Android之startActivity、startActivityForResult和setResult詳解

[PS:還是以GOOGLE原生的BluetoothChat為例]

1. startActivity與startActivityForResult的區別

startActivity( ) 僅僅是跳轉到目標頁面，若是想跳回當前頁面，則必須再使用一次startActivity( )。

startActivityForResult( ) 可以一次性完成這項任務，當程序執行到這段代碼的時候，假若從T1Activity跳轉到下一個Text2Activity，

而當這個Text2Activity調用了finish()方法以后，程序會自動跳轉回T1Activity，並調用前一個T1Activity中的onActivityResult( )方法。

例如在BluetoothChat類中，對跳轉到DeviceListActivity類采用的是startActivityForResult( ) 方式：

　　@Override
    public boolean onOptionsItemSelected(MenuItem item) {
        Intent serverIntent = null;
        switch (item.getItemId()) {
        case R.id.secure_connect_scan:
            // Launch the DeviceListActivity to see devices and do scan
            serverIntent = new Intent(this, DeviceListActivity.class);
            startActivityForResult(serverIntent, REQUEST_CONNECT_DEVICE_SECURE);
            return true;
        case R.id.insecure_connect_scan:
            // Launch the DeviceListActivity to see devices and do scan
            serverIntent = new Intent(this, DeviceListActivity.class);
            startActivityForResult(serverIntent, REQUEST_CONNECT_DEVICE_INSECURE);
            return true;
        case R.id.discoverable:
            // Ensure this device is discoverable by others
            ensureDiscoverable();
            return true;
        }
        return false;
    }

在DeviceActivity中則使用setResult()方法返回值：

　　// The on-click listener for all devices in the ListViews
    private OnItemClickListener mDeviceClickListener = new OnItemClickListener() {
        public void onItemClick(AdapterView<?> av, View v, int arg2, long arg3) {
            // Cancel discovery because it's costly and we're about to connect
            mBtAdapter.cancelDiscovery();

            // Get the device MAC address, which is the last 17 chars in the View
            String info = ((TextView) v).getText().toString();
            String address = info.substring(info.length() - 17);

            // Create the result Intent and include the MAC address
            Intent intent = new Intent();
            intent.putExtra(EXTRA_DEVICE_ADDRESS, address);

            // Set result and finish this Activity
 setResult(Activity.RESULT_OK, intent);
            finish();
        }
    };

但是，要注意，如果在startActivityForResult跳轉到的Activity里面設置setResult，結果並不會馬上返回給parent的Activity，只有當前Activity被finish，

結果才會被發送給parent的onActivityResult去處理：

　　public void onActivityResult(int requestCode, int resultCode, Intent data) {
        if(D) Log.d(TAG, "onActivityResult " + resultCode);
        switch (requestCode) {
        case REQUEST_CONNECT_DEVICE_SECURE:
            // When DeviceListActivity returns with a device to connect
            if (resultCode == Activity.RESULT_OK) {
                connectDevice(data, true);
            }
            break;
        case REQUEST_CONNECT_DEVICE_INSECURE:
            // When DeviceListActivity returns with a device to connect
            if (resultCode == Activity.RESULT_OK) {
                connectDevice(data, false);
            }
            break;
        case REQUEST_ENABLE_BT:
            // When the request to enable Bluetooth returns
            if (resultCode == Activity.RESULT_OK) {
                // Bluetooth is now enabled, so set up a chat session
                setupChat();
            } else {
                // User did not enable Bluetooth or an error occurred
                Log.d(TAG, "BT not enabled");
                Toast.makeText(this, R.string.bt_not_enabled_leaving, Toast.LENGTH_SHORT).show();
                finish();
            }
        }
    }

 2. setResult()的退出時機

這里的一個例子是提前調用setResult()設置好退出機制，以應對用戶按"BACK"的情況：

　　@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Setup the window
        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
        setContentView(R.layout.device_list);

        // Set result CANCELED in case the user backs out
 setResult(Activity.RESULT_CANCELED); // Initialize the button to perform device discovery
        Button scanButton = (Button) findViewById(R.id.button_scan);
        scanButton.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                doDiscovery();
                v.setVisibility(View.GONE);
            }
        });

        // Initialize array adapters. One for already paired devices and
        // one for newly discovered devices
        mPairedDevicesArrayAdapter = new ArrayAdapter<String>(this, R.layout.device_name);
        mNewDevicesArrayAdapter = new ArrayAdapter<String>(this, R.layout.device_name);

        // Find and set up the ListView for paired devices
        ListView pairedListView = (ListView) findViewById(R.id.paired_devices);
        pairedListView.setAdapter(mPairedDevicesArrayAdapter);
        pairedListView.setOnItemClickListener(mDeviceClickListener);

        // Find and set up the ListView for newly discovered devices
        ListView newDevicesListView = (ListView) findViewById(R.id.new_devices);
        newDevicesListView.setAdapter(mNewDevicesArrayAdapter);
        newDevicesListView.setOnItemClickListener(mDeviceClickListener);

        // Register for broadcasts when a device is discovered
        IntentFilter filter = new IntentFilter(BluetoothDevice.ACTION_FOUND);
        this.registerReceiver(mReceiver, filter);

        // Register for broadcasts when discovery has finished
        filter = new IntentFilter(BluetoothAdapter.ACTION_DISCOVERY_FINISHED);
        this.registerReceiver(mReceiver, filter);

        // Get the local Bluetooth adapter
        mBtAdapter = BluetoothAdapter.getDefaultAdapter();

        // Get a set of currently paired devices
        Set<BluetoothDevice> pairedDevices = mBtAdapter.getBondedDevices();

        // If there are paired devices, add each one to the ArrayAdapter
        if (pairedDevices.size() > 0) {
            findViewById(R.id.title_paired_devices).setVisibility(View.VISIBLE);
            for (BluetoothDevice device : pairedDevices) {
                mPairedDevicesArrayAdapter.add(device.getName() + "\n" + device.getAddress());
            }
        } else {
            String noDevices = getResources().getText(R.string.none_paired).toString();
            mPairedDevicesArrayAdapter.add(noDevices);
        }
    }