核心数据 willSave: 方法 [英] Core-Data willSave: method

问题描述

我的 Entity A 中有一个属性 modificationDate. 我想在保存 NSManagedObject 时设置它的值.但是，如果我尝试在 NSManagedObject willSave: 方法中执行此操作，则会出现错误:

I have an attribute modificationDate in my Entity A. I want to set its value whenever NSManagedObject is saved. However, if i try to do that in NSManagedObject willSave: method, i get an error:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Failed to process pending changes before save.  The context is still dirty after 100 attempts.  Typically this recursive dirtying is caused by a bad validation method, -willSave, or notification handler.' ***

那么，我想知道，设置 modificationDate 值的最佳方法是什么?

So, i'm wondering, what's the best way to set the value of modificationDate?

推荐答案

来自 willSave 的 NSManagedObject 文档:

From the NSManagedObject docs for willSave:

如果要更新持久属性值，通常应该在进行更改之前测试任何新值与现有值的相等性.如果您使用标准访问器方法更改属性值，Core Data 将观察结果更改通知，因此在保存对象的托管对象上下文之前再次调用 willSave.如果你继续修改 willSave 中的一个值，willSave 将继续被调用，直到你的程序崩溃.

If you want to update a persistent property value, you should typically test for equality of any new value with the existing value before making a change. If you change property values using standard accessor methods, Core Data will observe the resultant change notification and so invoke willSave again before saving the object’s managed object context. If you continue to modify a value in willSave, willSave will continue to be called until your program crashes.

例如，如果您设置了最后修改的时间戳，您应该检查您之前是否在同一个保存操作中设置了它，或者现有的时间戳不小于当前时间的小增量.通常，最好为所有正在保存的对象计算一次时间戳(例如，响应 NSManagedObjectContextWillSaveNotification).

For example, if you set a last-modified timestamp, you should check whether either you previously set it in the same save operation, or that the existing timestamp is not less than a small delta from the current time. Typically it’s better to calculate the timestamp once for all the objects being saved (for example, in response to an NSManagedObjectContextWillSaveNotification).

所以也许是这样的:

-(void)willSave {
    NSDate *now = [NSDate date];
    if (self.modificationDate == nil || [now timeIntervalSinceDate:self.modificationDate] > 1.0) {
        self.modificationDate = now;
    }
}

您可以在哪里调整 1.0 以反映您预期的保存请求之间的最小增量.

Where you can adjust the 1.0 to reflect the minimum delta between your expected save requests.

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