Question

情况如何？我在ViewPager中有4个片段，它们看起来完全相同。但我想要实现的是使每个片段不同（在更改ImageView和相关文本方面）。但是，我不知道在哪里放置代码来更改ImageViewResource，因为它总是说我正在尝试调用null对象引用上的方法。所以直截了当地说，我必须在哪里放置代码行来更改Image资源（不是在java中的xml中），这样我就不会得到错误。如果您需要进一步澄清，请随时提出。任何帮助表示赞赏。

爪哇：包含ViewPager的活动

package com.mediatechindo.wahyu.materialuikit.template.WalktroughCategory.Style14;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.content.ContextCompat;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.ImageView;
import android.widget.Toast;

import com.bumptech.glide.DrawableRequestBuilder;
import com.bumptech.glide.Glide;
import com.bumptech.glide.load.resource.drawable.GlideDrawable;
import com.bumptech.glide.request.animation.GlideAnimation;
import com.bumptech.glide.request.target.SimpleTarget;
import com.mediatechindo.wahyu.materialuikit.BuildConfig;
import com.mediatechindo.wahyu.materialuikit.R;

public class WalkthroughStyle14Activity extends AppCompatActivity implements View.OnClickListener {
    private ViewPager viewPager;
    ViewPagerAdapter viewPagerAdapter = null;
    private View indicator1;
    private View indicator2;
    private View indicator3;
    private View indicator4;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.walkthrough14_layout);

        indicator1 = findViewById(R.id.indicator1);
        indicator2 = findViewById(R.id.indicator2);
        indicator3 = findViewById(R.id.indicator3);
        indicator4 = findViewById(R.id.indicator4);

        //Layout manager that allows the user to flip left and right through pages of data. You supply an implementation of a PagerAdapter to generate the pages that the view shows.
        viewPager = (ViewPager) findViewById(R.id.viewPager);
        viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());
        viewPager.setAdapter(viewPagerAdapter);
        viewPager.addOnPageChangeListener(new WizardPageChangeListener());


        View image = findViewById(R.id.activity_walkthrough_style14);
        String url = BuildConfig.IMAGE_URL + "walkthrough/style-14/Welcome-14-960.jpg";
        String urlThumb = BuildConfig.IMAGE_URL + "walkthrough/style-14/Welcome-14-960-thumb.jpg";
        loadImageRequest(image, url, urlThumb);



        updateIndicators(0);




    }


    private void loadImageRequest(final View bg, String url, String urlThumb) {
        DrawableRequestBuilder<String> thumbnailRequest = Glide
                .with(this)
                .load(urlThumb);

        Glide.with(this)
                .load(url)
                .thumbnail(thumbnailRequest)
                .into(new SimpleTarget<GlideDrawable>() {
                    @Override
                    public void onResourceReady(GlideDrawable resource, GlideAnimation<? super GlideDrawable> glideAnimation) {
                        bg.setBackground(resource);
                    }
                });
    }


    //Implementation of PagerAdapter that represents each page as a Fragment that is persistently kept in the fragment manager as long as the user can return to the page.
    //Class that holds all the frames to swipe through
    private class ViewPagerAdapter extends FragmentPagerAdapter {

        private int WIZARD_PAGES_COUNT = 4;

        public ViewPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            WalkthroughStyle14Fragment walkthroughStyle14Fragment = new WalkthroughStyle14Fragment(position);
            switch (position){
                case 1:
                    walkthroughStyle14Fragment.imageView.setImageResource(R.drawable.ic_store);
                    break;
                default:

                    break;
            }
            return walkthroughStyle14Fragment;
        }

        @Override
        public int getCount() {
            return WIZARD_PAGES_COUNT;
        }

    }

    //Change listener to call action when page is changed
    private class WizardPageChangeListener implements ViewPager.OnPageChangeListener {

        @Override
        public void onPageScrollStateChanged(int position) {
            // TODO Auto-generated method stub

        }

        @Override
        public void onPageScrolled(int position, float positionOffset,
                                   int positionOffsetPixels) {
            // TODO Auto-generated method stub

        }

        @Override
        public void onPageSelected(int position) {
            updateIndicators(position);
        }
    }

    //Method used to change the color of the dots when page is changed
    public void updateIndicators(int position) {
        switch (position) {
            case 0:
                indicator1.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot));
                indicator2.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                indicator3.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                indicator4.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                break;
            case 1:
                indicator1.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                indicator2.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot));
                indicator3.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                indicator4.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                break;
            case 2:
                indicator1.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                indicator2.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                indicator3.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot));
                indicator4.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                break;
            case 3:
                indicator1.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                indicator2.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                indicator3.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot_grey));
                indicator4.setBackground(ContextCompat.getDrawable(this, R.drawable.ic_dot));
                break;
        }
    }

    @Override
    public void onClick(View view) {
        switch (view.getId()){
            case R.id.btnLoginSignupBack:
                onBackPressed();
                break;
            case R.id.btnSignIn:
                Toast.makeText(this, "Button Sign In clicked!", Toast.LENGTH_SHORT).show();
                break;
            case R.id.btnSignUp:
                Toast.makeText(this, "Button Sign Up clicked!", Toast.LENGTH_SHORT).show();
                break;
            default:
                break;
        }
    }
}

包含我现在尝试更改图像资源的片段的活动：

package com.mediatechindo.wahyu.materialuikit.template.WalktroughCategory.Style14;

import android.annotation.SuppressLint;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;

import com.mediatechindo.wahyu.materialuikit.R;

/**
 * Created by wahyu on 15/11/16.
 */

@SuppressLint("ValidFragment")
public class WalkthroughStyle14Fragment extends Fragment {
    int wizard_page_position;
    public ImageView imageView = null;

    public WalkthroughStyle14Fragment(int position) {
        this.wizard_page_position = position;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState){
        View view = inflater.inflate(R.layout.walkthrough14_fragment, container, false);

        imageView = (ImageView) view.findViewById(R.id.imagePage1);
        imageView.setImageResource(R.drawable.ic_store);

        return view;
    }

}

片段布局

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/activity_walkthrough_style14"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/walkthrough14Background"
    android:orientation="vertical">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior">

        <android.support.v4.view.ViewPager
            android:id="@+id/viewPager"
            android:layout_marginBottom="110dp"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentBottom="true"
            android:layout_marginBottom="124dp"
            android:animateLayoutChanges="true"
            android:gravity="center"
            android:orientation="horizontal">

            <View
                android:id="@+id/indicator1"
                android:layout_width="15dp"
                android:layout_height="15dp"
                android:layout_margin="2dp"
                android:background="@drawable/ic_dot_grey" />

            <View
                android:id="@+id/indicator2"
                android:layout_width="15dp"
                android:layout_height="15dp"
                android:layout_margin="2dp"
                android:background="@drawable/ic_dot_grey" />

            <View
                android:id="@+id/indicator3"
                android:layout_width="15dp"
                android:layout_height="15dp"
                android:layout_margin="2dp"
                android:background="@drawable/ic_dot_grey" />

            <View
                android:id="@+id/indicator4"
                android:layout_width="15dp"
                android:layout_height="15dp"
                android:layout_margin="2dp"
                android:background="@drawable/ic_dot_grey" />
        </LinearLayout>

        <LinearLayout
            android:id="@+id/buttonContainer"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentBottom="true"
            android:background="@color/walkthrough4Background"
            android:orientation="horizontal">

            <Button
                android:id="@+id/btnSignUp"
                style="@style/ButtonWhite"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:onClick="onClick"
                android:text="SIGN UP" />

            <Button
                android:id="@+id/btnSignIn"
                style="@style/ButtonMagenta"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:onClick="onClick"
                android:text="SIGN IN"
                android:textColor="@android:color/white" />

        </LinearLayout>

    </RelativeLayout>

</FrameLayout>

错误堆栈跟踪

06-26 16:53:55.397 7240-7240/com.mediatechindo.wahyu.materialuikit E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                     Process: com.mediatechindo.wahyu.materialuikit, PID: 7240
                                                                                     Theme: themes:{default=overlay:com.cyngn.hexo, iconPack:com.cyngn.hexo, fontPkg:com.cyngn.hexo, com.android.systemui=overlay:com.cyngn.hexo, com.android.systemui.navbar=overlay:com.cyngn.hexo}
                                                                                     java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageResource(int)' on a null object reference
                                                                                         at com.mediatechindo.wahyu.materialuikit.template.WalktroughCategory.Style14.WalkthroughStyle14Activity$ViewPagerAdapter.getItem(WalkthroughStyle14Activity.java:94)
                                                                                         at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:101)
                                                                                         at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1034)
                                                                                         at android.support.v4.view.ViewPager.populate(ViewPager.java:1248)
                                                                                         at android.support.v4.view.ViewPager.populate(ViewPager.java:1116)
                                                                                         at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1642)
                                                                                         at android.view.View.measure(View.java:18799)
                                                                                         at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:715)
                                                                                         at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:461)
                                                                                         at android.view.View.measure(View.java:18799)
                                                                                         at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                         at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                                                                                         at android.view.View.measure(View.java:18799)
                                                                                         at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                         at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                                                                                         at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
                                                                                         at android.view.View.measure(View.java:18799)
                                                                                         at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                         at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1458)
                                                                                         at android.widget.LinearLayout.measureVertical(LinearLayout.java:746)
                                                                                         at android.widget.LinearLayout.onMeasure(LinearLayout.java:629)
                                                                                         at android.view.View.measure(View.java:18799)
                                                                                         at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                         at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                                                                                         at android.view.View.measure(View.java:18799)
                                                                                         at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                         at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1458)
                                                                                         at android.widget.LinearLayout.measureVertical(LinearLayout.java:746)
                                                                                         at android.widget.LinearLayout.onMeasure(LinearLayout.java:629)
                                                                                         at android.view.View.measure(View.java:18799)
                                                                                         at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                         at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                                                                                         at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2899)
                                                                                         at android.view.View.measure(View.java:18799)
                                                                                         at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2108)
                                                                                         at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1224)
                                                                                         at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1460)
                                                                                         at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1115)
                                                                                         at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6023)
                                                                                         at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
                                                                                         at android.view.Choreographer.doCallbacks(Choreographer.java:670)
                                                                                         at android.view.Choreographer.doFrame(Choreographer.java:606)
                                                                                         at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
                                                                                         at android.os.Handler.handleCallback(Handler.java:739)
                                                                                         at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                         at android.os.Looper.loop(Looper.java:148)
                                                                                         at android.app.ActivityThread.main(ActivityThread.java:5461)
                                                                                         at java.lang.reflect.Method.invoke(Native Method)
                                                                                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Answer 1

您的公共变量ImageView为空，因为您在创建父WalkthroughStyle14Fragment的实例后尝试立即访问它。布局尚未膨胀，因此尚未调用onCreateView。在FragmentPagerAdapter.getItem(int)中创建片段是正确的设计，但访问成员变量并不适合您。

一个很好的面向对象的解决方案是从WalkthroughStyle14Fragment定义4个派生类，每个派生类都有自己的背景/前景图像或任何其他自定义。例如。 WalkthroughStyle14FragmentOne，WalkthroughStyle14FragmentTwo，WalkthroughStyle14FragmentThree，WalkthroughStyle14FragmentFour。在onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)中设置适当的图像/文字。您可以将每个派生片段的数量添加到适配器中显示的片段集。您可以根据传入的位置在适配器中实例化相应的片段。

Fragment中的Imageview一直为空

1 个答案: