[Feature] Add checks requiring intrinsic size of track and thumb.

Bug: #30

README.md

@@ -71,7 +71,7 @@ For more customization, please use the methods on [`FastScrollerBuilder`](librar
 - `setViewHelper()` allows providing a custom `ViewHelper` to support more views.
 - `setPopupTextProvider()` allows providing a custom `PopupTextProvider` if your `RecyclerView.Adapter` cannot implement that interface.
 - `setPadding()` allows setting a custom padding for the scrollbar, instead of the padding of the view.
-- `setTrackDrawable()` and `setThumbDrawable()` allow setting custom drawables for the scrollbar. The `android:state_pressed` state will be updated for them so you can use a selector.
+- `setTrackDrawable()` and `setThumbDrawable()` allow setting custom drawables for the scrollbar. The `android:state_pressed` state will be updated for them so you can use a selector. The track drawable needs to have an intrinsic width and the thumb drawable needs to have an intrinsic size, in order to allow proper touch event handling.
 - `setPopupStyle()` allows customizing the popup view with a lambda that will receive the view.
 - `setAnimationHelper()` allows providing a custom `AnimationHelper` to use an alternative scrollbar animation.
 - `disableScrollbarAutoHide()` allows disabling the auto hide animation for scrollbar. This implies using a `DefaultAnimationHelper`.

library/src/main/java/me/zhanghai/android/fastscroll/FastScroller.java

@@ -93,9 +93,12 @@ public FastScroller(@NonNull ViewGroup view, @NonNull ViewHelper viewHelper,
         mUserPadding = padding;
         mAnimationHelper = animationHelper;
 
-        mTrackWidth = trackDrawable.getIntrinsicWidth();
-        mThumbWidth = thumbDrawable.getIntrinsicWidth();
-        mThumbHeight = thumbDrawable.getIntrinsicHeight();
+        mTrackWidth = requireNonNegative(trackDrawable.getIntrinsicWidth(),
+                "trackDrawable.getIntrinsicWidth() < 0");
+        mThumbWidth = requireNonNegative(thumbDrawable.getIntrinsicWidth(),
+                "thumbDrawable.getIntrinsicWidth() < 0");
+        mThumbHeight = requireNonNegative(thumbDrawable.getIntrinsicHeight(),
+                "thumbDrawable.getIntrinsicHeight() < 0");
 
         mTrackView = new View(context);
         mTrackView.setBackground(trackDrawable);
@@ -119,6 +122,13 @@ public FastScroller(@NonNull ViewGroup view, @NonNull ViewHelper viewHelper,
         mViewHelper.addOnTouchEventListener(this::onTouchEvent);
     }
 
+    private static int requireNonNegative(int value, @NonNull String message) {
+        if (value < 0) {
+            throw new IllegalArgumentException(message);
+        }
+        return value;
+    }
+
     public void setPadding(int left, int top, int right, int bottom) {
         if (mUserPadding != null && mUserPadding.left == left && mUserPadding.top == top
                 && mUserPadding.right == right && mUserPadding.bottom == bottom) {