How to Access and Execute Methods in ViewPager Fragments from an Activity?

Access ViewPager Fragment Method from Activity

In this scenario, a ViewPager with six fragments is implemented within an Activity. Swiping between fragments is disabled, so a button is used to change the current fragment. A GET request is sent to the server and the response data is displayed in each fragment after the swipe is completed.

First Approach (Using setUserVisibleHint)

Initially, the setUserVisibleHint method was employed to trigger the GET request whenever a fragment became visible. However, this approach resulted in a lag in the swipe animation due to the immediate execution of the request.

Second Approach (Using OnPageChangeListener)

To overcome the animation lag, an OnPageChangeListener was added to the ViewPager. This listener tracks the page scroll state and sends the GET request when the desired fragment is completely visible (i.e., scroll state equals IDLE).

Problem and Solution

The second approach worked well, but a NullPointerException occurred when executing sendGetRequest(). This was because the fragment reference was not properly obtained. To resolve this, it was necessary to store references to the fragments in the FragmentPagerAdapter's instantiateItem method. Then, within the OnPageChangeListener, instead of creating a new fragment, the referenced fragment from the ViewPagerPagerAdapter could be used.

Updated Code for ViewPager2 (Kotlin)

<code class="kotlin">viewPager.registerOnPageChangeCallback(
            object: ViewPager2.OnPageChangeCallback() {
                override fun onPageSelected(position: Int) {
                    super.onPageSelected(position)
                    val frag: Fragment = mAdapter.fragments[position]
                    if (frag != null && frag is FragmentTwo) {
                        (frag as FragmentTwo).sendGetRequest()
                    }
                }
            }
    )</code>

Here, mAdapter refers to the FragmentPagerAdapter used within the ViewPager2.

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