ViewPager2 default position

Question

I'm creating a slideshow with ViewPager2. For example, the slideshow has 3 items and I want to show the second item when the activity opens. I use setCurrentItem(int item, boolean smoothScroll) method but it doesn't work and nothing happens. How can I achieve it?

viewPager.adapter = adapter
viewPager.setCurrentItem(1, true)

Answer 1

I think an easier more reliable fix is to defer to next run cycle instead of unsecure delay e.g

viewPager.post {
  viewPager.setCurrentItem(1, true)
}

Answer 2

setCurrentItem(int item, boolean smoothScroll) works correctly in ViewPager but in ViewPager2 it does not work as expected. Finally, I faced this problem by adding setCurrentItem(int item, boolean smoothScroll) method into a delay like this:

Handler().postDelayed({
     view.viewPager.setCurrentItem(startPosition, false)
}, 100)

Answer 3

Do not use timers, you will run into a lot of probable states in which the user has a slow phone and it actually takes a lot longer than 100 ms to run, also, you wouldn't want too slow of a timer making it ridiculously un-reliable.

Below we do the following, we set a listener to our ViewTreeObserver and wait until a set number of children have been laid out in our ViewPager2's RecyclerView (it's inner working). Once we are sure x number of items have been laid out, we start our no-animation scroll to start at the position.

val recyclerView = (Your ViewPager2).getChildAt(0)

recyclerView.apply {
   val itemCount = adapter?.itemCount ?: 0
   if(itemCount >= #(Position you want to scroll to)) {
      viewTreeObserver.addOnGlobalLayoutListener(object: ViewTreeObserver.OnGlobalLayoutListener {
      override fun onGlobalLayout() {
         viewTreeObserver.removeOnGlobalLayoutListener(this)

         // False for without animation scroll
         (Your ViewPager2).scrollToPosition(#PositionToStartAt, false)
      }
   }
}

Answer 4

First off, I think that the accepted answer shouldn't be @hosseinAmini 's, since it's suggesting to use a delay to work around the problem. You should first be looking for what the assumed bug is caused by, rather than trusting unreasonable solutions like that.

@Rune's proposal is correct, instead; so I'm quoting their code in my answer:

viewPager.post {
    viewPager.setCurrentItem(1, true)
}

The only thing I'd argue about is the aforementioned one's belief that their solution is just deferring the execution of that lambda in the next run cycle. This wouldn't make anything buggy work properly. Rather, what it is actually being done is deferring the execution of that lambda to once the view has been attached to a window, which implies it's also been added to a parent view. Indeed, there looks to be an issue as to changing the current ViewPager2 item before being attached to a window. Some evidence to support this claim follows:

• Using whichever Handler won't work nearly as effectively.

  Handler(Looper.getMainLooper()).post {
      viewPager.setCurrentItem(1, true) // Not working properly
  }
  

  From a theoretical standpoint, it might incidentally work due to the ViewPager2 being attached to a window acquiring priority in the message queue of the main looper, but this shouldn't ever be relied upon as there's just no guarantee that it'll work (it's even more likely it won't) and if it even turned out to be working, further investigation running multiple tests should make my point clear.

• View.handler gets null, which means the view hasn't been attached to any window yet.

  View.handler // = null
  

  Despite Android UI being tied to the main looper, which will always uniquely correspond to the main thread –hence also called the UI thread,– a weird design choice stands in the handler not being associated to each view until they get attached to a window. A reason why this may lay on the consequent inability of views to schedule any work on the main thread while they're not part of the hierarchy, which may turn useful when implementing a view controller that schedules view updates while unaware of their lifecycle (in which case it would employ the View's handler, if any — or just skip scheduling whatever it was going to if none).

EDIT:

Also, @josias has pointed out in a comment that it'd be clearer to use:

viewPager.doOnAttach {
    viewPager.setCurrentItem(1, true)
}

Thanks for that suggestion! It expresses better the actual intent, rather than relying on the behavior of the View.post method.

Answer 5

Do not use timers and all that stuff with 'post', it's not the reliable solution and just a piece of code that smells.

Instead, try use viewPager.setCurrentItem(1, false). That 'false' is about smoothScroll, you can't smooth scroll your viewPager2 when your activity is just opened. Tested it on a fragment in onViewCreated() method, it also didn't work with "true", but works with "false"

Answer 6

As it was mentioned above you have to use setCurrentItem(position, smoothScroll) method on ViewPager2 in order to show selected item. To make it work you have to define a callback, here is an example:

ViewPager2.OnPageChangeCallback callback = new ViewPager2.OnPageChangeCallback() {
    @Override
    public void onPageSelected(int position) {
        super.onPageSelected(position);
    }
};

And then you have to register it as follow:

viewPager.registerOnPageChangeCallback(callback);

Also do not forget to unregister it:

viewPager.unregisterOnPageChangeCallback(callback);

When you call setCurrentItem(position) method it will call onPageSelected(int position) method from your callback passing your argument, and then method createFragment(int position) from FragmentStateAdapter class will be called to show your fragment.

Answer 7

I tried changing viewpager2 page in Handler().dely() and viewPager2.post{} and even 'viewPager2.get(0).post all didn't work for me, I'm using ViewPager with FragmentStateAdapter with Tablayout.

What worked for me is changing the position of the RecylerView in ViewPager2 after binding FragmentStateAdapter to yourViewPager2View.adapter manually:

 (yourViewPager2View[0] as RecyclerView).scrollToPosition(moveToTabNumber)

Why

My problem is onCreateFragment(position:Int):Fragmeet function in FragmentStateAdapter always starting fragment at 0 position no matter what pageNumber I set the page

viewPager.setCurrentItem = pageNumber

I checked where it's called in FragmentStateAdapter it's called in FragmentStateAdapter:

onBindViewHolder(final @NonNull FragmentViewHolder holder, int position)`

so all I needed is to force onBindViewHolder to call onCreateFragment(position:Int) with the page number I wanted.

Answer 8

mViewPager.setCurrentItem(1, true); ---> this is sufficient as you written above

That should work, in doubt, just check your position:

        @Override
        public void onPageSelected(int i) {
            if (LOG_DEBUG) Log.v(TAG, " ++++++++    onPageSelected:  " + i);
            mViewPager.setCurrentItem(i);
            //TODO You can use this position: to write other dependent logic

        }

and also check

getItem(int position) in PagerAdapter

or else paste your code.

Answer 9

I noticed that it works fine when the view is initially created if you opt to not animate it.

viewPager2.setCurrentItem(index, false)

This is usually fine depending on your use case - this initial/default item probably doesn't need to be animated in.

Answer 10

I met the same problem. In my case, I make the viewPager2 Gone by default until network requests succeed, I fix it by setting the CurrentItem after I make the viewPager2 visible.

Answer 11

My answer may not be helpful now but i see no harm to post my expreince, i just came to this problem using ViewPager and ViewPager2 and unexpectedly solved it by just changing some line codes order.

Here is (java) solution for ViewPager:

    reviewWordViewPager.addOnPageChangeListener(changeListener);
    reviewWordViewPager.setCurrentItem(viewPosition, true/false);
    reviewWordTabIndicator.setupWithViewPager(reviewWordViewPager, true);

(Java) solution for ViewPager2:

wordViewPager.registerOnPageChangeCallback(viewPager2OnPageChangeCallback);
wordViewPager.setCurrentItem(vpPosition, true/false);
new TabLayoutMediator(tabIndicator, wordViewPager,
            ((tab, position) -> tab.setText(viewPagerTitle[position]))).attach();

I did not look up for ViewPager2 whether it needs the following old code used in ViewPager

    @Override
public int getItemPosition(@NonNull Object object) {
    // refresh all fragments when data set changed
    return POSITION_NONE;
}

But surprisingly no need for it in ViewPager2 to solve the problem i've been having, hope it helps others

In case you use context.startActivity to start new activities no need to use wordViewPager.setCurrentItem(item, smoothScroll) in your onResume function to get back to the last selected tab before you started new activity you just save ViewPager/ViewPager2 position like vpPisition = wordViewPager.getCurrentItem(); in onStop function.

vpPisition is a global variable.

Answer 12

as @Daniel Kim but a java version

RecyclerView rvOfViewPager2 = (RecyclerView) viewPager2.getChildAt(0);
    rvOfViewPager2.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener()
    {
        @Override
        public void onGlobalLayout()
        {
            rvOfViewPager2.getViewTreeObserver().removeOnGlobalLayoutListener(this);
            viewPager2.setCurrentItem(currentTabId, false);
        }
    });

Answer 13

First You need to Initilaze the Main activity under any listener or button You want then After that You need to put this Line.. here MainActvity is the Viewpager Main Class You are using and and 2 is the position where you want to move

 MainActivity main = (MainActivity ) mContext;
    main.selectTab(2, true);