返回到屏幕的先前版本

Question

I am working on a BlackBerry application that has a number of screens. I am able to navigate back and forth from each screen easily. However, I am trying to navigate "back" to the previous version of the screen. For instance, there are two screens A (Home Screen) and B (Second Screen). I navigate from A to B as follows:

UiApplication.getUiApplication().pushScreen(new MsgScreen());

I navigate back to A from B:

public boolean onClose() {
         UiApplication.getUiApplication().popScreen(this);
            return true;
        }

Whenever I navigate from A to B, a new instance of B is created. However, screen B has a button that allows it to add LabelFields to a VerticalFieldManager . Whenever I navigate from A to B, the fields added are not viewed as it was not part of the constructor. Therefore, I would like to view the last or "previous" version of the screen instead of getting a newly created screen each time. Please help.

Answer 1

I think you have a couple different choices here (at least):

Keep Screen B in Memory

Here, when you create a new instance of B from within class A, you will save it in a member variable (of the A class). Then, whenever you need to go back to B from A, you will use the member variable, if there is one:

Class A:

 private MsgScreen _bScreen;

 private MsgScreen getBScreen() {
     // don't actually create the screen until we need it,
     //   but don't create more than one instance
     if (_bScreen == null) {
         _bScreen = new MsgScreen();
     }
     return _bScreen;
 }

 public void showB() {
     UiApplication.getUiApplication().pushScreen(getBScreen());
 }

Note: see here for a more sophisticated way to implement this technique in many Screen classes, with the use of a reusable base class .

Store the State of B and Load it When (Re-)Displayed

Another option is to let the MsgScreen (B) get destroyed, and re-created every time, as you originally had. But, instead of the original design, make sure that when the MsgScreen is closed, you use something like the PersistentStore to save its state. In other words, if the user can add labels on the B screen, then save the content of all the labels. Maybe you save a String[] (or Vector ) to your PersistentStore . Then, in the constructor of MsgScreen , or maybe in ( MsgScreen.java ):

protected void onUiEngineAttached(boolean attached) {
    if (attached) {
        // load the String[] from the persistent store, 
        //  and create new LabelFields
    } else {
        // save the String[] to the persistent store
    }
}

you can reload the array of Strings from the persistent store, and create new LabelFields , exactly as you had before.

Or, you could choose to save the LabelField contents to the store as soon as they are created by the user, instead of when the screen is closed. That's up to you.

You might also consider what the content of those labels are, and what should happen if the user leaves your app, and doesn't return (before rebooting). Is it ok that the labels' strings are lost?

Also, the first alternative can use up a lot of memory, if you're doing this same thing with many Screens (or if the Screens are complicated, and have many child Fields ). If you only have this issue with the MsgScreen , then it's probably not a big problem, and the simplest solution would be the first one I suggested. But, if you have many screens in your app (A, B, C, D, E, F, etc.), then you might not want to save them all, when they're not actually being shown.

These kind of questions should help you determine when, or if, you should save the state of your UI.