allow changing grid column count by pinching

app/src/main/kotlin/com/simplemobiletools/gallery/activities/MediaActivity.kt

@@ -6,6 +6,7 @@ import android.content.Intent
 import android.graphics.Bitmap
 import android.net.Uri
 import android.os.Bundle
+import android.support.v7.widget.GridLayoutManager
 import android.util.Log
 import android.view.Menu
 import android.view.MenuItem
@@ -20,6 +21,7 @@ import com.simplemobiletools.gallery.dialogs.ChangeSortingDialog
 import com.simplemobiletools.gallery.extensions.getHumanizedFilename
 import com.simplemobiletools.gallery.helpers.*
 import com.simplemobiletools.gallery.models.Medium
+import com.simplemobiletools.gallery.views.MyScalableRecyclerView
 import kotlinx.android.synthetic.main.activity_media.*
 import java.io.File
 import java.io.IOException
@@ -54,6 +56,23 @@ class MediaActivity : SimpleActivity(), MediaAdapter.MediaOperationsListener {
         mShowAll = mConfig.showAll
         if (mShowAll)
             supportActionBar?.setDisplayHomeAsUpEnabled(false)
+
+        val layoutManager = media_grid.layoutManager as GridLayoutManager
+        MyScalableRecyclerView.mListener = object : MyScalableRecyclerView.ZoomListener {
+            override fun zoomIn() {
+                if (layoutManager.spanCount > 1) {
+                    layoutManager.spanCount--
+                    MediaAdapter.actMode?.finish()
+                }
+            }
+
+            override fun zoomOut() {
+                if (layoutManager.spanCount < 10) {
+                    layoutManager.spanCount++
+                    MediaAdapter.actMode?.finish()
+                }
+            }
+        }
     }
 
     override fun onResume() {

app/src/main/kotlin/com/simplemobiletools/gallery/views/MyScalableRecyclerView.kt

@@ -0,0 +1,58 @@
+package com.simplemobiletools.gallery.views
+
+import android.content.Context
+import android.support.v7.widget.RecyclerView
+import android.util.AttributeSet
+import android.view.MotionEvent
+import android.view.ScaleGestureDetector
+
+class MyScalableRecyclerView : RecyclerView {
+    var mScaleDetector: ScaleGestureDetector
+
+    companion object {
+        var mListener: ZoomListener? = null
+        var mCurrScaleFactor = 1.0f
+    }
+
+    constructor(context: Context) : super(context) {
+    }
+
+    constructor(context: Context, attrs: AttributeSet) : super(context, attrs) {
+    }
+
+    init {
+        mScaleDetector = ScaleGestureDetector(context, GestureListener())
+    }
+
+    override fun dispatchTouchEvent(ev: MotionEvent): Boolean {
+        super.dispatchTouchEvent(ev)
+        if (ev.action == MotionEvent.ACTION_UP) {
+            mCurrScaleFactor = 1.0f
+        }
+
+        return mScaleDetector.onTouchEvent(ev)
+    }
+
+    class GestureListener : ScaleGestureDetector.SimpleOnScaleGestureListener() {
+        val ZOOM_IN_THRESHOLD = -0.7f
+        val ZOOM_OUT_THRESHOLD = 0.3f
+
+        override fun onScale(detector: ScaleGestureDetector): Boolean {
+            val diff = mCurrScaleFactor - detector.scaleFactor
+            if (diff < ZOOM_IN_THRESHOLD && mCurrScaleFactor == 1.0f) {
+                mListener?.zoomIn()
+                mCurrScaleFactor = detector.scaleFactor
+            } else if (diff > ZOOM_OUT_THRESHOLD && mCurrScaleFactor == 1.0f) {
+                mListener?.zoomOut()
+                mCurrScaleFactor = detector.scaleFactor
+            }
+            return false
+        }
+    }
+
+    interface ZoomListener {
+        fun zoomOut()
+
+        fun zoomIn()
+    }
+}

app/src/main/res/layout/activity_media.xml

@@ -6,7 +6,7 @@
     android:layout_width="match_parent"
     android:layout_height="wrap_content">
 
-    <android.support.v7.widget.RecyclerView
+    <com.simplemobiletools.gallery.views.MyScalableRecyclerView
         android:id="@+id/media_grid"
         android:layout_width="match_parent"
         android:layout_height="match_parent"