Android and design patterns-Decorator and androiddecorator

Android and design patterns-Decorator and androiddecorator

The Decorator mode is described as follows in Dr. Yan Hong's book "JAVA and patterns:
　　Decoration mode, also known as Wrapper mode. The decoration mode extends the object functions in a transparent way to the client. It is an alternative to the inheritance relationship.
Decoration mode structure
Decoration modeDynamically attaches more responsibilities to an object in a transparent manner to the customer. In other words, the client does not think that objects are different before and after decoration. The decoration mode can extend the functions of objects without creating more child classes. (The above is from the Network)

The decoration mode class diagram is as follows:

Roles in the decoration mode include:
● Abstract component (Context) Role: provides an abstract interface to standardize the objects for receiving additional responsibilities.
● ContextImpl role: defines a class that will receive additional responsibilities.
● ContextWrapper: holds an instance of a Component object and defines an interface consistent with the abstract Component Interface.
● Specific Decoration (Activity/Service/Application) Role: "attaches" an additional responsibility to the component object.

ContextImpl is the concrete implementation of the Context abstract class. ContextWrapper and the Context held by all its subclass objects are ContextImpl objects. Take the Context of the Activity as an example. The Activity is created in ActivityThread. When the Activity is created:

public final class ActivityThread {

     .......     private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {          ......            if (activity != null) {                Context appContext = createBaseContextForActivity(r, activity);                CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());                Configuration config = new Configuration(mCompatConfiguration);                if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "                        + r.activityInfo.name + " with config " + config);                activity.attach(appContext, this, getInstrumentation(), r.token,                        r.ident, app, r.intent, r.activityInfo, title, r.parent,                        r.embeddedID, r.lastNonConfigurationInstances, config);

              .......            }

            ........    }

    private Context createBaseContextForActivity(ActivityClientRecord r,            final Activity activity) {        ContextImpl appContext = new ContextImpl();        appContext.init(r.packageInfo, r.token, this);        appContext.setOuterContext(activity);        // For debugging purposes, if the activity's package name contains the value of        // the "debug.use-second-display" system property as a substring, then show        // its content on a secondary display if there is one.        Context baseContext = appContext;        String pkgName = SystemProperties.get("debug.second-display.pkg");        if (pkgName != null && !pkgName.isEmpty()                && r.packageInfo.mPackageName.contains(pkgName)) {            DisplayManagerGlobal dm = DisplayManagerGlobal.getInstance();            for (int displayId : dm.getDisplayIds()) {                if (displayId != Display.DEFAULT_DISPLAY) {                    Display display = dm.getRealDisplay(displayId, r.token);                    baseContext = appContext.createDisplayContext(display);                    break;                }            }        }        return baseContext;    }}

After the createBaseContextForActivity () method returns the ContextImpl object, it associates the ContextImpl object with the activity through Activity. attach.

Differences between the decoration mode and Agent Mode:

Decoration mode: the object function is extended in a transparent way to the client, which is an alternative to the inheritance relationship;
Proxy mode: provides a proxy object for an object and has a proxy object to control reference to the original object;
The decoration mode should enhance the function of the decorated object. The proxy mode controls the object of the proxy and does not provide the enhancement function of the object itself.
The implementation mechanisms of the two are indeed the same. We can see that there are many duplicate instance codes. However, in terms of semantics, the two functions are the opposite. An important role of the model is to simplify the understanding of your program by other programmers,
If you write Decoration in one place, you will know that this is adding a function. If you write proxy, you will know that it is restricted. Although the code is likely to be the same, if you call them decoration, others will be confused. (Transfer)

Not yet resumed. please correct me if there is anything wrong.