java - 想要在 viewpager 页面上启动动画，但页面正在预加载

Question

在我的项目中，我有几个屏幕( fragment )，我想为 fragment 元素( View )提供动画。我使用过 view pager 和 viewpager adapter 和 fragments。我面临的主要问题-

1. 开始时加载 2 个页面，就像开始时加载第 1 页和第 2 页一样。如果滑动到第二页，则它会自动加载第 3 页。
2. 我需要为使用 Universal tween engine 的 View (RelativeLayout) 显示动画.

代码有问题吗？

这是 MainActivity.java 代码-

    public class MainActivity extends FragmentActivity {    
    ViewPager viewPager;
    PagerAdapter adapter;
    CirclePageIndicator mIndicator;
    private int mWidthScreen;
    private int mHeightScreen;
    private Bundle bundle;
    private List<Fragment> frgScreens;
    private int selectedtheme;
    private Handler mHandler = new Handler();


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.viewpager_main); // Get the view from viewpager_main.xml        
        buildargument();        
        initialisePaging(); // Page creating function       
    }

    private void initialisePaging(int theme) {
        viewPager = (ViewPager) findViewById(R.id.pager);       
        frgScreens = new Vector<Fragment>(); // clearing old entries        
        viewPager.setOffscreenPageLimit(0);
viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

                //Loading fragment # position

                //Instead of starting animation from onCreate start it from here 
                //Only for the first time (for page 1) this callback will not trigger
            }

            @Override
            public void onPageSelected(int position) {
            }

            @Override
            public void onPageScrollStateChanged(int state) {
            }
        });

        frgScreens = this.basicThemes();
        // Bind result to ViewPagerAdapter
        adapter = new ViewPagerAdapter(this.getSupportFragmentManager(), frgScreens);
        this.viewPager.setAdapter(adapter);
    }

    /*
     * Basic fragments
     */
    private List<Fragment> basicThemes() {
        // declaring fragments Group Basic
        Fragment frgBasic1 = Fragment.instantiate(this, Basic1.class.getName());
        Fragment frgBasic2 = Fragment.instantiate(this, Basic2.class.getName());
        Fragment frgBasic3 = Fragment.instantiate(this, Basic3.class.getName());
        Fragment frgBasic4 = Fragment.instantiate(this, Basic4.class.getName()); 
        Fragment frgBasic5 = Fragment.instantiate(this, Basic5.class.getName()); 

        // Passing arguments
        frgBasic1.setArguments(bundle);
        frgBasic2.setArguments(bundle);
        frgBasic3.setArguments(bundle);
        frgBasic4.setArguments(bundle); 
        frgBasic5.setArguments(bundle);



        // Loading screens into Fragment list
        List<Fragment> frgBasicScreens = new Vector<Fragment>();

        frgBasicScreens.add(frgBasic1);
        frgBasicScreens.add(frgBasic2);
        frgBasicScreens.add(frgBasic3);
        frgBasicScreens.add(frgBasic4); 
        frgBasicScreens.add(frgBasic5);

    return frgBasicScreens;

}

private void getScreenHeightWidhth() {
    DisplayMetrics localDisplayMetrics = new DisplayMetrics();
    getWindowManager().getDefaultDisplay().getMetrics(localDisplayMetrics);
    this.mWidthScreen = localDisplayMetrics.widthPixels;
    this.mHeightScreen = localDisplayMetrics.heightPixels;
    mHandler.post(
            new DisplayToast(this, "Width Screen:" + mWidthScreen + ". Height Screen: " + mHeightScreen + "."));
}

private void buildargument() {
    this.bundle = new Bundle();
    bundle.putInt("mWidthScreen", this.mWidthScreen);
}   

@Override
protected void onResume() {
    super.onResume();
}

@Override
public void onDestroy() {
    super.onDestroy();
}}

现在ViewPagerAdapter.java

public class ViewPagerAdapter extends FragmentPagerAdapter {    
private List<Fragment> screens; 

public ViewPagerAdapter(FragmentManager fm, List<Fragment> fragments ) {
    super(fm);      
    this.screens = fragments;       
}

@Override
public Fragment getItem(int pos) {
    // getting the position of the item
    return this.screens.get(pos);       
}

@Override
public int getCount() {
    // Getting the size of screen passed
    return this.screens.size();
}}

skinbase.java 是所有基础屏幕一些共享方法使用的通用类。

public abstract class SkinsBase extends Fragment
{
 protected int mWidthScreen;
 protected int mHeightScreen;
 public int skinHeight;
 public int skinWidth;
 public final int textShadowSize = 2;
 .................
 }

现在 basic1.java

public class Basic1 extends SkinsBase { 
private RelativeLayout mBlueBackground;
private Handler mHandler = new Handler();
private static final String TAG = "Basic1";

public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    if (container == null) {
        return null;
    }       
    mContext = container.getContext();
    View v = inflater.inflate(R.layout.blank_viewpager_item, container, false);
    mLayoutForeground = (RelativeLayout) v.findViewById(R.id.relative);

    mBlueBackground = initshape(getResources().getColor(R.color.transparent_blue_80),this.mWidthScreen, (int) (0.10F * this.mWidthScreen), 0,(int) (0.12F *this.mWidthScreen));     
    this.mLayoutForeground.addView(this.mBlueBackground);   

    Log.e(TAG, "Inside basic 1 ");
    return v;
}



public RelativeLayout initshape(int intColor,int intWidthscreen, int intHeight, int intMerginLeft,int intMerginTop) {
    RelativeLayout localRelativeLayout = new RelativeLayout(this.mContext);             
    localRelativeLayout.setBackgroundColor(intColor);       
    RelativeLayout.LayoutParams localLayoutParams = new RelativeLayout.LayoutParams(intWidthscreen, intHeight);
    localLayoutParams.setMargins(intMerginLeft, intMerginTop, 0, 0);
    localRelativeLayout.setLayoutParams(localLayoutParams);         
    return localRelativeLayout;
}   

public void onResume() {
    super.onResume();
}
@Override
public void onPause() {
    super.onPause();
}}

其他 basic2.java, basic3.java, basic4.java..... 就像 basic1.java

我需要将动画应用到 RelativeLayout。是否可以？我希望在页面加载时开始动画，而不是在此之前。

Answer 1

view-pager 的默认行为是从当前页面的两侧预加载至少 1 个页面。请参阅以下链接

ViewPager.setOffscreenPageLimit(0) doesn't work as expected

所以你可以做的不是在 onCreate 开始动画，而是从 onPageScrolled 回调开始它

viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

        //Loading fragment # position

        //Instead of starting animation from onCreate start it from here 
        //Only for the first time (for page 1) this callback will not trigger
    }

    @Override
    public void onPageSelected(int position) {
    }

    @Override
    public void onPageScrollStateChanged(int state) {
    }
});

参见 setOffscreenPageLimit

Set the number of pages that should be retained to either side of the current page in the view hierarchy in an idle state. Pages beyond this limit will be recreated from the adapter when needed.

This is offered as an optimization. If you know in advance the number of pages you will need to support or have lazy-loading mechanisms in place on your pages, tweaking this setting can have benefits in perceived smoothness of paging animations and interaction. If you have a small number of pages (3-4) that you can keep active all at once, less time will be spent in layout for newly created view subtrees as the user pages back and forth.

You should keep this limit low, especially if your pages have complex layouts. This setting defaults to 1.