Text field stays under the custom keyboard in UIWebView for iOS 8

问题

I'm creating a web App for iOS and I have a custom keyboard. When I use the default keyboard, it immediately shows the keyboard and the text field is above the keyboard. I would have the same result if I change the keyboard to my custom keyboard. But the problem is for the time that I gives the focus to the text field when my custom keyboard is enabled. It has some delay to load and when it loads, text field stays under the keyboard. Here is my code for custom keybaord:

#import "KeyboardViewController.h"
#import "CustomKeyboardView.h"

@interface KeyboardViewController () <CustomKeyboardViewDelegate>
@property (nonatomic, strong) CustomKeyboardView *customKeyboardView;
@end

@implementation KeyboardViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.customKeyboardView = [[CustomKeyboardView alloc] init];
    self.inputView = (UIInputView *) self.customKeyboardView;

    self.customKeyboardView.delegate = self;
}


- (void)handleCharacter:(NSString *)character sender:(CustomKeyboardView *)sender
{
    [self.textDocumentProxy insertText:character];
}

-(void)handelDelete:(CustomKeyboardView *)sender
{
    [self.textDocumentProxy deleteBackward];
}
-(void)handleDismissKeyboard:(CustomKeyboardView *)sender
{
    [self dismissKeyboard];
}
-(void)handleChangeKeyboard:(CustomKeyboardView *)sender
{
    [self advanceToNextInputMode];
}


- (void)textWillChange:(id<UITextInput>)textInput {
    // The app is about to change the document's contents. Perform any preparation here.
}

- (void)textDidChange:(id<UITextInput>)textInput {
    // The app has just changed the document's contents, the document context has been updated.
    [[NSNotificationCenter defaultCenter] postNotificationName:UITextViewTextDidChangeNotification object:textInput];
}

@end

The implementation of CustomKeyboardView class is similar to this library. Any help would be appreciated.