NSPointerArray怪异压实

Question

我有一个弱的NSPointerArray和一些已经发布的NSObject。在调用compact之前，我看到的是：

(lldb) po [currentArray count]
1
(lldb) po [currentArray pointerAtIndex:0]
<nil>
(lldb) po [currentArray allObjects]
<__NSArrayM 0x16f04f00>(

)

这是有道理的，但真正奇怪的是，当我在那个数组上调用compact时，我会看到相同的值！Count仍然返回1，而pointerAtIndex:0是nil。

为什么零没有被移除？

编辑

下面是完整的代码(是的，这是XCTesting框架)：

- (void)testCompaction {
    __weak id testingPointer = nil;

    NSPointerArray *weakArray = [NSPointerArray weakObjectsPointerArray];

    @autoreleasepool {

        NSObject *someObj = [[NSObject alloc] init];

        testingPointer = someObj;

        [weakArray addPointer:(__bridge void*)testingPointer];

        NSLog(@"before compaction inside autorelease: testingPointer = %@ count = %d, allObjects = %@, pointerAtIndex:0 = %@, pointerAtIndex:0 class = %@", testingPointer, [weakArray count], [weakArray allObjects], [weakArray pointerAtIndex:0], [(id)[weakArray pointerAtIndex:0] class]);

        someObj = nil;
    }

    NSLog(@"before compaction outside autorelease: testingPointer = %@ count = %d, allObjects = %@, pointerAtIndex:0 = %@, pointerAtIndex:0 class = %@", testingPointer, [weakArray count], [weakArray allObjects], [weakArray pointerAtIndex:0], [(id)[weakArray pointerAtIndex:0] class]);

    [weakArray compact];

    NSLog(@"after compaction outside autorelease: testingPointer = %@ count = %d, allObjects = %@, pointerAtIndex:0 = %@, pointerAtIndex:0 class = %@", testingPointer, [weakArray count], [weakArray allObjects], [weakArray pointerAtIndex:0], [(id)[weakArray pointerAtIndex:0] class]);
}

和原木：

  before compaction inside autorelease: testingPointer = <NSObject: 0x7de7ff80> count = 1, allObjects = (
    "<NSObject: 0x7de7ff80>"
), pointerAtIndex:0 = <NSObject: 0x7de7ff80>, pointerAtIndex:0 class = NSObject
2015-07-20 14:27:14.062 AppetizeSuite copy[54144:9019054] before compaction outside autorelease: testingPointer = (null) count = 1, allObjects = (
), pointerAtIndex:0 = (null), pointerAtIndex:0 class = (null)
2015-07-20 14:27:22.615 AppetizeSuite copy[54144:9019054] after compaction outside autorelease: testingPointer = (null) count = 1, allObjects = (
), pointerAtIndex:0 = (null), pointerAtIndex:0 class = (null)   

为什么compact方法不删除第一个指针？在调用nil之前，这显然是一个compact。

Answer 1

我也见过同样的行为。下面是一个开放的雷达漏洞报告：http://www.openradar.me/15396578

Answer 2

发生这种情况的原因是-compact首先检查是否设置了内部标志'needsCompaction‘。如果不是的话，它就会提前出航。设置标志的唯一时间是通过公共接口将零指针直接插入到数组中。如果弱引用对象在指针插入数组后被解除分配(指针为零)，则不会设置该对象。

要解决此问题，一项工作是在调用-compact之前，有目的地向数组追加一个零指针。不太理想，但会成功的。

[pa addPointer:nil]; // forces the pointer array to do compaction next time
[pa compact];

Answer 3

已经过了几年了，使用示例代码没有任何变化。

但是，当使用方法-compact执行指针更新时，-replacePointerAtIndex:withPointer工作得很好：

在上面的代码中，替换

someObj = nil;

通过

 [weakArray replacePointerAtIndex:0 withPointer:nil];

提供[weakArray compact];后的预期结果，代码将在最后一个NSLog崩溃。