android - 为什么 notifyItemChanged 只能在 setAdapter 之后的 Post Runnable 中起作用？

Question

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    RecyclerView list = (RecyclerView) findViewById(R.id.list);
    list.setLayoutManager(new LinearLayoutManager(this));
    final RecyclerView.Adapter adapter = getAdapter();
    list.setAdapter(adapter);

    adapter.notifyItemChanged(1, new Object());//this doesn't work
    list.post(new Runnable() {
        @Override
        public void run() {
            adapter.notifyItemChanged(1, new Object());//this works
        }
    });


}

@NonNull
private RecyclerView.Adapter getAdapter() {
    return new RecyclerView.Adapter() {
        @Override
        public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            return new ItemViewHolder( LayoutInflater.from(parent.getContext())
                    .inflate(R.layout.list_view_item, parent, false));
        }

        @Override
        public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
            ItemViewHolder item = (ItemViewHolder) holder;
            item.tv.setText("test");
        }

        @Override
        public void onBindViewHolder(RecyclerView.ViewHolder holder, int position, List payloads) {
            if(payloads.isEmpty())
                onBindViewHolder(holder, position);
            else{
                ItemViewHolder item = (ItemViewHolder) holder;
                item.tv.setText("changed!!!!!! ");
            }
        }

        @Override
        public int getItemCount() {
            return 40;
        }

        class ItemViewHolder extends RecyclerView.ViewHolder{
            TextView tv;
            ItemViewHolder(View itemView) {
                super(itemView);
                tv = (TextView) itemView.findViewById(R.id.tv);
            }
        }
    };
}}

这里很简单的例子只是为了测试notifyItemChanged，它只在发送到消息队列时有效，而不是在setAdapter之后直接调用。setAdapter 触发对 requeSTLayout() 的调用，这是否意味着 notifyItemChanged 不会在项目布局过程中发生？

Answer 1

经过一番调查后发现 notifyItemChanged仅在 RecyclerView 时有效附上，实际上已经完成onLayout发生在 onCreate 之后.

内部 RecyclerView.onLayout()由 notifyItemChanged -> requestLayout() 调用, processAdapterUpdatesAndSetAnimationFlags()被称为检查要更新的项目是否有可用的 ViewHolder在这种情况下是 null因为在 onCreate() 期间, RecyclerView没有贴在 window 上，因此没有对 RecyclerView 进行测量和布局。

显然是 RecyclerView 的完整绘图在 onCreate() 之后的某个时间发生和 OnResume()

@Override
protected void onResume() {
    super.onResume();
    boolean a = list.isAttachedToWindow();//this is false!
}

所以要notifyItemChanged致力于 onCreate

list.addOnLayoutChangeListener(new View.OnLayoutChangeListener() {
        @Override
        public void onLayoutChange(View v, int left, int top, int right, int bottom, int oldLeft, int oldTop, int oldRight, int oldBottom) {
            adapter.notifyItemChanged(1, new Object());//this will update the item
        }
    });



new Thread((new Runnable() {
        @Override
        public void run() {
            try {
                Thread.sleep(28);//small amount of delay,below 20 doesn't seem to work
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    adapter.notifyItemChanged(1, new Object());//also works
                }
            });
        }
    })).start();

我猜这是因为 ViewRootImpl.performTraversal()由系统控制，它发生在 DecorView 之后的某个时间附加到 onCreate 期间的窗口

我假设这只会在 onCreate 期间发生和 onResume ，如果稍后调用它可能不会发生。