记录一波三折的ViewPager与PagerAdapter

本来很简单的一个东西弄了几个小时,好吧记录一下遇到的种种问题,以便以后出现同样的错误可以迅速解决

ViewPager是android3.0中新加的一个组件,用来右滑动切换多个View,这东西很适合做启动页面与导航页面之类的东西.

要使用ViewPager首先需要做如下事情:

1.导入android-support-v4.jar

2.一个包含ViewPager的布局文件

3.最后制作一个每个切换的View的Layout文件,infalte起来比较方便(可选)

4.实现一个Adapter 继承与 PagerAdapter  ,在instantiateItem方法中生成View,并将这个view添加到container中 . container.addView(v)

PagerAdapter必须实现如下方法: (带星号的是必须实现的方法)

* public int getCount() 获取总View的数量

public void startUpdate(ViewGroup container)

* public Object instantiateItem(ViewGroup container, int position)   实例化当前的View,添加到contrainer中,然后返回生成的View或关联的Object (用于在isViewFromObject中检查)

* public void destroyItem(ViewGroup container, int position, Object object)  从container中移除指定的View (释放内存)

public void finishUpdate(ViewGroup container)

* public boolean isViewFromObject(View view, Object object) 检查View是否与一个Object关联,

在开发过程中遇到很多问题总结如下:

1.通过gradle 添加编译依赖包 ,由两种方式可以添加

(第一种导入方式)通过jar包放入到lib目录下,然后添加

compile fileTree(dir: ‘libs‘, include: [‘*.jar‘])compile ‘com.android.support:android-support-v7-appcompat‘compile ‘com.android.support:android-support-v4‘

(第二种导入方式)通过sdk目录下 android-sdk-linux/extras/android/m2repository/com/android/support/support-v4/ 目录下对应的版本导入

compile ‘com.android.support:support-v4:22.2.0‘compile ‘com.android.support:appcompat-v7:22.2.0‘

导入完整后编译总是提示

Unable to start activity ComponentInfo{linuxeye.com.knowledgeispower/linuxeye.com.knowledgeispower.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method ‘void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)‘ on a null object reference

开始以为是因为包没有导入,到处google,bing,baidu, 看到一片stackoverflow上的问答,说是Activity没有生成,突然间想到是不是Activity的代码有问题.

经过反复调试,发现findViewById竟然在setViewContent之前被调用,导致findViewById一致返回null引起的.我靠这个错误的代价是一个小时!这个一个小时反复的清理项目从新导入包..反复的检查问题.

之间还想过是不是ViewPager的布局外面需要一个Layout之类的.真的是累死人了.ViewPager是不需要外容器的,可以直接作为Layout使用的.

布局文件如下 . mypager.xml

<?xml version="1.0" encoding="utf-8"?>

<android.support.v4.view.ViewPager    android:id="@+id/my_pager"    xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent"    ></android.support.v4.view.ViewPager>

修改完Activity后可以findViewById了,接下来的问题就是Activity确实显示出来了,就是什么内容都没有,一片漆黑呀!这个过程也是浪费了我好长时间,反复baidu看别人的代码和文章,发现一个问题,我的isViewFromObject方法是android Studio自动生成的,代码自动返回false,看到文档里写着这个方法是用来比较View和Object是否有关联? 这个方法是在ViewPager中被调用的,ViewPager将每一个View添加到mItems中,mItem是一个ArrayList<ItemInfo>  , ItemInfo是每个View相关的信息,其中Object就是 instantiateItem 方法返回的哪个Object,

官方文档标注这个对象可以不是一个View,是用来让ViewPager检查是否和一个View对应的. 

static class ItemInfo {    Object object;    int position;    boolean scrolling;    float widthFactor;    float offset;}

修改了isViewFromObject方法为:

public boolean isViewFromObject(View view, Object object) {    return view == object;}

一切都ok了,图片终于显示出来了!大工告成!

AppPagerAdapter代码如下:

package linuxeye.com.knowledgeispower;

import android.content.Context;import android.content.Intent;import android.support.v4.view.PagerAdapter;import android.util.Log;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import android.widget.ImageButton;import android.widget.ImageView;

import java.util.ArrayList;

/** * Created by Raffeale on 15-7-7. */public class AppPagerAdapter extends PagerAdapter {

    Context ctx = null;    ArrayList<ImageView> pagerData = null;

    public AppPagerAdapter(Context ct , ArrayList<ImageView> data) {

        ctx = ct;        pagerData = data;

    }

    /**     * Return the number of views available.     */    @Override    public int getCount() {        return pagerData.size();    }

    /**     * Create the page for the given position.  The adapter is responsible     * for adding the view to the container given here, although it only     * must ensure this is done by the time it returns from     * {@link #finishUpdate(ViewGroup)}.     *     * @param container The containing View in which the page will be shown.     * @param position  The page position to be instantiated.     * @return Returns an Object representing the new page.  This does not     * need to be a View, but can be some other container of the page.     */    @Override    public Object instantiateItem(ViewGroup container, int position) {        Log.i("PagerAdapter" , "method instantiateItem be calling" );        View v = LayoutInflater.from(ctx).inflate(R.layout.pager , null);        ImageView img = (ImageView) v.findViewById(R.id.page_img);        img.setImageResource(R.drawable.pic0+position);        if(position+1 == pagerData.size()) {            ImageButton ib = (ImageButton)v.findViewById(R.id.ib);            ib.setVisibility(View.VISIBLE);            ib.setBackgroundResource(R.drawable.bt);            ib.setOnClickListener(new View.OnClickListener() {                @Override                public void onClick(View v) {                    ctx.startActivity(new Intent(ctx,ArticleActivity.class));                }            });        }

        container.addView(v);        return v;

        //return super.instantiateItem(container, position);    }

    /**     * Remove a page for the given position.  The adapter is responsible     * for removing the view from its container, although it only must ensure     * this is done by the time it returns from {@link #finishUpdate(ViewGroup)}.     *     * @param container The containing View from which the page will be removed.     * @param position  The page position to be removed.     * @param object    The same object that was returned by     *                  {@link #instantiateItem(View, int)}.     */    @Override    public void destroyItem(ViewGroup container, int position, Object object) {        Log.i("PagerAdapter" , "method destroyItem be calling");        container.removeView((View)object);    }

    /**     * Determines whether a page View is associated with a specific key object     * as returned by {@link #instantiateItem(ViewGroup, int)}. This method is     * required for a PagerAdapter to function properly.     *     * @param view   Page View to check for association with <code>object</code>     * @param object Object to check for association with <code>view</code>     * @return true if <code>view</code> is associated with the key object <code>object</code>     */    @Override    public boolean isViewFromObject(View view, Object object) {        return view == object;    }

}

Activity代码如下:

package linuxeye.com.knowledgeispower;

import android.app.Activity;import android.os.Bundle;import android.support.v4.view.ViewPager;import android.widget.ImageView;

import java.util.ArrayList;

public class MainActivity extends Activity {    ViewPager vp = null;

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

        //vp = new ViewPager(this);        setContentView(R.layout.activity_main);        ArrayList<ImageView> data = new ArrayList<ImageView>();        for(int i=0;i<5;i++) {            ImageView img = new ImageView(this);            img.setImageResource(R.drawable.pic0+i);            data.add(img);        }

        AppPagerAdapter myadapter = new AppPagerAdapter(this , data);

        vp = (ViewPager)findViewById(R.id.my_pager);

        //LayoutInflater inflater = getLayoutInflater();        try{

            vp.setAdapter(myadapter);        }catch (Exception ex) {            ex.printStackTrace();        }

    }

}