Question

当用户点击按钮时，我试图从MainActivity类中的一个名为QrScann的类中执行一个方法，但每次单击按钮时应用程序崩溃，它就会给我这个日志崩溃输出

 06-02 17:00:24.890    7067-7084/t.n.app W/EGL_emulation﹕ eglSurfaceAttrib not implemented
06-02 17:00:24.890    7067-7084/t.n.app W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xb3ee6da0, error=EGL_SUCCESS
06-02 17:00:27.473    7067-7067/t.n.app D/AndroidRuntime﹕ Shutting down VM
06-02 17:00:27.474    7067-7067/t.n.app E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: snet.tuberlin.app, PID: 7067
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.app.ActivityThread$ApplicationThread android.app.ActivityThread.getApplicationThread()' on a null object reference
            at android.app.Activity.startActivityForResult(Activity.java:3745)
            at android.app.Activity.startActivityForResult(Activity.java:3706)
            at snet.tuberlin.app.QRCode.scanQR(QRCode.java:23)
            at snet.tuberlin.app.MainActivity.onClick(MainActivity.java:48)
            at android.view.View.performClick(View.java:4780)
            at android.view.View$PerformClick.run(View.java:19866)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

这是具有我想要执行的方法（扫描程序）的类：

public class QrScann extends Activity {

static final String ACTION_SCAN = "com.google.zxing.client.android.SCAN";

public void Scanner(View v) {
    try {
        Intent intent = new Intent(ACTION_SCAN);
        intent.putExtra("SCAN_MODE", "QR_CODE_MODE");
        startActivityForResult(intent, 0);
    } catch (ActivityNotFoundException anfe) {
        showDialog(QrScann.this, "No Scanner Found", "Download a scanner code activity?", "Yes", "No").show();
    }
}

该课程还有其他方法。这就是我在MainActivity类中所做的：

 public class MainActivity extends Activity implements View.OnClickListener {



public Button Scanner;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Scanner =(Button)findViewById(R.id.buttonScanQR);
        Scanner.setOnClickListener(this);

    }


    @Override
    public void onClick(View v) {
        //Intent QrScann = new Intent(this, QrScann.class);
        //startActivity(QrScann );
        QrScann QRScanner =new QrScann();
        QRScanner.Scanner(v);

    }
}

我真的很感激我可能缺少的解释。

Answer 1

您不创建“new”活动，通过提供类来启动它们Intents，然后通过在启动Intent中提供额外内容和操作来处理生命周期方法中的操作

因此，基本上，QrScann QRScanner = new QRScann();是尝试创建系统组件的错误方法。您应该使用Intent intent = new Intent(MainActivity.this, QrScann.class);并在意图中提供操作/附加功能。

Answer 2

问题是您的QrScann课程不是有效Activity。您不能简单地使用Activity关键字实例化新的new类 - 您需要允许框架创建活动，并通过startActivity()上的QrScann启动活动生命周期。有效的上下文。

看起来您startActivityForResult()延长了活动的唯一目的是能够致电QrScann。相反，MainActivity应该引用其startActivityForResult()并在其上调用MainActivity，或者回拨View v=inflater.inflate(R.layout.fragment_b,container,false); View v = findViewById(R.id.fragB);。

Answer 3

public class QrScann {

static final String ACTION_SCAN = "com.google.zxing.client.android.SCAN";
private MainActivity mContext;
public QrScann(MainActivity context) {
   mContext = context;
}

public void Scanner(View v) {
    try {
        Intent intent = new Intent(ACTION_SCAN);
        intent.putExtra("SCAN_MODE", "QR_CODE_MODE");
        mContext.startActivityForResult(intent, 0);
    } catch (ActivityNotFoundException anfe) {
        //show dialog is deprecated , try DialogFragments
        mContext.showDialog(QrScann.this, "No Scanner Found", "Download a scanner code activity?", "Yes", "No").show();
    }
}

来自活动

@Override
    public void onClick(View v) {

        QrScann QRScanner =new QrScann(this);
        QRScanner.Scanner(v);

    }

Answer 4

您不能仅像Activity new QrScann()那样实例，您必须这样做：

Intent intent = new Intent(this, QrScann.class);
startActivity(intent);

从mainActivity（Android）中的类执行方法

4 个答案: