Android基礎--ListView（一）

ListView最主要的功能是展示數據

listview的簡單使用

ListView 與數據直接需要一個adapter來適配關係圖如下：

有來adapter就可以減少listview與數據直接的耦合度

ListView中的setAdapter方法中需要設置的是ListAdapter
public void setAdapter(ListAdapter adapter)
ListAdapter是一個接口，需要實現的方法太多。所以Android有給我們封裝了一層BaseAdapter，BaseAdapter繼承了ListAdapter接口。

實現BaseAdapter
BaseAdapter是個抽象類，實現BaseAdapter需要實現裏面的四個方法：

	//獲取數據源的總量
	public abstract int getCount()
	//獲取指定位置的列表項數據
    public abstract Object getItem(int position)   
	//獲取指定位置列表項數據的ID
    public abstract long getItemId(int position)
	//獲取指定位置的列表項視圖
    public abstract View getView(int position, View convertView, ViewGroup parent)

BaseAdapter的基本實現如下：

public class MyBaseAdapter extends BaseAdapter {

    private List<TextMessage> messageList;

    public MyBaseAdapter(List<TextMessage> messageList) {
        this.messageList = messageList;
    }

    @Override
    public int getCount() {
        return messageList.size();
    }

    @Override
    public Object getItem(int position) {
        return messageList.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

//    @Override
//    public boolean hasStableIds() {
////        return super.hasStableIds(); //默認返回false
//        return true; //返回true的話效率更高 需要getItemId的時候 返回數據本身獨有的ID messageList.get(position).getId()
//    }

    @Override
    public View getView(int position, View view, ViewGroup viewGroup) {
        LayoutInflater layoutInflater = LayoutInflater.from(viewGroup.getContext());
//        View messageView = layoutInflater.inflate(R.layout.text_message_item,null);//會導致listview的item的高度是自適應，不能顯示設置的高度
        View messageView = layoutInflater.inflate(R.layout.text_message_item,viewGroup,false);//下面這個方法即可顯示我們設置的高度
        TextView textView = messageView.findViewById(R.id.message_content);
        TextMessage message = messageList.get(position);
        textView.setText(message.getText());
        return messageView;
    }

}

TextMessage的代碼如下：

public class TextMessage {

    private int id;
    private String text;

    public TextMessage(int id, String text) {
        this.id = id;
        this.text = text;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }
}

ListView中設置BaseAdapter

		ListView listView = findViewById(R.id.list_view);
        List<TextMessage> messageList = new ArrayList<>();
        for (int i = 0; i < 100; i++){
            messageList.add(new TextMessage(i,"我是文本消息"+i));
        }
        MyBaseAdapter adapter = new MyBaseAdapter(messageList);
        listView.setAdapter(adapter);

ListView中實現多種樣式佈局

需要實現以下兩個方法

1）重寫 getViewTypeCount() – 該方法返回多少個不同的佈局

2）重寫 getItemViewType(int) – 根據position返回相應的Item

其他用法與上面一樣代碼如下：

public class MyBaseAdapter extends BaseAdapter {

    private static final int ViewTypeCount = 2;
    private interface ViewType {
        int TEXT = 0;
        int IMAGE = 1;
    }


    private List<Object> messageList;

    public MyBaseAdapter(List<Object> messageList) {
        this.messageList = messageList;
    }

    @Override
    public int getCount() {
        return messageList.size();
    }

    @Override
    public Object getItem(int position) {
        return messageList.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View view, ViewGroup viewGroup) {
        LayoutInflater inflater = LayoutInflater.from(viewGroup.getContext());
        if (getItemViewType(position) == ViewType.TEXT){
            View convertView = inflater.inflate(R.layout.text_message_item,viewGroup,false);
            TextView textView = convertView.findViewById(R.id.message_content);
            TextMessage textMessage = (TextMessage)messageList.get(position);
            textView.setText(textMessage.getText());
            return convertView;
        }else {
            View convertView = inflater.inflate(R.layout.image_message_item,viewGroup,false);
            ImageView imageView = convertView.findViewById(R.id.image_head);
            ImageMessage imageMessage = (ImageMessage)messageList.get(position);
            imageView.setImageResource(imageMessage.getImage());
            return convertView;
        }
    }

    @Override
    public int getItemViewType(int position) {
        if (getItem(position) instanceof TextMessage){
            return ViewType.TEXT;
        }else {
            return ViewType.IMAGE;
        }
    }

    @Override
    public int getViewTypeCount() {
        return ViewTypeCount;
    }
}