實現代理方法

- (BOOL)textField:(UITextField*)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString*)string
{
    NSString *blank = [[string componentsSeparatedByCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] componentsJoinedByString:@""];
    
    if(![string isEqualToString:blank]) {
        return NO;
    }
    
    return YES;
}