NSTextField doesn't respond to keyboard input - objective-c

Question

I tried to write a single-window application using Objective-C in OS X 10.8 to test some thing. Everything works fine except that the NSTextField I created does not respond to keyboard input. The code is as follows:
#import <Cocoa/Cocoa.h>
#interface customizedView:NSView
+(customizedView *)aCustomizedView; // changed method name to follow convention
#end
#implementation customizedView
+(customizedView *)aCustomizedView{
customizedView *newView = [[[customizedView alloc] initWithFrame:NSMakeRect(0,0,400,300)] autorelease];
NSTextField *tf = [[[NSTextField alloc] initWithFrame:NSMakeRect(200,150,150,50)] autorelease];
[newView addSubview: tf positioned:NSWindowBelow relativeTo:newView];
return newView;
}
-(void)windowWillClose:(NSNotification *)aNot{
[NSApp terminate:self];
}
#end
void setup(){
NSWindow *aWindow = [[[NSWindow alloc] initWithContentRect:NSMakeRect(0,0,400,300) styleMask:NSTitledWindowMask | NSClosableWindowMask | NSResizableWindowMask backing:2 defer:NO] autorelease];
customizedView *aView = [customizedView aCustomizedView];
[aWindow setContentView:aView];
[aWindow setDelegate:aView];
[aView setNeedsDisplay:YES];
[aWindow makeKeyAndOrderFront:nil];
}
int main(int argc, char **argv){
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSApplication *NSApp = [NSApplication sharedApplication];
setup();
[NSApp run]; [NSApp release]; [pool release];
exit(EXIT_SUCCESS);
}
Compile using
gcc -w filename.m -o tmp -framework Cocoa
Everything works except that I cannot input using keyboard to input into the textfield in my customized NSView. Paste using mouse is okay. So far I still have no ideal why this happens and I give my thanks in advance to all who read this tip sincerely.