android - 保存的游戏快照初始化(空对象引用)

Question

我正在尝试使用新的 Saved Games (Snapshot) API

但我一直收到这个错误:

java.lang.NullPointerException: Attempt to invoke interface method 'boolean com.google.android.gms.games.snapshot.Snapshot.writeBytes(byte[])' on a null object reference

这是我正在尝试的代码

Snapshot snapshot = new Snapshot();
snapshot.writeBytes(my_app_state);

// Save the snapshot.
SnapshotMetadataChange metadataChange
        = new SnapshotMetadataChange.Builder()
        .setDescription("Completed %" + (levelscompleted/totallevels) + " of levels.")
        .build();
Games.Snapshots.commitAndClose(getApiClient(), snapshot,
        metadataChange);

我正在尝试弄清楚如何初始化快照，因为 = new Snapshot(); 不起作用。

Answer 1

Snapshot API 对您必须如何做事有非常明确的规定。以下是快照保存的高级过程:

• 打开快照
• 解决冲突
• 保存

以下代码来自Google Play Games Snapshot sample其中向您展示了如何在 Android 和 iOS 之间跨平台使用 Snapshots。

首先，您必须打开快照并解决打开时的冲突。

/**
 * Prepares saving Snapshot to the user's synchronized storage, conditionally resolves errors,
 * and stores the Snapshot.
 */
void saveSnapshot() {
    AsyncTask<Void, Void, Snapshots.OpenSnapshotResult> task =
            new AsyncTask<Void, Void, Snapshots.OpenSnapshotResult>() {
                @Override
                protected Snapshots.OpenSnapshotResult doInBackground(Void... params) {
                    Snapshots.OpenSnapshotResult result = Games.Snapshots.open(getApiClient(),
                            currentSaveName, true).await();
                    return result;
                }

                @Override
                protected void onPostExecute(Snapshots.OpenSnapshotResult result) {
                    Snapshot toWrite = processSnapshotOpenResult(result, 0);

                    Log.i(TAG, writeSnapshot(toWrite));
                }
            };

    task.execute();
}

接下来，您必须处理冲突解决:

/**
 * Conflict resolution for when Snapshots are opened.
 * @param result The open snapshot result to resolve on open.
 * @return The opened Snapshot on success; otherwise, returns null.
 */
Snapshot processSnapshotOpenResult(Snapshots.OpenSnapshotResult result, int retryCount){
    Snapshot mResolvedSnapshot = null;
    retryCount++;
    int status = result.getStatus().getStatusCode();

    Log.i(TAG, "Save Result status: " + status);

    if (status == GamesStatusCodes.STATUS_OK) {
        return result.getSnapshot();
    } else if (status == GamesStatusCodes.STATUS_SNAPSHOT_CONTENTS_UNAVAILABLE) {
        return result.getSnapshot();
    } else if (status == GamesStatusCodes.STATUS_SNAPSHOT_CONFLICT){
        Snapshot snapshot = result.getSnapshot();
        Snapshot conflictSnapshot = result.getConflictingSnapshot();

        // Resolve between conflicts by selecting the newest of the conflicting snapshots.
        mResolvedSnapshot = snapshot;

        if (snapshot.getMetadata().getLastModifiedTimestamp() <
                conflictSnapshot.getMetadata().getLastModifiedTimestamp()){
            mResolvedSnapshot = conflictSnapshot;
        }

        Snapshots.OpenSnapshotResult resolveResult = Games.Snapshots.resolveConflict(
                getApiClient(), result.getConflictId(), mResolvedSnapshot)
                .await();

        if (retryCount < MAX_SNAPSHOT_RESOLVE_RETRIES){
            return processSnapshotOpenResult(resolveResult, retryCount);
        }else{
            String message = "Could not resolve snapshot conflicts";
            Log.e(TAG, message);
            Toast.makeText(getBaseContext(), message, Toast.LENGTH_LONG);
        }

    }
    // Fail, return null.
    return null;
}

以下代码是如何在 the Google Play Games Snapshots sample app 中完成的:

/**
 * Generates metadata, takes a screenshot, and performs the write operation for saving a
 * snapshot.
 */
private String writeSnapshot(Snapshot snapshot){
    // Set the data payload for the snapshot.
    snapshot.writeBytes(mSaveGame.toBytes());

    // Save the snapshot.
    SnapshotMetadataChange metadataChange = new SnapshotMetadataChange.Builder()
            .setCoverImage(getScreenShot())
            .setDescription("Modified data at: " + Calendar.getInstance().getTime())
            .build();
    Games.Snapshots.commitAndClose(getApiClient(), snapshot, metadataChange);
    return snapshot.toString();
}