解决ListView列表setEmptyView无效问题及源码解析

ListView的setEmptyView方法是用来设置列表为空时显示的View，但是有时候我们在使用该方法时发现并没有生效，可能是以下几个原因：

1. 没有给ListView设置Adapter，因为只有当Adapter没有数据时才会显示EmptyView。

2. 在设置EmptyView之前已经给ListView设置了数据源，需要在设置EmptyView之前将数据源清空。

3. 在设置EmptyView之前需要先inflate出EmptyView的布局，并将其添加到ListView的父布局中。

以下是一个setEmptyView无效的例子：

ListView listView = findViewById(R.id.list_view);
View emptyView = findViewById(R.id.empty_view);
listView.setEmptyView(emptyView);
String[] data = {"item1", "item2", "item3"};
ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, data);
listView.setAdapter(adapter);

上述代码中，设置了EmptyView，但是并没有生效，因为在设置EmptyView之前已经给ListView设置了数据源，需要将数据源清空，即在setAdapter之前添加如下代码：

listView.setAdapter(null);

另外，还需要注意，EmptyView需要先inflate出来并添加到ListView的父布局中，例如：

View emptyView = LayoutInflater.from(this).inflate(R.layout.empty_view, (ViewGroup) listView.getParent(), false);

至于ListView的源码实现，可以参考以下几个方法：

1. setEmptyView方法：

public void setEmptyView(View emptyView) {
    mEmptyView = emptyView;

    final T adapter = mAdapter;
    final boolean empty = ((adapter == null) || adapter.isEmpty());
    updateEmptyStatus(empty);
}

private void updateEmptyStatus(boolean empty) {
    if (isInFilterMode()) {
        empty = false;
    }
    if (empty) {
        if (mEmptyView != null) {
            mEmptyView.setVisibility(View.VISIBLE);
            setVisibility(View.GONE);
        } else {
            // If the caller just removed our empty view, make sure the list view is visible
            setVisibility(View.VISIBLE);
        }
        // We are now GONE, so pending layouts will not be dispatched.
        // Force one here to ensure that the state of the list matches
        // the state of the adapter.
        if (mDataChanged) {
            this.onLayout(false, getLeft(), getTop(), getRight(), getBottom());
        }
    } else {
        if (mEmptyView != null) {
            mEmptyView.setVisibility(View.GONE);
        }
        setVisibility(View.VISIBLE);
    }
}

2. setAdapter方法：

public void setAdapter(ListAdapter adapter) {
    if (mAdapter != null) {
        mAdapter.unregisterDataSetObserver(mDataSetObserver);
        resetList();
    }

    mAdapter = adapter;

    if (mAdapter != null) {
        mOldSelectedPosition = INVALID_POSITION;
        mOldSelectedRowId = INVALID_ROW_ID;
        mItemCount = mAdapter.getCount();
        mDataChanged = true;

        // Detect the case where a cursor that was previously invalidated has
        // been repopulated with new data.
        mHasStableIds = mAdapter.hasStableIds();
        if (mChoiceMode != CHOICE_MODE_NONE && mAdapter.hasStableIds()) {
            mChoiceMode = CHOICE_MODE_SINGLE;
            if (mCheckStates == null) {
                mCheckStates = new SparseBooleanArray();
            }
            if (mCheckedIdStates == null) {
                mCheckedIdStates = new LongSparseArray<Boolean>();
            }
        }

        if (!mPopupHidden) {
            // Show the popup if we have one
            if (mAdapter.getCount() > 0) {
                showPopup();
            }
        }
    } else {
        mItemCount = 0;
        mDataChanged = false;
        mHasStableIds = false;
        mOldSelectedPosition = INVALID_POSITION;
        mOldSelectedRowId = INVALID_ROW_ID;
        mChoiceMode = CHOICE_MODE_NONE;
        mCheckStates.clear();
        mCheckedIdStates.clear();
        // Hide the currently visible popup
        dismissPopup();
    }

    if (mFilter != null) {
        mAdapter.getFilter().filter(mFilter);
    }

    requestLayout();
}