how can i share view code for both datafield and watch-app

It isn't going to be ideal to determine at runtime if your app is a datafield or a watch-app, and I'm not sure I understand how this will be helpful. At some point you will not be able to share code. You've discovered the datafield app type requires your view extend DataField, so that is a pretty clear line in the sand.

If all you want to do is share code while fitting into the framework by using the right view class, the easiest thing to do is to extract the code that you want to reuse, and then reference them from the derived View or DataField. Something like this...

// Common.mc: shared by watch-app and datafield
class MyCommon
{
    // all shared data goes here

    function initialize() {
    }

    // all shared functions go here
}


//
//
//

// MyWatchApp.mc: used only by the watch app
class MyWatchApp extends Application.AppBase
{
    function initialize() {
        AppBase.initialize();
    }
    
    function getInitialView() {
        var common = new Common();
        return [ new MyWatchAppView(common) ];
    }
}

// MyWatchAppView.mc: use only by the watch-app
class MyWatchAppView extends WatchUi.View
{
    hidden var _common;
    
    function initialize(common) {
        View.initalize();
        _common = common;
    }
    
    // use _common
}

//
//
//

// MyDataFieldApp.mc: used only by the datafield app
class MyDataFieldApp extends Application.AppBase
{
    function initialize() {
        AppBase.initialize();
    }
    
    function getInitialView() {
        var common = new Common();
        return [ new MyDataFieldView(common) ];
    }
}

// MyDataField.mc: used only by the datafield app
class MyDataField extends WatchUi.DataField
{
    hidden var _common;
    
    function initialize(common) {
        DataField.initalize();
        _common = common;
    }
    
    // use _common
}

Typically this shared class would be the Model in a Model-View-Controller implementation, but you could also do something where it was just common view code for displaying data.

4643287 said:

 2) It looks like the initial view either has to extend a WatchUI View or Datafield, now sure how to do both.  Can I just extend View and at runtime during initialization either call DataField.initialize or View.Initialize (again, then I would need to figure out how to determine what I am at run time).

You can't have a single application that is both a watch-app and a datafield and changes its behavior at runtime based on how it is loaded. When your app is compiled the manifest.xml specifies the app type. A compiled .PRG can only be on application type and each application must have a unique manifest identifier.

As shown above, you can certainly share code. You can do this with a barrel, a barrel project, revision control system (e.g., a git submodule), or by just copying files around on your system. Beyond the shared code there will be code/files that are not going to be sharable.

As Travis.ConnectIQ, there's no such thing as a "hybrid" .prg that can be both a DF and a devices app, so no need to do any runtime logic.  The code is built as either a DF or a device app - different app types in the manifest, as well as different mainifest IDs.

Something that may of be of concern is that things are not shared - Storage or settings - the device app can't save something that's available to the DF and v.v.  In the same way, if you use app settings, they are separate, so if a user chances something for the device app, they must also change it for the DF.

There's also some really significant differences between how a DF and a device app work.  In a device app, there's a whole bunch of things you need to deal with, like starting/stopping/pausing the recording, enabling the sensors, etc.  In a DF, compute gets called every second, and when visible, onUpdate is also called every second.  In a device app, you need a timer to trigger things like this.  There's also the whole max size.  DF's are typically much low than device apps.

When something is a DF, if a user wants to see other data, they can just add another DF for that, but in a device app, your apps data is all they can see, so there may be things your device app needs to display that wouldn't be needed in a DF, so you probably wouldn't use the same onUpdate() code, as a device app can also have multiple screens.

When it gets down to it, your may find the amount of shared code between a DF and device app isn't that much

Perhaps I wasn't clear.  Right now I am weighing the pros and cons and developing either a device field or an app.  I would like to just be able to change the manifest file, and at run time see what type I am and then execute a slightly different code path.  My question was mostly about if there is anyone to do that, or do I have to make syntactical changes in my View/DataField subclass (besides changing the manifest).