

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