Persist PDF zoom with Nutrient

Persisting zoom level across a PDF document improves the experience for users who want consistent viewing or have visual impairments. This tutorial shows how to implement persistent zoom in your Android app using Nutrient Android SDK.

Requirements

To get started, you’ll need:

• Android Studio(opens in a new tab) — The official integrated development environment (IDE) for Android.
• Nutrient Android Catalog(opens in a new tab) — Clone and run this repository before proceeding.

Persisting zoom

Once you have Catalog up and running, navigate to app/src/main/java/com/pspdfkit/catalog/examples/kotlin/ZoomExample.kt, which is the file you’ll be modifying. More specifically, you’ll be modifying the ZoomExampleActivity class in the file. You can see how this example works by installing the app and opening Zoom Example in your device or emulator.

To persist the zoom, you need to do the following things:

• Listen to page changes in the document as a user scrolls between the pages. For this, you’ll use DocumentListener.
• Keep track of the scale factor once the zoom has been modified.
• Apply the zoom to the document after the page has changed. For this, you’ll use the zoomTo method.
• Finally, clean up.

In the ZoomExampleActivity class, paste the following as a class member variable:

private var documentListener:DocumentListener? = null

In the companion object, add the following constant:

private const val ZOOM_DURATION = 100L

The listener above will be used to track the changes, while the ZOOM_DURATION is a constant to determine how long the zoom animation should take. Next, add the following method in the ZoomExampleActivity class:

    private fun persistZoom() {
        val fragment = requirePdfFragment()
        var zoom = 1.0f

        documentListener = object : DocumentListener {
            override fun onPageChanged(document: PdfDocument, pageIndex: Int) {
                val size = fragment.document?.getPageSize(pageIndex)
                fragment.zoomTo(0, size?.height?.toInt() ?: 0, pageIndex, zoom, ZOOM_DURATION)
            }

            override fun onDocumentZoomed(document: PdfDocument, pageIndex: Int, scaleFactor: Float) {
                zoom = scaleFactor
            }
        }

        documentListener?.let { listener ->
            fragment.addDocumentListener(listener)
        }
    }

In the method above, you get PdfFragment and declare a zoom variable to track the current zoom factor. The zoom factor is always updated once the document zoom has changed in the onDocumentZoomed method. You then set the zoom on the document using the onPageChanged method. By setting the zoom as shown above, the document will always zoom to the top-left corner. Then, you call this method at the end of onDocumentLoaded. To have the document zoom to the center, for example, you can use the following code instead:

fragment.zoomTo((size?.width?.toInt() ?: 0) / 2, (size?.height?.toInt() ?: 0) / 2, pageIndex, zoom, ZOOM_DURATION)

Feel free to play around with the values above to get the desired behavior.

Finally, inside the onDestroy method, clean up as shown below to remove the listener you added:

    override fun onDestroy() {
        super.onDestroy()
        ...

        documentListener?.let { listener ->
            requirePdfFragment().removeDocumentListener(listener)
        }
    }

Installing Catalog again in a device or emulator will now show you a consistent zoom across the document as you swipe from one page to another.

Conclusion

Persistent zoom improves the reading experience by maintaining a consistent zoom level as users scroll between pages. The DocumentListener tracks page changes and zoom factors, while zoomTo applies the saved zoom to each new page.

If you run into issues, contact our Support team. Start a free trial or check out the demo.

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