在NSUserDefaults保存的NSMutableArray中插入对象 [英] insert object in an NSMutableArray saved with NSUserDefaults

问题描述

我有一个NSUserDefaults保存的NSMutableArray。
这个数组是我用户可以保存的最喜欢的项目，所以当我想添加一个项目时，我需要读取数组（来自NSuserDefault）并保存在第一个空闲位置。

I have an NSMutableArray saved with NSUserDefaults. This array is my "favourite" items that the user can saves, so when i want to add one item, i need to read the array (from NSuserDefault) and save in the first free position.

我正在使用此方法在NSMutableArray中添加值

I'm using this method to add a value in the NSMutableArray

-(IBAction)save{
   NSMutableArray *abc = [[NSUserDefaults standardUserDefaults] objectForKey:@"12345"];
   int n = [abc count];
   [abc insertObject:@"aaa" atIndex:n];
   NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
   [[NSUserDefaults standardUserDefaults] setObject:abc forKey:@"12345"];
   [defaults synchronize];
   [abc release];
}

这笔交易是什么？
如果用户调用此方法两次，则应用程序第二次与此日志崩溃：

what's the deal? That if the user call this method two times, the second time the app crashes with this log:

* 由于未捕获的异常'NSInternalInconsistencyException'而终止应用程序，原因：' - [__ NSCFArray insertObject：atIndex：]：发送到不可变对象的mutating方法'

* Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFArray insertObject:atIndex:]: mutating method sent to immutable object'

为什么？
和为什么只是第二次？
第一次工作正常！

why? and why just the second time? The first time works fine!

推荐答案

NSUserDefaults 总是返回不可变对象，即使原始对象是可变的。它位于 objectForKey 的文档中：

NSUserDefaults always returns immutable objects, even if the original object was mutable. It's in the documentation for objectForKey:

返回的对象是不可变的，即使您最初设置的值是可变的。

The returned object is immutable, even if the value you originally set was mutable.

您需要在修改之前创建返回对象的副本，使用 [NSMutableArray arrayWithArray：]

You will need to create a copy of the returned object before you modify it, using [NSMutableArray arrayWithArray:]

也可能最好使用 arrayForKey 如果您正在检索数组，则 NSUserDefaults 的方法。文档： https://developer.apple.com/documentation/foundation/userdefaults

Probably also best to use the arrayForKey method of NSUserDefaults if you're retrieving an array. Docs here: https://developer.apple.com/documentation/foundation/userdefaults

这篇关于在NSUserDefaults保存的NSMutableArray中插入对象的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！