使用 Core Data 时的向后兼容性

Question

有人能解释一下为什么我的应用程序在 iPhone OS 2.2.1 上崩溃并出现以下错误吗？

dyld: Symbol not found: _OBJC_CLASS_$_NSPredicate
  Referenced from: /var/mobile/Applications/456F243F-468A-4969-9BB7-A4DF993AE89C/AppName.app/AppName
  Expected in: /System/Library/Frameworks/Foundation.framework/Foundation

我的 CoreData.framework 链接很弱，并且将 Base SDK 设置为 3.0，将部署目标设置为 SDK 2.2

该应用程序已经使用其他3.0 功能可用时，我对这些功能没有任何问题。但显然用于其他功能的向后兼容方法不适用于 Core Data。

应用程序在调用应用程序委托的 applicationDidFinishLaunching 之前崩溃。

这是调试器日志：

[Session started at 2010-05-25 20:17:03 -0400.]
GNU gdb 6.3.50-20050815 (Apple version gdb-1119) (Thu May 14 05:35:37 UTC 2009)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "--host=i386-apple-darwin --target=arm-apple-darwin".tty /dev/ttys001
Loading program into debugger…
sharedlibrary apply-load-rules all
warning: Unable to read symbols from "MessageUI" (not yet mapped into memory).
warning: Unable to read symbols from "CoreData" (not yet mapped into memory).
Program loaded.
target remote-mobile /tmp/.XcodeGDBRemote-12038-42
Switching to remote-macosx protocol
mem 0x1000 0x3fffffff cache
mem 0x40000000 0xffffffff none
mem 0x00000000 0x0fff none
run
Running…
[Switching to thread 10755]
[Switching to thread 10755]
Re-enabling shared library breakpoint 1
Re-enabling shared library breakpoint 2
Re-enabling shared library breakpoint 3
Re-enabling shared library breakpoint 4
Re-enabling shared library breakpoint 5
(gdb) continue
warning: Unable to read symbols for ""/Users/alex/iPhone Projects/AppName/build/Debug-iphoneos"/AppName.app/AppName" (file not found).
dyld: Symbol not found: _OBJC_CLASS_$_NSPredicate
  Referenced from: /var/mobile/Applications/456F243F-468A-4969-9BB7-A4DF993AE89C/AppName.app/AppName
  Expected in: /System/Library/Frameworks/Foundation.framework/Foundation

(gdb) 

Answer 1

显然，如果您不向后兼容 iPhone OS 2.2，则无法通过文字类符号引用 NSPredicate

因此，将我对 NSPredicate 的引用更改为：

[fetchRequest setPredicate:[NSClassFromString(@"NSPredicate") predicateWithFormat:@"EntryId == %@", [NSNumber numberWithInt: index]]];

现在它可以工作了。

请参阅https://devforums.apple.com/message/225591#225591

Answer 2

抱歉，请重新阅读您的问题。看起来框架本身不能弱链接。我想这意味着您不能使用 Core Data 框架创建 2.x 目标:(

即整个框架在目标平台上不可用，并且弱链接不适用。