Code analysis of Android Adapter (Adapter) and Observer (Observer) patterns (picture)

Adapter details: //m.sbmmt.com/java-article-355851.html

Observer details: //m.sbmmt.com /java-article-356024.html

AdapterView system

AdapterView will use Adapter

Adapter system

BaseAdapter implements SpinnerAdapter and ListAdapter

This form is the application of the adapter pattern, which is adopted here It is an interface adapter

Observer mode

The method of registering and unregistering DataSetObserver is defined in the Adapter

public interface Adapter {
  
    void registerDataSetObserver(DataSetObserver observer);

    void unregisterDataSetObserver(DataSetObserver observer);

    ...
}

public abstract class DataSetObserver {

    public void onChanged() {
        // Do nothing
    }

    public void onInvalidated() {
        // Do nothing
    }
}

public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {
    private final DataSetObservable mDataSetObservable = new DataSetObservable();

    public void registerDataSetObserver(DataSetObserver observer) {
        mDataSetObservable.registerObserver(observer);
    }

    public void unregisterDataSetObserver(DataSetObserver observer) {
        mDataSetObservable.unregisterObserver(observer);
    }

    public void notifyDataSetChanged() {
        mDataSetObservable.notifyChanged();
    }

    public void notifyDataSetInvalidated() {
        mDataSetObservable.notifyInvalidated();
    }
    ...
}

public class DataSetObservable extends Observable<DataSetObserver> {

    public void notifyChanged() {
        synchronized(mObservers) {
            for (int i = mObservers.size() - 1; i >= 0; i--) {
                mObservers.get(i).onChanged();
            }
        }
    }

    public void notifyInvalidated() {
        synchronized (mObservers) {
            for (int i = mObservers.size() - 1; i >= 0; i--) {
                mObservers.get(i).onInvalidated();
            }
        }
    }
}

public abstract class Observable<T> {

    protected final ArrayList<T> mObservers = new ArrayList<T>();

    public void registerObserver(T observer) {
        if (observer == null) {
            throw new IllegalArgumentException("The observer is null.");
        }
        synchronized(mObservers) {
            if (mObservers.contains(observer)) {
                throw new IllegalStateException("Observer " + observer + " is already registered.");
            }
            mObservers.add(observer);
        }
    }

    public void unregisterObserver(T observer) {
        if (observer == null) {
            throw new IllegalArgumentException("The observer is null.");
        }
        synchronized(mObservers) {
            int index = mObservers.indexOf(observer);
            if (index == -1) {
                throw new IllegalStateException("Observer " + observer + " was not registered.");
            }
            mObservers.remove(index);
        }
    }

    public void unregisterAll() {
        synchronized(mObservers) {
            mObservers.clear();
        }
    }
}

DataSetObserver Observer (observe data set: changed onChanged, invalid onInvalidated)

Observable handles observers (add, remove, register, unregister)

DataSetObservable extends Observable4864d1769940fac3918b0cc0f2557122 Added two methods to notify data changes and invalidity

In AdapterView

class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;

        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = getAdapter().getCount();

            // Detect the case where a cursor that was previously invalidated has
            // been repopulated with new data.
            if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
                    && mOldItemCount == 0 && mItemCount > 0) {
                AdapterView.this.onRestoreInstanceState(mInstanceState);
                mInstanceState = null;
            } else {
                rememberSyncState();
            }
            checkFocus();
            requestLayout();
        }

        @Override
        public void onInvalidated() {
            mDataChanged = true;

            if (AdapterView.this.getAdapter().hasStableIds()) {
                // Remember the current state for the case where our hosting activity is being
                // stopped and later restarted
                mInstanceState = AdapterView.this.onSaveInstanceState();
            }

            // Data is invalid so we should reset our state
            mOldItemCount = mItemCount;
            mItemCount = 0;
            mSelectedPosition = INVALID_POSITION;
            mSelectedRowId = INVALID_ROW_ID;
            mNextSelectedPosition = INVALID_POSITION;
            mNextSelectedRowId = INVALID_ROW_ID;
            mNeedSync = false;

            checkFocus();
            requestLayout();
        }

        public void clearSavedState() {
            mInstanceState = null;
        }
    }

Customized AdapterDataSetObserver, requestLayout() is called in both implementation methods to reset the layout

The entire process of observing data changes leading to interface changes:
1. Customize the AdapterDataSetObserver in the AdapterView system and register it (that is, add the observer to the observable).

2. After calling adapter.notifyDataSetChanged(), observable's notifyChanged() is executed internally,

will execute each observer's onChanged() also achieves the effect of updating the interface.

notifyDataSetInvalidated is the same as

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