objective-c - 为什么使用cocoa库的HelloWorld代码无法构建？

Question

代码如下:

#import <Foundation/Foundation.h>
#import <Cocoa/Cocoa.h>
int main(int argc, const char * argv[])
{

    @autoreleasepool {

        // insert code here...

        NSLog(@"Hello, World!");
    }
    NSAlert *alert = [[NSAlert alloc] init];
    [alert setMessageText:@"Hi there."];
    [alert runModal];


    return 0;
}

我想要的是:当从命令行调用时，当我关闭警报框时，该程序会弹出一个警报框。程序退出。

但是在构建时，它会这样提示:

Ld /Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Products/Debug/KeyCatcher normal x86_64
    cd /Users/hanfei/Desktop/KeyCatcher
    setenv MACOSX_DEPLOYMENT_TARGET 10.9
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk -L/Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Products/Debug -F/Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Products/Debug -filelist /Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Intermediates/KeyCatcher.build/Debug/KeyCatcher.build/Objects-normal/x86_64/KeyCatcher.LinkFileList -mmacosx-version-min=10.9 -fobjc-arc -fobjc-link-runtime -framework Foundation -Xlinker -dependency_info -Xlinker /Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Intermediates/KeyCatcher.build/Debug/KeyCatcher.build/Objects-normal/x86_64/KeyCatcher_dependency_info.dat -o /Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Products/Debug/KeyCatcher

Undefined symbols for architecture x86_64:
  "_OBJC_CLASS_$_NSAlert", referenced from:
      objc-class-ref in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

当我为新项目选择模板时，我选择命令行工具而不是CoCoa Application，因为我只需要CoCoa来显示警报框。有没有人对此有想法..

Answer 1

那是因为 NSAlert 不是#imported。

AppKit.h当您创建命令行工具时，默认情况下不包含此内容，正如您在链接中看到的，AppKit 导入 NSAlert.h。

编辑:
首先，要编译，您需要在项目中添加 Cocoa 框架。

其次，所有代码都应包含在 @autoreleasepool 部分

@autoreleasepool {

    // insert code here...
    NSLog(@"Hello, World!");
    NSAlert *alert = [[NSAlert alloc] init];
    [alert setMessageText:@"Hi there."];
    [alert runModal];    
}

您的代码将编译并运行，但我认为您还会遇到一些其他运行时错误。