umano/AndroidSlidingUpPanel · error · java.lang.IllegalArgumentException

captureChildView: parameter must be a descendant of the…

Error message

captureChildView: parameter must be a descendant of the ViewDragHelper's tracked parent view (mParentView)

What it means

captureChildView() requires the child's direct parent to be the mParentView the helper was created with; otherwise it throws IllegalArgumentException. This ensures drag coordinate math is done in the tracked parent's coordinate space.

Solutions

1. Create the helper with the ViewGroup that directly contains the draggable children
2. In tryCaptureView, return true only for direct children of mParentView
3. Verify the captured view hasn't been re-parented before capture

Example fix

// before
mDragHelper = ViewDragHelper.create((ViewGroup) getParent(), 1f, cb); // wrong parent
// after
mDragHelper = ViewDragHelper.create(this, 1f, cb); // direct parent of dragged children

@Override
public boolean tryCaptureView(View child, int pointerId) {
    return child.getParent() == this; // only direct children of mParentView
}

boolean isDirectChild(View v, ViewGroup p) { return v.getParent() == p; }

try { mDragHelper.captureChildView(v, pointerId); } catch (IllegalArgumentException e) { Log.w(TAG, "view not child of tracked parent", e); }

When it happens

Trigger: tryCaptureViewForDrag (via shouldCaptureView/callback) handing a view whose getParent() != mParentView — e.g. the Callback.tryCaptureView returns true for a child of a different container, or the helper was constructed with the wrong parent (getParent() instead of the view itself).

Common situations: Custom views where the drag helper is created with getParent() but the callback captures a nested child; view re-parenting at runtime (RecyclerView recycling) between helper creation and capture.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Parent view may not be null
• Callback may not be null
• Cannot settleCapturedViewAt outside of a call to…
• Cannot flingCapturedView outside of a call to…
• gravity must be set to either top or bottom

AI-assisted analysis of umano/AndroidSlidingUpPanel@45a460435b (2026-09-11). Data as JSON: /api/errors/9d325e42cceeafba. Report an issue: GitHub.

Appendix: source

Thrown at library/src/main/java/com/sothree/slidinguppanel/ViewDragHelper.java:492

     *
     * @return The size of an edge in pixels
     * @see #setEdgeTrackingEnabled(int)
     */
    public int getEdgeSize() {
        return mEdgeSize;
    }

    /**
     * Capture a specific child view for dragging within the parent. The callback will be notified
     * but {@link Callback#tryCaptureView(android.view.View, int)} will not be asked permission to
     * capture this view.
     *
     * @param childView Child view to capture
     * @param activePointerId ID of the pointer that is dragging the captured child view
     */
    public void captureChildView(View childView, int activePointerId) {
        if (childView.getParent() != mParentView) {
            throw new IllegalArgumentException("captureChildView: parameter must be a descendant " +
                    "of the ViewDragHelper's tracked parent view (" + mParentView + ")");
        }

        mCapturedView = childView;
        mActivePointerId = activePointerId;
        mCallback.onViewCaptured(childView, activePointerId);
        setDragState(STATE_DRAGGING);
    }

    /**
     * @return The currently captured view, or null if no view has been captured.
     */
    public View getCapturedView() {
        return mCapturedView;
    }

    /**
     * @return The ID of the pointer currently dragging the captured view,

View on GitHub (pinned to 45a460435b)