android - 如何理解BaseAdapter.getVeiw()参数convertView的null与非null

Question

public View getView(int position, View convertView, ViewGroup parent) {
    ImageView imageview;
    if (convertView==null){
        imageview=new ImageView(MyActivity.this);
        imageview.setScaleType(ImageView.ScaleType.CENTER_INSIDE);
        imageview.setPadding(5,0,5,0);

    }else{
        imageview=(ImageView)convertView;
    }

    imageview.setImageResource(imageId[position]);
    return imageview;
}

当一张图片滑进屏幕的时候，调用这个getview()？那么其中第一个if条件null是什么？如何理解这里面的条件语句？

Answer 1

To put it simply, it is for reuse and avoids generating a new view from the layout resource file (or generating a new view through code) every time .
For example, like ListView or GridView, if N items can be displayed on the screen, then getView will be called N times to provide a view of the corresponding position. Reusing previously generated views can improve efficiency.

Parameter description of the getView() method in the android.widget.Adapter source code file:

@convertView
The old view to reuse, if possible. Note: You should check that this view is non-null and of an appropriate type before using.

But you still need to check if it is empty and of the appropriate type.

Answer 2

This is to avoid creating duplicate objects. It is for better reuse

Answer 3

getView creates a view object through the adapter to fill in the listview, etc. to determine whether the convertview is cached. If it is cached, there is no need to create a new view and the purpose of reuse can be achieved. Nowadays, the getview method is generally not written like this. There is a more efficient way to write it. It has been too long since I wrote about Android