Android Support Library 25.0.0 版本中，新增加了一个API –> BottomNavigationView – 底部导航视图。

先来看看这个控件的实现效果。

1.gif

2.gif

基本使用

使用起来也很简单
首先在xml中引入该控件

<android.support.design.widget.BottomNavigationView
        android:id="@+id/bottom_navi_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        app:itemBackground="@android:color/white"
        app:menu="@menu/menu_bottom_navi" />

该控件的基本属性有：

3.png

app:itemIconTint : 设置菜单图标着色
app:itemTextColor : 设置菜单文本颜色
app:menu : 设置菜单
app:itemBackground : 设置导航栏的背景色

@menu/menu_buttom_navi

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/menu_recent"
        android:icon="@drawable/ic_history_black_24dp"
        android:title="@string/menu_recents" />
    <item
        android:id="@+id/menu_favorites"
        android:icon="@drawable/ic_favorite_black_24dp"
        android:title="@string/menu_favorites" />
    <item
        android:id="@+id/menu_nearby"
        android:icon="@drawable/ic_place_black_24dp"
        android:title="@string/menu_nearby" />
    <item
        android:id="@+id/menu_navi"
        android:icon="@drawable/ic_navigation_black_24dp"
        android:title="@string/menu_navigation" />
</menu>

与定义普通menu布局一样。

接下来是Java代码

bottomNaviView = (BottomNavigationView) findViewById(R.id.bottom_navi_view);
        bottomNaviView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                switch (item.getItemId()) {
                    case R.id.menu_recent:
                        break;
                    case R.id.menu_favorites:
                        break;
                    case R.id.menu_nearby:
                        break;
                    case R.id.menu_navi:
                        break;
                }
                return true;
            }
        });

对BottomNavigationView设置选择监听器就可以做一些item切换事件了。

注意事项

• 底部导航栏默认高度是56dp
• 菜单只能是3-5个

源码分析

BottomNavigationView 有几个先关的重要类

• BottomNavigationView
• BottomNavigationMenu
• BottomNavigationMenuView
• BottomNavigationPresenter

它的设计有点类似于开发中的 MVP模式。

先来看 BottomNavigationView 的构造函数

public BottomNavigationView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        ThemeUtils.checkAppCompatTheme(context); //检测当前主题
        // Create the menu
        mMenu = new BottomNavigationMenu(context);
        mMenuView = new BottomNavigationMenuView(context);
        FrameLayout.LayoutParams params = new FrameLayout.LayoutParams(
                ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
        params.gravity = Gravity.CENTER;
        mMenuView.setLayoutParams(params);
        mPresenter.setBottomNavigationMenuView(mMenuView);
        mMenuView.setPresenter(mPresenter);
        mMenu.addMenuPresenter(mPresenter);
        // Custom attributes

        // ...省略若干代码

        if (a.hasValue(R.styleable.BottomNavigationView_menu)) {
            inflateMenu(a.getResourceId(R.styleable.BottomNavigationView_menu, 0)); // 加载menu
        }
        a.recycle();
        addView(mMenuView, params); //
        mMenu.setCallback(new MenuBuilder.Callback() { // 设置监听器
            @Override
            public boolean onMenuItemSelected(MenuBuilder menu, MenuItem item) {
                return mListener != null && mListener.onNavigationItemSelected(item);
            }
            @Override
            public void onMenuModeChange(MenuBuilder menu) {}
        });
    }

ThemeUtils.checkAppCompatTheme(context) 是用来检测当前主题的。代码很简单，就是判断是否有 colorPrimary 属性。

class ThemeUtils {
    private static final int[] APPCOMPAT_CHECK_ATTRS = {
            android.support.v7.appcompat.R.attr.colorPrimary
    };
    static void checkAppCompatTheme(Context context) {
        TypedArray a = context.obtainStyledAttributes(APPCOMPAT_CHECK_ATTRS);
        final boolean failed = !a.hasValue(0);
        if (a != null) {
            a.recycle();
        }
        if (failed) {
            throw new IllegalArgumentException("You need to use a Theme.AppCompat theme "
                    + "(or descendant) with the design library.");
        }
    }
}

构造函数中接下来调用了 inflateMenu()

public void inflateMenu(int resId) {
        mPresenter.setUpdateSuspended(true);
        getMenuInflater().inflate(resId, mMenu);
        mPresenter.initForMenu(getContext(), mMenu);
        mPresenter.setUpdateSuspended(false);
        mPresenter.updateMenuView(true);
    }

可以看出都是调用的BottomNavigationPresenter的函数。

setUpdateSuspended(true) – 暂停修改menu

setUpdateSuspended(false) — 可以修改menu

在initForMenu()一前一后，设置一个标志来表示当前正在操作menu。

重点来看 BottomNavigationPresenter.initForMenu()

@Override
    public void initForMenu(Context context, MenuBuilder menu) {
        mMenuView.initialize(mMenu);
        mMenu = menu;
    }

函数内部又是调用的 BottomNavigationMenuView.initialize()

@Override
    public void initialize(MenuBuilder menu) {
        mMenu = menu;
        if (mMenu == null) return;
        if (mMenu.size() > mActiveButton) {
            mMenu.getItem(mActiveButton).setChecked(true);
        }
    }

代码中就是一些简单的初始化操作。

接下来看 BottomNavigationPresenter.updateMenuView(true)

@Override
    public void updateMenuView(boolean cleared) {
        if (mUpdateSuspended) return;
        if (cleared) {
            mMenuView.buildMenuView();
        } else {
            mMenuView.updateMenuView();
        }
    }

第一次创建Menu时，调用的是buildMenuView方法。

BottomNavigationMenuView.buildMenuView()

public void buildMenuView() {
        if (mButtons != null) {
            for (BottomNavigationItemView item : mButtons) {
                sItemPool.release(item);
            }
        }
        removeAllViews();
        mButtons = new BottomNavigationItemView[mMenu.size()];

        // 当menu item大于3个的时候，会出现缩放动画
        mShiftingMode = mMenu.size() > 3;

        for (int i = 0; i < mMenu.size(); i++) {
            mPresenter.setUpdateSuspended(true);
            mMenu.getItem(i).setCheckable(true);
            mPresenter.setUpdateSuspended(false);

            BottomNavigationItemView child = getNewItem();

            mButtons[i] = child;
            child.setIconTintList(mItemIconTint);
            child.setTextColor(mItemTextColor);
            child.setItemBackground(mItemBackgroundRes);
            child.setShiftingMode(mShiftingMode);

            // 单个item -- BottomNavigationItenView 的初始化操作
            child.initialize((MenuItemImpl) mMenu.getItem(i), 0);
            child.setItemPosition(i);

            // 设置点击事件
            child.setOnClickListener(mOnClickListener);

            // 添加子视图
            addView(child);
        }
    }

在 BottomNavigationMenuView类中定义了一个Pool对象，用来缓存BottomNavigationItemView对象。

private static final Pools.Pool<BottomNavigationItemView> sItemPool = new Pools.SynchronizedPool<>(5);

通过for循环创建了nMenu.size() 个BottomNavigationItemView 对象。

BottomNavigationItemView child = getNewItem();
mButtons[i] = child;

getNewItem()

private BottomNavigationItemView getNewItem() {
        BottomNavigationItemView item = sItemPool.acquire();
        if (item == null) {
            item = new BottomNavigationItemView(getContext());
        }
        return item;
    }

getNewItem() 类似于 **Message.obtain() **的机制。

接着看buildMenuView() ，方法最后面，调用了 **BottomNavigationItemView.initialize() **，然后调用 **addView(child) ** 来添加子item视图。

@Override
    public void initialize(MenuItemImpl itemData, int menuType) {
        mItemData = itemData;
        setCheckable(itemData.isCheckable());
        setChecked(itemData.isChecked());
        setEnabled(itemData.isEnabled());
        setIcon(itemData.getIcon());
        setTitle(itemData.getTitle());
        setId(itemData.getItemId());
    }

看一下 BottomNavigationItemView的构造函数

public BottomNavigationItemView(@NonNull Context context) {
        this(context, null);
    }
    public BottomNavigationItemView(@NonNull Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }
    public BottomNavigationItemView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        final Resources res = getResources();
        int inactiveLabelSize =
                res.getDimensionPixelSize(R.dimen.design_bottom_navigation_text_size);
        int activeLabelSize = res.getDimensionPixelSize(
                R.dimen.design_bottom_navigation_active_text_size);
        mDefaultMargin = res.getDimensionPixelSize(R.dimen.design_bottom_navigation_margin);
        mShiftAmount = inactiveLabelSize - activeLabelSize;
        mScaleUpFactor = 1f * activeLabelSize / inactiveLabelSize;
        mScaleDownFactor = 1f * inactiveLabelSize / activeLabelSize;
        // 注意下面的代码
        LayoutInflater.from(context).inflate(R.layout.design_bottom_navigation_item, this, true);
        setBackgroundResource(R.drawable.design_bottom_navigation_item_background);
        mIcon = (ImageView) findViewById(R.id.icon);
        mSmallLabel = (TextView) findViewById(R.id.smallLabel);
        mLargeLabel = (TextView) findViewById(R.id.largeLabel);
    }

代码中映射了一个布局文件

design_bottom_navigation_item.xml

<merge xmlns:android="http://schemas.android.com/apk/res/android">
    <ImageView
        android:id="@+id/icon"
        android:layout_width="24dp"
        android:layout_height="24dp"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="@dimen/design_bottom_navigation_margin"
        android:layout_marginBottom="@dimen/design_bottom_navigation_margin"
        android:duplicateParentState="true" />
    <android.support.design.internal.BaselineLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="@dimen/design_bottom_navigation_margin"
        android:layout_gravity="bottom|center_horizontal"
        android:duplicateParentState="true">
        <TextView
            android:id="@+id/smallLabel"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="@dimen/design_bottom_navigation_text_size"
            android:duplicateParentState="true" />
        <TextView
            android:id="@+id/largeLabel"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:visibility="invisible"
            android:textSize="@dimen/design_bottom_navigation_active_text_size"
            android:duplicateParentState="true" />
    </android.support.design.internal.BaselineLayout>
</merge>

该布局文件由系统提供，包括一个ImageView和两个TextView。
当菜单项大于3个，切换item时，被选中的item会将largeLabel显示，将smallLabel隐藏。然后改变ImageView和TextView的margin值达到动画效果。
在BottomNavigationMenuView的构造函数中对mAnimationHelper进行了实例化

private final BottomNavigationAnimationHelperBase mAnimationHelper;

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
            mAnimationHelper = new BottomNavigationAnimationHelperIcs();
        } else {
            mAnimationHelper = new BottomNavigationAnimationHelperBase();
        }

当版本小于14（Android 4.0）时，是没有动画效果的。

class BottomNavigationAnimationHelperBase {
    void beginDelayedTransition(ViewGroup view) {
        // Do nothing.
    }
}

Android 4.0及之上的版本

class BottomNavigationAnimationHelperIcs extends BottomNavigationAnimationHelperBase {
    private static final long ACTIVE_ANIMATION_DURATION_MS = 115L;
    private final TransitionSet mSet;
    BottomNavigationAnimationHelperIcs() {
        mSet = new AutoTransition();
        mSet.setOrdering(TransitionSet.ORDERING_TOGETHER);
        mSet.setDuration(ACTIVE_ANIMATION_DURATION_MS);
        mSet.setInterpolator(new FastOutSlowInInterpolator());
        TextScale textScale = new TextScale();
        mSet.addTransition(textScale);
    }
    void beginDelayedTransition(ViewGroup view) {
        TransitionManager.beginDelayedTransition(view, mSet);
    }
}

代码中定义了文本缩放动画 – TextScale

然后来看 BottomNavigationItemView的点击事件。

在初始化这些子itemView的时候就设置了onClickListener

child.initialize((MenuItemImpl) mMenu.getItem(i), 0);
child.setItemPosition(i);
// 设置点击事件
child.setOnClickListener(mOnClickListener);

mOnClickListener = new OnClickListener() {
            @Override
            public void onClick(View v) {
                final BottomNavigationItemView itemView = (BottomNavigationItemView) v;
                final int itemPosition = itemView.getItemPosition();
                activateNewButton(itemPosition);
                mMenu.performItemAction(itemView.getItemData(), mPresenter, 0);
            }
        };

activateNewButtom()

private void activateNewButton(int newButton) {
        if (mActiveButton == newButton) return;
        mAnimationHelper.beginDelayedTransition(this); // 产生动画
        mPresenter.setUpdateSuspended(true);
        mButtons[mActiveButton].setChecked(false); // 旧的被激活的按钮切换成未被激活状态
        mButtons[newButton].setChecked(true); // 将新点击的按钮切换到被激活状态
        mPresenter.setUpdateSuspended(false);
        mActiveButton = newButton; // 记录当前被激活按钮的位置
    }

先产生动画，然后切换被选中和之前选中的item的状态。

@Override
    public void setChecked(boolean checked) {
        mItemData.setChecked(checked);
        ViewCompat.setPivotX(mLargeLabel, mLargeLabel.getWidth() / 2);
        ViewCompat.setPivotY(mLargeLabel, mLargeLabel.getBaseline());
        ViewCompat.setPivotX(mSmallLabel, mSmallLabel.getWidth() / 2);
        ViewCompat.setPivotY(mSmallLabel, mSmallLabel.getBaseline());
        if (mShiftingMode) {
            if (checked) {
                LayoutParams iconParams = (LayoutParams) mIcon.getLayoutParams();
                iconParams.gravity = Gravity.CENTER_HORIZONTAL | Gravity.TOP;
                iconParams.topMargin = mDefaultMargin;
                mIcon.setLayoutParams(iconParams);
                mLargeLabel.setVisibility(VISIBLE);
                ViewCompat.setScaleX(mLargeLabel, 1f);
                ViewCompat.setScaleY(mLargeLabel, 1f);
            } else {
                LayoutParams iconParams = (LayoutParams) mIcon.getLayoutParams();
                iconParams.gravity = Gravity.CENTER;
                iconParams.topMargin = mDefaultMargin;
                mIcon.setLayoutParams(iconParams);
                mLargeLabel.setVisibility(INVISIBLE);
                ViewCompat.setScaleX(mLargeLabel, 0.5f);
                ViewCompat.setScaleY(mLargeLabel, 0.5f);
            }
            mSmallLabel.setVisibility(INVISIBLE);
        } else {
            if (checked) {
                LayoutParams iconParams = (LayoutParams) mIcon.getLayoutParams();
                iconParams.gravity = Gravity.CENTER_HORIZONTAL | Gravity.TOP;
                iconParams.topMargin = mDefaultMargin + mShiftAmount;
                mIcon.setLayoutParams(iconParams);
                mLargeLabel.setVisibility(VISIBLE);
                mSmallLabel.setVisibility(INVISIBLE);
                ViewCompat.setScaleX(mLargeLabel, 1f);
                ViewCompat.setScaleY(mLargeLabel, 1f);
                ViewCompat.setScaleX(mSmallLabel, mScaleUpFactor);
                ViewCompat.setScaleY(mSmallLabel, mScaleUpFactor);
            } else {
                LayoutParams iconParams = (LayoutParams) mIcon.getLayoutParams();
                iconParams.gravity = Gravity.CENTER_HORIZONTAL | Gravity.TOP;
                iconParams.topMargin = mDefaultMargin;
                mIcon.setLayoutParams(iconParams);
                mLargeLabel.setVisibility(INVISIBLE);
                mSmallLabel.setVisibility(VISIBLE);
                ViewCompat.setScaleX(mLargeLabel, mScaleDownFactor);
                ViewCompat.setScaleY(mLargeLabel, mScaleDownFactor);
                ViewCompat.setScaleX(mSmallLabel, 1f);
                ViewCompat.setScaleY(mSmallLabel, 1f);
            }
        }
        refreshDrawableState();
    }

mShiftingMode 表示是否偏移值。当菜单个数大于3时，为true。