从 mainActivity (Android) 中的 class 执行一个方法

Question

我正在尝试在用户单击按钮时从 MainActivity class 中名为 QrScann 的 class 执行一个方法，但每次单击按钮时应用程序都会崩溃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)

这是具有我要执行的方法（扫描仪）的 class：

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();
    }
}

class还有其他方法。很好，这就是我在 MainActivity class 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);

    }
}

如果您能解释我可能遗漏的内容，我将不胜感激。

Answer 1

您不创建“new”活动，您通过提供 class 以 Intents 启动它们，然后通过提供额外的和在您的初始 Intent 中执行的操作。

所以，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 actions/extras。

Answer 2

问题是您的 QrScann class 不是有效的 Activity。您不能简单地使用 new 关键字实例化一个新的 Activity class - 您需要允许框架创建 Activity 并通过 Activity通过 startActivity() 在有效上下文中启动生命周期。

看起来您 QrScann 扩展 Activity 的唯一目的是能够调用 startActivityForResult()。相反，QrScann 应该获取对其 MainActivity 的引用并调用 startActivityForResult()，或者回调 MainActivity.

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Activity

@Override
    public void onClick(View v) {

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

    }

Answer 4

你不能像 new QrScann() 那样只实例化一个 Activity，你必须这样做：

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

从 mainActivity (Android) 中的 class 执行一个方法

Execute a method from a class in mainActivity (Android)

java

methods

android

call