如何获取NSArray的最后一个对象 [英] How to get the last object of an NSArray
问题描述
所以我有一个数组，项目的数量可以变化。我想知道如果有反正我可以得到一个 NSArray 的最后一个对象？我想到有一个 int 计数器来跟踪数组中的项目数量，这似乎太麻烦了。
So I have an array which the amount of items could vary. I was wondering if there is anyway I can get the last object of an NSArray? I did think of having like an int counter to trace the amount of items in the array however that seems too much of a hassle.
有没有人知道比这更好？
Does anyone know anyway that's better than this?
推荐答案
扩展一点问题，有几种情况下，可能需要一个数组的最后一个对象，
Expanding a bit on the question, there are several situations in which one could need the last object of an array, with different ways to obtain it.
如果只是想获取对象的标准Cocoa程序，则这将执行：
If one just wants to get the object in course of a standard Cocoa program, then this will do it:
[myArray lastObject]
为了解决计数器的问题 - 不需要自己实现：
To address the concern of the counter - no need to implement one's own, either:
NSUInteger myCount = [myArray count];
使用键值编码（KVC）
如果需要通过KVC访问数组的最后一个对象，该故事需要一点解释。
Using key-value coding (KVC)
In case one needs to access the last object of an array through KVC, the story requires a bit of explanation.
首先，请求正常键的值数组将创建一个新数组，该数组由数组中每个对象的键的值组成。换句话说，从数组请求键 lastObject 将使数组向每个数组发送 valueForKey：对象使用 lastObject 键。除了不是预期的结果，它也可能会抛出异常。
First, requesting the value of a normal key from an array will create a new array consisting of the values of that key for each of the objects in the array. In other words, a request for the key lastObject from an array will make the array send valueForKey: to each of the objects using the key lastObject on them. Apart from not being the intended result, it will also likely throw an exception.
因此，如果真的需要发送一个键到数组本身内容），密钥需要在前面加上 @ -sign。这告诉数组，键是为数组本身，而不是其内容。
So in case one really needs to send a key to the array itself (as opposed to its contents), the key needs to be prepended with an @-sign. This tells the array that the key is intended for the array itself, and not its contents.
键因此必须具有 @lastObject 的形式，并使用如下：
The key therefore has to have the form @lastObject, and be used like this:
NSArray *arr = @[@1, @2, @3];
NSNumber *number = [arr valueForKey: @"@lastObject"];
在类别描述符
如何在实际程序中使用这个键是一种情况，其中数组数组需要通过每个内部数组中的 last 对象进行排序。
上面的键只用于排序描述符：
The above key is simply used in the sort descriptor:
NSArray *arrayOfArrays = @[@[@5, @7, @8], @[@2, @3, @4, @6], @[@2, @5]];
NSSortDescriptor *sd = [NSSortDescriptor sortDescriptorWithKey: @"@lastObject" ascending: YES];
NSArray *sorted = [arrayOfArrays sortedArrayUsingDescriptors: @[sd]];
在谓词
为了过滤数组数组，键可以直接在谓词中使用：
In a predicate
Likewise, in order to filter an array of arrays, the key can be used directly in a predicate:
NSPredicate *pred = [NSPredicate predicateWithFormat: @"self.@lastObject > 5"];
NSArray *filtered = [arrayOfArrays filteredArrayUsingPredicate: pred];
人们更喜欢离开 self out可以简单地使用格式 @@ lastObject> 5
People preferring to leave the self-part out can simply use the format @"@lastObject > 5"
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