記錄一波三折的ViewPager與PagerAdapter

本來很簡單的一個東西弄了幾個小時,好吧記錄一下遇到的種種問題,以便以后出現同樣的錯誤可以迅速解決

ViewPager是android3.0中新加的一個組件,用來右滑動切換多個View,這東西很適合做啟動頁面與導航頁面之類的東西.

要使用ViewPager首先需要做如下事情:

1.導入android-support-v4.jar

2.一個包含ViewPager的布局文件

3.最后制作一個每個切換的View的Layout文件,infalte起來比較方便(可選)

4.實現一個Adapter 繼承與 PagerAdapter  ,在instantiateItem方法中生成View,並將這個view添加到container中 . container.addView(v)

 

PagerAdapter必須實現如下方法: (帶星號的是必須實現的方法)

* public int getCount() 獲取總View的數量

public void startUpdate(ViewGroup container) 

* public Object instantiateItem(ViewGroup container, int position)   實例化當前的View,添加到contrainer中,然后返回生成的View或關聯的Object (用於在isViewFromObject中檢查)

* public void destroyItem(ViewGroup container, int position, Object object)  從container中移除指定的View (釋放內存)

public void finishUpdate(ViewGroup container) 

* public boolean isViewFromObject(View view, Object object) 檢查View是否與一個Object關聯,

 

在開發過程中遇到很多問題總結如下:

1.通過gradle 添加編譯依賴包 ,由兩種方式可以添加

(第一種導入方式)通過jar包放入到lib目錄下,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'

(第二種導入方式)通過sdk目錄下 android-sdk-linux/extras/android/m2repository/com/android/support/support-v4/ 目錄下對應的版本導入

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

導入完整后編譯總是提示

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

開始以為是因為包沒有導入,到處google,bing,baidu, 看到一片stackoverflow上的問答,說是Activity沒有生成,突然間想到是不是Activity的代碼有問題.

經過反復調試,發現findViewById竟然在setViewContent之前被調用,導致findViewById一致返回null引起的.我靠這個錯誤的代價是一個小時!這個一個小時反復的清理項目從新導入包..反復的檢查問題.

之間還想過是不是ViewPager的布局外面需要一個Layout之類的.真的是累死人了.ViewPager是不需要外容器的,可以直接作為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,接下來的問題就是Activity確實顯示出來了,就是什么內容都沒有,一片漆黑呀!
這個過程也是浪費了我好長時間,反復baidu看別人的代碼和文章,發現一個問題,我的isViewFromObject方法是android Studio自動生成的,代碼自動返回false,看到文檔里寫着這個方法是用來比較View和Object是否有關聯? 
這個方法是在ViewPager中被調用的,ViewPager將每一個View添加到mItems中,mItem是一個ArrayList<ItemInfo> , ItemInfo是每個View相關的信息,其中Object就是 instantiateItem 方法返回的哪個Object,

官方文檔標注這個對象可以不是一個View,是用來讓ViewPager檢查是否和一個View對應的. 

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

修改了isViewFromObject方法為:

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

一切都ok了,圖片終於顯示出來了!大工告成!

AppPagerAdapter代碼如下:

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代碼如下:

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();
 }


 }

}