没有静态引用，Android自定义视图事件侦听器将无法正常工作

Question

I'm creating a custom compound view in android which contains two TextViews within a relative layout. There are two classes, one is BarcodeScannerView which is the compound view and another activity that uses the custom view. The code is as follows.

BarcodeScannerView.java

    public class BarcodeScannerView extends RelativeLayout implements View.OnClickListener {

    private int boxRadius;
    private String boxText;
    private int boxColor, boxEntityColor;
    private TextView entityName;
    private TextView manualEntry;
    private ImageView scannerReadyImv;
    private RelativeLayout scannerBox;
    private OnClickListener listener;
    private ScannerListener scanner;
    private ScannerState state;
   // private static BarcodeScannerView instance;

    public enum ScannerState {
        IDLE, READY, COMPLETE;
    }

    public interface ScannerListener {

        void onClickScannerBox();
        void scannerIdle();
    }

    public BarcodeScannerView(Context context) {
        super(context);
    }

    public BarcodeScannerView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(context, attrs);
    }

    private void init(Context context, AttributeSet attrs) {

        View.inflate(context, R.layout.activity_barcode_scanner, this);
        setDescendantFocusability(FOCUS_BLOCK_DESCENDANTS);

        entityName = (TextView) findViewById(R.id.entityNameTV);
        manualEntry = (TextView) findViewById(R.id.manualEntryTV);
        scannerReadyImv = (ImageView) findViewById(R.id.scannerReadyImv);
        scannerBox = (RelativeLayout) findViewById(R.id.scannerBox);

        manualEntry.setOnClickListener(this);
        scannerBox.setOnClickListener(this);

        // Assign custom attributes
        TypedArray a = context.getTheme().obtainStyledAttributes(attrs, R.styleable.barcodeviewer, 0, 0);

        try {

            boxEntityColor = a.getInteger(R.styleable.barcodeviewer_boxEntityColor, 0);
            setBoxEntityColor(boxEntityColor);

        } finally {
            a.recycle();
        }

    }

    // Set custom attributes
    private void setBoxEntityColor(int color) {
        this.entityName.setTextColor(color);
    }

    // Set event handlers
    public void setOnClickListener(OnClickListener listener) {
        this.listener = listener;
    }

    public void setOnScannerListener(ScannerListener scanner) {
        this.scanner = scanner;
    }


    // Setters and getters
    public void setScannerState(ScannerState state) {
        this.state = state;
    }

    public ScannerState getScannerState() {
        return this.state;
    }

    private void setScannerReadyState() {
        scannerBox.setBackgroundResource(R.drawable.bc_active_4);
        scannerReadyImv.setVisibility(View.VISIBLE);
    }

    public void onClick(View v) {

        if (v.getId() == R.id.scannerBox) {
            this.scanner.onClickScannerBox();
            if (this.getScannerState() == ScannerState.IDLE) {
                setScannerReadyState();
            }
        }
    }
}

MainActivity.java

public class MainActivity extends AppCompatActivity implements BarcodeScannerView.ScannerListener {

private BarcodeScannerView barcodeScannerView;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main_activity);
    /*barcodeScannerView = BarcodeScannerView.instance(MainActivity.this);
    barcodeScannerView.setOnScannerListener(this);*/
    barcodeScannerView = new BarcodeScannerView(this);
    barcodeScannerView.setOnScannerListener(this);
}

@Override
public void onClickScannerBox() {
    scannerIdle();
}

@Override
public void scannerIdle(){
    barcodeScannerView.setScannerState(BarcodeScannerView.ScannerState.IDLE);
}

}

The issue is when I call the onClick() method of the BarcodeScannerView class it gives a null point exception.

java.lang.NullPointerException: Attempt to invoke interface method 'void [packagename]$ScannerListener.onClickScannerBox()' on a null object reference

In debugging I noticed that when the onClickScannerBox() is called, the scanner object get initialized. But after onClickScannerBox() is executed in BarcodeScannerView the scanner object gets null.

According to my observation the reason is when onClickScannerBox() is called one BarcodeScannerView object instance is initialized and when the onClickScannerBox() finishes and comes out of the method since it is in the onClick() method of another BarcodeScannerView object instance the scanner object gets null.

I can simply correct this issue using static scanner object. (The code is not provided here since the gets too long) I don't think having static references is the best approach for this problem. Is there any approach to correct the issue without using any static references? Any suggestions are appreciated.

Answer 1

In your implementation there are two instances of your custom compound view comes into play, one is in the layout (activity layout) and other one is explicitly created one in the activity onCreate method.

barcodeScannerView = new BarcodeScannerView(this);
barcodeScannerView.setOnScannerListener(this);

You have set the listener to the explicitly created one which doesn't have clue about view used in the layout, simply they are two instances. You should use the instance created (inflated) in the xml as follows.

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

    barcodeScannerView = (BarcodeScannerView) findViewById(R.id.barcode_scanner);
    barcodeScannerView.setOnScannerListener(this);
}

barcode_scanner should be the id you have given to the compound view used in the activity layout.