Why is onActivityResult triggered before activity even starts?

I have an app that needs to scan a barcode to get a code before it can continue. I use this code to start the scanning activity:

finish = (Button) findViewById(R.id.finishButton);
        finish.setOnClickListener(new OnClickListener() {
            public void onClick(View viewParam) {

                /*Prompt 开发者_如何学运维the user to scan the barcode */
                new AlertDialog.Builder(Visit.this)
                .setMessage("Please Scan the clients barcode to complete the visit")
                .setPositiveButton("Scan Barcode", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        //Start the scan application
                        Intent intent = new Intent("com.google.zxing.client.android.SCAN");
                        startActivityForResult(intent, 0);
                    }
                })

                .setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        //Execute some method call
                        Toast.makeText(Visit.this, "Scan declined...", Toast.LENGTH_SHORT).show();
                    }
                })
                .show();
                /* End of Scan prompt */
            }
        });

The above code sets a listener on a button labeled finished. When the button is clicked, it displays a prompt asking the user to scan a barcode or cancel.

Clicking on the Scan Barcode button starts a new activity which starts the scan.

I have the following code set up to read the result of the scan on the return from the scan:

/* Return from scanning barcode */
    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
;
      if (resultCode == Activity.RESULT_OK && requestCode == 0) {
        Bundle extras = data.getExtras();
        String result = extras.getString("SCAN_RESULT");
      }
      Toast.makeText(Visit.this, "request code: "+requestCode+" result code = "+resultCode+ "\nRESULT_OK: "+Activity.RESULT_OK, Toast.LENGTH_SHORT).show();
    }

Which very simply (for now) outputs the activity result in a toast message.

The problem I'm having is that the onActivityResult method is triggered as soon as I press the scan barcode button.

The scanning process works fine as I can see the results of the scan in the logcat., however because it is triggered too soon the onActivityResult method never gets the scan result and the result code is always -1

Am I missing a step here? is there someway of getting to onActivityResult to wait until the activity actual finishes?

It seems like the barcode scanner I am using has a particular bug in it for tab interfaces with single instance in the manifest which is how I had it.

Removing the single instance stipulation allowed me to do a hack on it thats working perfectly.