应用程序不适用于 ViewHolder

Question

我在 Android Studio 中的 ImageAdapter 有这个问题。当我在 phone 上启动应用程序时，我遇到了巨大的性能问题。所以，人们建议，我应该使用 ViewHolder。我看了几十个其他人是如何做到的例子。尝试了很多版本。不过，当我尝试打开 activity 时，我的应用程序崩溃了。这是我的代码：

public class ImageAdap extends BaseAdapter {
private Context mContext;

public ImageAdap(Context c) {
    mContext = c;
}

public int getCount() {
    return mThumbIds.length;
}

public Object getItem(int position) {
    return null; //mThumbIds[position];
}

public long getItemId(int position) {
    return position;
}

static class ViewHolder {
    ImageView image;
}

public View getView(int position, View convertView, ViewGroup parent) {

    View vi = convertView;
    ViewHolder holder;

    if(convertView == null) {
        holder = new ViewHolder();
        holder.image = new ImageView(mContext);
        vi.setTag(holder);        //Here is the point the app crashes
    }
    else  {
        holder = (ViewHolder) vi.getTag();
    }

    holder.image.setLayoutParams(new GridView.LayoutParams(800, 600));
    holder.image.setScaleType(ImageView.ScaleType.CENTER_CROP);
    holder.image.setPadding(8, 8, 8, 8);

    holder.image.setImageResource(mThumbIds[position]);

    return vi;
}

// references to my images
private Integer[] mThumbIds = {
        R.drawable.sampleda_01,
        R.drawable.sampleda_02,
        R.drawable.sampleda_03,
        R.drawable.sampleda_04,
        R.drawable.sampleda_05
}; }

程序没有任何问题。但是当我打开 activity 时它崩溃了。所以我使用调试器来查看应用程序何时崩溃。应用程序崩溃，当

vi.setTag(holder);

应该执行。我感谢各种帮助。

logcat:

--------- beginning of crash 07-21 13:42:19.229 3059-3059/com.b.a.test E/AndroidRuntime: FATAL EXCEPTION: main Process: com.b.a.test, PID: 3059 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setTag(java.lang.Object)' on a null object reference at com.b.a.test.ImageAdap.getView(ImageAdap.java:59) at android.widget.AbsListView.obtainView(AbsListView.java:2346) at android.widget.GridView.onMeasure(GridView.java:1065) at android.view.View.measure(View.java:18788) 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:135) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465) at android.widget.LinearLayout.measureVertical(LinearLayout.java:748) at android.widget.LinearLayout.onMeasure(LinearLayout.java:630) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465) at android.widget.LinearLayout.measureVertical(LinearLayout.java:748) at android.widget.LinearLayout.onMeasure(LinearLayout.java:630) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643) at android.view.View.measure(View.java:18788) at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2100) at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1216) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1452) at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107) at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013) 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:5417) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 07-21 13:42:21.620 3059-3059/com.b.a.test I/Process: Sending signal. PID: 3059 SIG: 9 07-21 13:42:22.939 3315-3315/com.b.a.test W/System: ClassLoader referenced unknown path: /data/app/com.b.a.test-1/lib/x86 07-21 13:42:23.815 3315-3315/com.b.a.test W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable 07-21 13:42:23.971 3315-3348/com.b.a.test D/OpenGLRenderer: Use EGL_SWAP_BEHAVIOR_PRESERVED: true
                                                                       [ 07-21 13:42:24.003  3315: 3315 D/         ]
                                                                       HostConnection::get() New Host Connection established 0xad1b97e0, tid
3315

Answer 1

View vi = convertView;

if(convertView == null) {
    vi.setTag(holder); 
}

请仔细检查您的代码。简单地说，您从 null 调用 setTag，因此 NullPointerException 抛出。

Answer 2

if(convertView == null)

当该断言为真时，您应该扩充布局。因为你的观点是空的。就那样；

LayoutInflater inflater = (LayoutInflater)   getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE); 
view = inflater.inflate(R.layout.mylayout, null);

如果您不想扩充布局，您可以这样使用。

View vi = convertView;
ViewHolder holder;

if(convertView == null) {
    vi = new ImageView(mContext);
    holder = new ViewHolder();
    holder.image =(ImageView) vi;
    vi.setTag(holder);        //Here is the point the app crashes
}

Answer 3

您的代码应如下所示

if(convertView == null) {
    holder = new ViewHolder();
    ImageView imageView = new ImageView(mContext);
    vi = imageView;
    holder.image = imageView
    vi.setTag(holder); 
}

您需要先设置视图

应用程序不适用于 ViewHolder

App doesn't work with ViewHolder

android

gridview

android-studio

android-viewholder