Android Support V4官方Demo概览（二）

2016-03-17

本文介绍了Support V4包中TransportMediator、ViewPager等组件的使用。

Fragment

框架Fragment的静态支持类库版本。用于兼容Android 3.0之前的版本。当运行在Android 3.0及之后的版本，还会使用这个实现；它不会切换到框架的实现。查看框架Fragment文档。

当使用这个支持版本而不是框架的版本最大的不同是：

activity必须继承FragmentActivity
必须调用getSupportFragmentManager()获取FragmentManager

DrawableCompat

API4引入的用向后兼容的方式帮助访问Drawable的特性。

CursorLoader

框架CursorLoader的静态支持类库版本。用于兼容Android 3.0之前的版本。当运行在Android 3.0及之后的版本，还会使用这个实现；它不会切换到框架的实现。类概览请查看框架SDK文档。

TransportController之TransportMediator

帮助实现媒体传输控制（播放，暂停，跳过和其它功能）。控制关键事件和高级功能就像RemoteControlClient。这个类在传输控制（在屏幕上控制，实体按键，远程控制）和实际的播放器之间作为中介服务。播放器通过一个单独的TransportPerformer表现，必须提供这个类。在屏幕想要控制和显示播放器的状态应该调用TransportController接口。

这有一个简单但很完整的视频播放器的例子。注意这里用到的MediaController类没有包含在标准的Android框架中，而是一个自定义的实现。真正的应用通常实现自己的传输控制，或者你可以复制Support4Demos中的实现。

import android.support.v4.media.TransportMediator;
import android.support.v4.media.TransportPerformer;
import com.example.android.supportv4.R;

import android.app.ActionBar;
import android.content.Context;
import android.media.MediaPlayer;
import android.os.Handler;
import android.util.AttributeSet;
import android.view.KeyEvent;
import android.view.View;

import android.app.Activity;
import android.net.Uri;
import android.os.Bundle;
import android.widget.VideoView;

public class TransportControllerActivity extends Activity {

    /**
     * TODO: Set the path variable to a streaming video URL or a local media
     * file path.
     */
    private Content mContent;
    private TransportMediator mTransportMediator;
    private MediaController mMediaController;

    /**
     * Handle actions from on-screen media controls.  Most of these are simple re-directs
     * to the VideoView; some we need to capture to update our state.
     */
    TransportPerformer mTransportPerformer = new TransportPerformer() {
        @Override public void onStart() {
            mContent.start();
        }

        @Override public void onStop() {
            mContent.pause();
        }

        @Override public void onPause() {
            mContent.pause();
        }

        @Override public long onGetDuration() {
            return mContent.getDuration();
        }

        @Override public long onGetCurrentPosition() {
            return mContent.getCurrentPosition();
        }

        @Override public void onSeekTo(long pos) {
            mContent.seekTo((int)pos);
        }

        @Override public boolean onIsPlaying() {
            return mContent.isPlaying();
        }

        @Override public int onGetBufferPercentage() {
            return mContent.getBufferPercentage();
        }

        @Override public int onGetTransportControlFlags() {
            int flags = TransportMediator.FLAG_KEY_MEDIA_PLAY
                    | TransportMediator.FLAG_KEY_MEDIA_PLAY_PAUSE
                    | TransportMediator.FLAG_KEY_MEDIA_STOP;
            if (mContent.canPause()) {
                flags |= TransportMediator.FLAG_KEY_MEDIA_PAUSE;
            }
            if (mContent.canSeekBackward()) {
                flags |= TransportMediator.FLAG_KEY_MEDIA_REWIND;
            }
            if (mContent.canSeekForward()) {
                flags |= TransportMediator.FLAG_KEY_MEDIA_FAST_FORWARD;
            }
            return flags;
        }
    };

    /**
     * This is the actual video player.  It is the top-level content of
     * the activity's view hierarchy, going under the status bar and nav
     * bar areas.
     */
    public static class Content extends VideoView implements
            View.OnSystemUiVisibilityChangeListener, View.OnClickListener,
            ActionBar.OnMenuVisibilityListener, MediaPlayer.OnPreparedListener,
            MediaPlayer.OnCompletionListener, MediaPlayer.OnErrorListener {
        Activity mActivity;
        TransportMediator mTransportMediator;
        MediaController mMediaController;
        boolean mAddedMenuListener;
        boolean mMenusOpen;
        boolean mPaused;
        boolean mNavVisible;
        int mLastSystemUiVis;

        Runnable mNavHider = new Runnable() {
            @Override public void run() {
                setNavVisibility(false);
            }
        };

        Runnable mProgressUpdater = new Runnable() {
            @Override public void run() {
                mMediaController.updateProgress();
                getHandler().postDelayed(this, 1000);
            }
        };

        public Content(Context context, AttributeSet attrs) {
            super(context, attrs);
            setOnSystemUiVisibilityChangeListener(this);
            setOnClickListener(this);
            setOnPreparedListener(this);
            setOnCompletionListener(this);
            setOnErrorListener(this);
        }

        public void init(Activity activity, TransportMediator transportMediator,
                MediaController mediaController) {
            // This called by the containing activity to supply the surrounding
            // state of the video player that it will interact with.
            mActivity = activity;
            mTransportMediator = transportMediator;
            mMediaController = mediaController;
            pause();
        }

        @Override protected void onAttachedToWindow() {
            super.onAttachedToWindow();
            if (mActivity != null) {
                mAddedMenuListener = true;
                mActivity.getActionBar().addOnMenuVisibilityListener(this);
            }
        }

        @Override protected void onDetachedFromWindow() {
            super.onDetachedFromWindow();
            if (mAddedMenuListener) {
                mActivity.getActionBar().removeOnMenuVisibilityListener(this);
            }
            mNavVisible = false;
        }

        @Override public void onSystemUiVisibilityChange(int visibility) {
            // Detect when we go out of nav-hidden mode, to clear our state
            // back to having the full UI chrome up.  Only do this when
            // the state is changing and nav is no longer hidden.
            int diff = mLastSystemUiVis ^ visibility;
            mLastSystemUiVis = visibility;
            if ((diff&SYSTEM_UI_FLAG_HIDE_NAVIGATION) != 0
                    && (visibility&SYSTEM_UI_FLAG_HIDE_NAVIGATION) == 0) {
                setNavVisibility(true);
            }
        }

        @Override protected void onWindowVisibilityChanged(int visibility) {
            super.onWindowVisibilityChanged(visibility);

            // When we become visible or invisible, play is paused.
            pause();
        }

        @Override public void onClick(View v) {
            // Clicking anywhere makes the navigation visible.
            setNavVisibility(true);
        }

        @Override public void onMenuVisibilityChanged(boolean isVisible) {
            mMenusOpen = isVisible;
            setNavVisibility(true);
        }

        @Override
        public void onPrepared(MediaPlayer mp) {
            mMediaController.setEnabled(true);
        }

        @Override
        public void onCompletion(MediaPlayer mp) {
            mTransportMediator.pausePlaying();
            pause();
        }

        @Override
        public boolean onError(MediaPlayer mp, int what, int extra) {
            mTransportMediator.pausePlaying();
            pause();
            return false;
        }

        @Override public void start() {
            super.start();
            mPaused = false;
            setKeepScreenOn(true);
            setNavVisibility(true);
            mMediaController.refresh();
            scheduleProgressUpdater();
        }

        @Override public void pause() {
            super.pause();
            mPaused = true;
            setKeepScreenOn(false);
            setNavVisibility(true);
            mMediaController.refresh();
            scheduleProgressUpdater();
        }

        void scheduleProgressUpdater() {
            Handler h = getHandler();
            if (h != null) {
                if (mNavVisible && !mPaused) {
                    h.removeCallbacks(mProgressUpdater);
                    h.post(mProgressUpdater);
                } else {
                    h.removeCallbacks(mProgressUpdater);
                }
            }
        }

        void setNavVisibility(boolean visible) {
            int newVis = SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                    | SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
                    | SYSTEM_UI_FLAG_LAYOUT_STABLE;
            if (!visible) {
                newVis |= SYSTEM_UI_FLAG_LOW_PROFILE | SYSTEM_UI_FLAG_FULLSCREEN
                        | SYSTEM_UI_FLAG_HIDE_NAVIGATION;
            }

            // If we are now visible, schedule a timer for us to go invisible.
            if (visible) {
                Handler h = getHandler();
                if (h != null) {
                    h.removeCallbacks(mNavHider);
                    if (!mMenusOpen && !mPaused) {
                        // If the menus are open or play is paused, we will not auto-hide.
                        h.postDelayed(mNavHider, 3000);
                    }
                }
            }

            // Set the new desired visibility.
            setSystemUiVisibility(newVis);
            mNavVisible = visible;
            mMediaController.setVisibility(visible ? VISIBLE : INVISIBLE);
            scheduleProgressUpdater();
        }
    }

    @Override
    public void onCreate(Bundle icicle) {
        super.onCreate(icicle);
        setContentView(R.layout.videoview);

        // Find the video player in our UI.
        mContent = (Content) findViewById(R.id.content);

        // Create transport controller to control video, giving the callback
        // interface to receive actions from.
        mTransportMediator = new TransportMediator(this, mTransportPerformer);

        // Create and initialize the media control UI.
        mMediaController = (MediaController) findViewById(R.id.media_controller);
        mMediaController.setMediaPlayer(mTransportMediator);

        // We're just playing a built-in demo video.
        mContent.init(this, mTransportMediator, mMediaController);
        mContent.setVideoURI(Uri.parse("android.resource://" + getPackageName() +
                "/" + R.raw.videoviewdemo));
    }

    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        // We first dispatch keys to the transport controller -- we want it
        // to get to consume any media keys rather than letting whoever has focus
        // in the view hierarchy to potentially eat it.
        if (mTransportMediator.dispatchKeyEvent(event)) {
            return true;
        }

        return super.dispatchKeyEvent(event);
    }
}

BidiFormatter

用于可能反向显示格式化文本的实用类。

ViewPager

布局管理器允许用户左右翻动查看页面数据。提供PagerAdapter的实现生成显示view的页面。

注意这个类目前为早期的设计和开发。API很可能在后面的兼容库更新中修改，当使用新版本编译时需要修改应用的源码。

ViewPager经常和Fragment配合使用，这是一种很方便的方法对于提供和管理每个页面的生命周期。这有一个标准的使用fragment和ViewPager的adapter的实现，覆盖了最常见的情况。这有FragmentPagerAdapter 和 FragmentStatePagerAdapter两个类；每个类都有怎样使用它们构建用户界面的示例代码。

更多关于怎样使用ViewPager的信息，阅读 Creating Swipe Views with Tabs。

下面是一个ViewPager的更复杂的例子，使用它和ActionBar进行关联。你可以在API 4+ Support Demos 和 API 13+ Support Demos的示例代码中找到其它使用ViewPager的例子。

public class ActionBarTabsPager extends Activity {
    ViewPager mViewPager;
    TabsAdapter mTabsAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        mViewPager = new ViewPager(this);
        mViewPager.setId(R.id.pager);
        setContentView(mViewPager);

        final ActionBar bar = getActionBar();
        bar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
        bar.setDisplayOptions(0, ActionBar.DISPLAY_SHOW_TITLE);

        mTabsAdapter = new TabsAdapter(this, mViewPager);
        mTabsAdapter.addTab(bar.newTab().setText("Simple"),
                CountingFragment.class, null);
        mTabsAdapter.addTab(bar.newTab().setText("List"),
                FragmentPagerSupport.ArrayListFragment.class, null);
        mTabsAdapter.addTab(bar.newTab().setText("Cursor"),
                CursorFragment.class, null);

        if (savedInstanceState != null) {
            bar.setSelectedNavigationItem(savedInstanceState.getInt("tab", 0));
        }
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt("tab", getActionBar().getSelectedNavigationIndex());
    }

    /**
     * This is a helper class that implements the management of tabs and all
     * details of connecting a ViewPager with associated TabHost.  It relies on a
     * trick.  Normally a tab host has a simple API for supplying a View or
     * Intent that each tab will show.  This is not sufficient for switching
     * between pages.  So instead we make the content part of the tab host
     * 0dp high (it is not shown) and the TabsAdapter supplies its own dummy
     * view to show as the tab content.  It listens to changes in tabs, and takes
     * care of switch to the correct paged in the ViewPager whenever the selected
     * tab changes.
     */
    public static class TabsAdapter extends FragmentPagerAdapter
            implements ActionBar.TabListener, ViewPager.OnPageChangeListener {
        private final Context mContext;
        private final ActionBar mActionBar;
        private final ViewPager mViewPager;
        private final ArrayList<TabInfo> mTabs = new ArrayList<TabInfo>();

        static final class TabInfo {
            private final Class<?> clss;
            private final Bundle args;

            TabInfo(Class<?> _class, Bundle _args) {
                clss = _class;
                args = _args;
            }
        }

        public TabsAdapter(Activity activity, ViewPager pager) {
            super(activity.getFragmentManager());
            mContext = activity;
            mActionBar = activity.getActionBar();
            mViewPager = pager;
            mViewPager.setAdapter(this);
            mViewPager.setOnPageChangeListener(this);
        }

        public void addTab(ActionBar.Tab tab, Class<?> clss, Bundle args) {
            TabInfo info = new TabInfo(clss, args);
            tab.setTag(info);
            tab.setTabListener(this);
            mTabs.add(info);
            mActionBar.addTab(tab);
            notifyDataSetChanged();
        }

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

        @Override
        public Fragment getItem(int position) {
            TabInfo info = mTabs.get(position);
            return Fragment.instantiate(mContext, info.clss.getName(), info.args);
        }

        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
        }

        @Override
        public void onPageSelected(int position) {
            mActionBar.setSelectedNavigationItem(position);
        }

        @Override
        public void onPageScrollStateChanged(int state) {
        }

        @Override
        public void onTabSelected(Tab tab, FragmentTransaction ft) {
            Object tag = tab.getTag();
            for (int i=0; i<mTabs.size(); i++) {
                if (mTabs.get(i) == tag) {
                    mViewPager.setCurrentItem(i);
                }
            }
        }

        @Override
        public void onTabUnselected(Tab tab, FragmentTransaction ft) {
        }

        @Override
        public void onTabReselected(Tab tab, FragmentTransaction ft) {
        }
    }
}

从Android SDK目录可找到源码，或从这里下载