Przykład kodu programu dla platformy Android, który wyświetla listę elementów. Jest tutaj zaimplentowane dwie ciekawe rzeczy: a) BetterArrayAdapter, który wymuszę stosowanie wzorca ViewHolder na klasach potomnych, co zwiększa wydajność oraz rozdziela dwie funkcje metody (create i bind) na dwie metody, co usprawnia kod b) wykorzystanie CheckableR…

activity_student_list.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context="com.laboratories.bregula.myapplication.StudentListActivity">

    <ListView
        android:id="@android:id/list"
        android:layout_width="fill_parent"
        android:layout_height="0dp"
        android:layout_weight="1.0" />

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:id="@+id/button_bar_normal"
        style="?android:buttonBarStyle">

        <Button
            android:id="@+id/button_save"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Add"
            android:layout_weight="1.0"
            style="?android:attr/borderlessButtonStyle"/>

        <Button
            android:id="@+id/button_remove"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1.0"
            android:text="Remove"
            style="?android:attr/borderlessButtonStyle"/>
    </LinearLayout>
    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:visibility="gone"
        android:id="@+id/button_bar_remove"
        style="?android:buttonBarStyle">

        <Button
            android:id="@+id/button_confirm_remove"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Remove"
            android:layout_weight="1.0"
            style="?android:attr/borderlessButtonStyle"/>

        <Button
            android:id="@+id/button_cancel"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1.0"
            android:text="Cancel"
            style="?android:attr/borderlessButtonStyle"/>
    </LinearLayout>
</LinearLayout>

BetterArrayAdapter.java

package com.laboratories.bregula.myapplication;

import android.content.Context;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;

import java.util.List;

public abstract class BetterArrayAdapter<T, VH extends BetterArrayAdapter.ViewHolder> extends ArrayAdapter<T> {

    abstract static class ViewHolder {
        View view;

        ViewHolder(View v) {
            this.view = v;
        }
    }

    public BetterArrayAdapter(@NonNull Context context, int resource) {
        super(context, resource);
    }

    public BetterArrayAdapter(@NonNull Context context, int resource, int textViewResourceId) {
        super(context, resource, textViewResourceId);
    }

    public BetterArrayAdapter(@NonNull Context context, int resource, @NonNull T[] objects) {
        super(context, resource, objects);
    }

    public BetterArrayAdapter(@NonNull Context context, int resource, int textViewResourceId, @NonNull T[] objects) {
        super(context, resource, textViewResourceId, objects);
    }

    public BetterArrayAdapter(@NonNull Context context, int resource, @NonNull List<T> objects) {
        super(context, resource, objects);
    }

    public BetterArrayAdapter(@NonNull Context context, int resource, int textViewResourceId, @NonNull List<T> objects) {
        super(context, resource, textViewResourceId, objects);
    }

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
        VH vh;
        View v;
        if (convertView == null) {
            int viewType = getItemViewType(position);
            v = createView(parent, viewType);
            vh = createViewHolder(v);
            v.setTag(vh);
        } else {
            v = convertView;
            vh = (VH) convertView.getTag();
        }

        T item = getItem(position);

        bindViewHolder(vh, position, item);

        return v;
    }

    protected abstract View createView(ViewGroup parent, int viewType);

    protected abstract void bindViewHolder(VH vh, int position, T item);

    protected abstract VH createViewHolder(View v);
}

CheckableRelativeLayout.java

package com.laboratories.bregula.myapplication;

import android.content.Context;
import android.util.AttributeSet;
import android.widget.CheckBox;
import android.widget.Checkable;
import android.widget.RelativeLayout;

/**
 * Created by andrzej on 06.11.17.
 */

public class CheckableRelativeLayout extends RelativeLayout implements Checkable {
    private CheckBox mCheckbox;

    public CheckableRelativeLayout(Context context) {
        super(context);
    }

    public CheckableRelativeLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public CheckableRelativeLayout(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        mCheckbox = findViewById(android.R.id.checkbox);
    }

    @Override
    public void setChecked(boolean checked) {
        mCheckbox.setChecked(checked);
    }

    @Override
    public boolean isChecked() {
        return mCheckbox.isChecked();
    }

    @Override
    public void toggle() {
        mCheckbox.toggle();
    }
}

simple_list_item_2_checked.xml

<?xml version="1.0" encoding="utf-8"?>
<com.laboratories.bregula.myapplication.CheckableRelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="?android:attr/listChoiceBackgroundIndicator"
    android:minHeight="?attr/listPreferredItemHeight"
    android:mode="twoLine"
    android:paddingEnd="?android:attr/listPreferredItemPaddingEnd"
    android:paddingLeft="?android:attr/listPreferredItemPaddingLeft"
    android:paddingRight="?android:attr/listPreferredItemPaddingRight"
    android:paddingStart="?android:attr/listPreferredItemPaddingStart">

    <TextView
        android:id="@android:id/text1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:layout_marginTop="8dp"
        android:layout_toLeftOf="@android:id/checkbox"
        android:layout_toStartOf="@android:id/checkbox"
        android:textAppearance="?attr/textAppearanceListItem" />

    <TextView
        android:id="@android:id/text2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@android:id/text1"
        android:layout_alignStart="@android:id/text1"
        android:layout_below="@android:id/text1"
        android:layout_toLeftOf="@android:id/checkbox"
        android:layout_toStartOf="@android:id/checkbox"
        android:textAppearance="?attr/textAppearanceListItemSecondary" />

    <CheckBox
        android:id="@android:id/checkbox"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentEnd="true"
        android:layout_alignParentRight="true"
        android:layout_marginTop="12dp"
        android:focusable="false"
        android:text="" />
</com.laboratories.bregula.myapplication.CheckableRelativeLayout>

StudentListActivity.java

package com.laboratories.bregula.myapplication;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.SparseBooleanArray;
import android.view.View;
import android.widget.ListView;

import java.util.ArrayList;
import java.util.Random;

public class StudentListActivity extends AppCompatActivity implements View.OnClickListener {
    public static final int VIEW_MODE_NORMAL = 1;
    public static final int VIEW_MODE_REMOVE = 2;
    
    private static final int REQUEST_CODE_ADD = 1;

    private ListView mListView;
    private Random random = new Random();
    private StudentListAdapter mAdapter;
    private View mBarNormal;
    private View mBarRemove;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_student_list);

        mListView = findViewById(android.R.id.list);
        ArrayList<Student> students = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            students.add(new Student("Kotek " + i, random.nextInt(50000) + ""));
        }
        mAdapter = new StudentListAdapter(this, students);
        mListView.setAdapter(mAdapter);

        findViewById(R.id.button_save).setOnClickListener(this);
        findViewById(R.id.button_remove).setOnClickListener(this);
        findViewById(R.id.button_confirm_remove).setOnClickListener(this);
        findViewById(R.id.button_cancel).setOnClickListener(this);

        mBarNormal = findViewById(R.id.button_bar_normal);
        mBarRemove = findViewById(R.id.button_bar_remove);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.button_save:
                startActivityForResult(new Intent(this, AddStudentActivity.class), REQUEST_CODE_ADD);
                break;
            case R.id.button_remove:
                setMode(VIEW_MODE_REMOVE);
                break;
            case R.id.button_confirm_remove:
                removeSelectedItems();
                setMode(VIEW_MODE_NORMAL);
                break;
            case R.id.button_cancel:
                setMode(VIEW_MODE_NORMAL);
                break;
        }
    }

    private void removeSelectedItems() {
        SparseBooleanArray ids = mListView.getCheckedItemPositions();
        if (ids != null && ids.size() != 0) {
            mAdapter.setNotifyOnChange(false);
            for (int i = ids.size(); i >= 0; i--) {
                if (ids.valueAt(i)) {
                    int position = ids.keyAt(i);
                    mAdapter.remove(mAdapter.getItem(position));
                }
            }
            mAdapter.setNotifyOnChange(true);
            mAdapter.notifyDataSetChanged();
        }
        mListView.clearChoices();
    }

    public void setMode(int viewMode) {
        if (viewMode == VIEW_MODE_NORMAL) {
            mBarNormal.setVisibility(View.VISIBLE);
            mBarRemove.setVisibility(View.GONE);
            mAdapter.setCheckboxVisible(false);
            mListView.setChoiceMode(ListView.CHOICE_MODE_NONE);
        } else if (viewMode == VIEW_MODE_REMOVE) {
            mBarNormal.setVisibility(View.GONE);
            mBarRemove.setVisibility(View.VISIBLE);
            mAdapter.setCheckboxVisible(true);
            mListView.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
        }
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == REQUEST_CODE_ADD) {
            if (resultCode == Activity.RESULT_OK) {
                String name = data.getStringExtra(AddStudentActivity.EXTRA_NAME);
                String phone = data.getStringExtra(AddStudentActivity.EXTRA_PHONE);
                mAdapter.add(new Student(name, phone));
            }
        }
    }
}