fix(android): support negative values for pageMargin (#301)

Supporting negative values let's us have pages peek from the previous and next pages.

Author: satya164

README.md

@@ -180,7 +180,7 @@ yarn lint --fix
 - `flex:1` does not work for child views, please use `width:”100%”, height:”100%” ` [instead](https://github.com/react-native-community/react-native-viewpager/issues/186#issuecomment-675320732)
 
 - [iOS]: In case of `UIViewControllerHierarchyInconsistency` error, please use below fix:
- 
+
 ```
 requestAnimationFrame(() => refViewPager.current?.setPage(index));
 ```

android/gradle.properties

@@ -1,7 +1,7 @@
 ReactNativeViewPager_compileSdkVersion=28
 ReactNativeViewPager_buildToolsVersion=28.0.3
 ReactNativeViewPager_targetSdkVersion=27
-ReactNativeViewPager_minSdkVersion=16
+ReactNativeViewPager_minSdkVersion=17
 
 android.enableJetifier=true
-android.useAndroidX=true
+android.useAndroidX=true

android/src/main/java/com/reactnativecommunity/viewpager/ReactViewPagerManager.java

@@ -5,15 +5,12 @@
 
 package com.reactnativecommunity.viewpager;
 
-import android.util.SparseArray;
 import android.view.View;
 
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 
 import androidx.fragment.app.FragmentActivity;
-import androidx.viewpager2.adapter.FragmentStateAdapter;
-import androidx.viewpager2.widget.MarginPageTransformer;
 import androidx.viewpager2.widget.ViewPager2;
 
 import com.facebook.infer.annotation.Assertions;
@@ -235,8 +232,25 @@ public void receiveCommand(@NonNull final ViewPager2 root, int commandId, @Nulla
 
     @ReactProp(name = "pageMargin", defaultFloat = 0)
     public void setPageMargin(ViewPager2 pager, float margin) {
-        int pageMargin = (int) PixelUtil.toPixelFromDIP(margin);
-        pager.setPageTransformer(new MarginPageTransformer(pageMargin));
+        final int pageMargin = (int) PixelUtil.toPixelFromDIP(margin);
+        final ViewPager2 vp = pager;
+
+        /**
+         * Don't use MarginPageTransformer to be able to support negative margins
+         */
+        pager.setPageTransformer(new ViewPager2.PageTransformer() {
+            @Override
+            public void transformPage(@NonNull View page, float position) {
+                float offset = pageMargin * position;
+
+                if (vp.getOrientation() == ViewPager2.ORIENTATION_HORIZONTAL) {
+                    boolean isRTL = vp.getLayoutDirection() == View.LAYOUT_DIRECTION_RTL;
+
+                    page.setTranslationX(isRTL ? -offset : offset);
+                } else {
+                    page.setTranslationY(offset);
+                }
+            }
+        });
     }
 }
-

example/android/build.gradle

@@ -3,7 +3,7 @@
 buildscript {
     ext {
         buildToolsVersion = "29.0.2"
-        minSdkVersion = 16
+        minSdkVersion = 17
         compileSdkVersion = 29
         targetSdkVersion = 29
     }

src/types.ts

@@ -84,6 +84,8 @@ export interface ViewPagerProps {
    * be recreated from the adapter when needed.
    * Defaults to RecyclerView's caching strategy.
    * The given value must either be larger than 0.
+   *
+   * Only supported on Android.
    */
   offscreenPageLimit?: number;