背景

自己写了一个自定义的ViewGroup的流式布局，但是调用addView方法添加xml中的View的时候报错：

The specified child already has a parent.You must call removeView() on the child's parent first."

探索

二话不说，点源码看一下，顺着addView的流程，最后走到了addViewInner()这个方法，看下代码

 private void addViewInner(View child, int index, LayoutParams params,
            boolean preventRequestLayout) {

        if (mTransition != null) {
            // Don't prevent other add transitions from completing, but cancel remove
            // transitions to let them complete the process before we add to the container
            mTransition.cancel(LayoutTransition.DISAPPEARING);
        }

        if (child.getParent() != null) { //这里抛出了我遇到的错误的异常
            throw new IllegalStateException("The specified child already has a parent. " +
                    "You must call removeView() on the child's parent first.");
        }

        if (mTransition != null) {
            mTransition.addChild(this, child);
        }

        if (!checkLayoutParams(params)) {
            params = generateLayoutParams(params);
        }

        if (preventRequestLayout) {
            child.mLayoutParams = params;
        } else {
            child.setLayoutParams(params);
        }

        if (index < 0) {
            index = mChildrenCount;
        }

        addInArray(child, index);

        // tell our children
        if (preventRequestLayout) {
            child.assignParent(this);
        } else {
            child.mParent = this;
        }

        if (child.hasFocus()) {
            requestChildFocus(child, child.findFocus());
        }

        AttachInfo ai = mAttachInfo;
        if (ai != null && (mGroupFlags & FLAG_PREVENT_DISPATCH_ATTACHED_TO_WINDOW) == 0) {
            boolean lastKeepOn = ai.mKeepScreenOn;
            ai.mKeepScreenOn = false;
            child.dispatchAttachedToWindow(mAttachInfo, (mViewFlags&VISIBILITY_MASK));
            if (ai.mKeepScreenOn) {
                needGlobalAttributesUpdate(true);
            }
            ai.mKeepScreenOn = lastKeepOn;
        }

        if (child.isLayoutDirectionInherited()) {
            child.resetRtlProperties();
        }

        dispatchViewAdded(child);

        if ((child.mViewFlags & DUPLICATE_PARENT_STATE) == DUPLICATE_PARENT_STATE) {
            mGroupFlags |= FLAG_NOTIFY_CHILDREN_ON_DRAWABLE_STATE_CHANGE;
        }

        if (child.hasTransientState()) {
            childHasTransientStateChanged(child, true);
        }

        if (child.getVisibility() != View.GONE) {
            notifySubtreeAccessibilityStateChangedIfNeeded();
        }

        if (mTransientIndices != null) {
            final int transientCount = mTransientIndices.size();
            for (int i = 0; i < transientCount; ++i) {
                final int oldIndex = mTransientIndices.get(i);
                if (index <= oldIndex) {
                    mTransientIndices.set(i, oldIndex + 1);
                }
            }
        }

        if (mCurrentDragStartEvent != null && child.getVisibility() == VISIBLE) {
            notifyChildOfDragStart(child);
        }
    }

从注释中可以看到代码哪里错了，瞬间明白，其实，在Android中，一个View只能属于一个特定的ViewGroup，我自定义了一个ViewGroup，然后又addView()加载xml中的一个view的时候，这个View的外层布局是一个LinearLayout。到了addInnerView的时候。进入if条件语句，抛出异常，崩溃。

    if (child.getParent() != null) { //这里抛出了我遇到的错误的异常
            throw new IllegalStateException("The specified child already has a parent. " +
                    "You must call removeView() on the child's parent first.");
        }

解决办法

把View的父ViewGroup解除关系，再在自己的ViewGroup添加就好。

     if (childView != null) {
            ViewGroup parentViewGroup = (ViewGroup) childView.getParent();
            if (parentViewGroup != null ) {
                 parentViewGroup.removeView(childView);
            }
        }

到这里，这个问题就搞定了。OK！收集小问题，一点一滴。