我们在ActivityStack类中可以看到某个方法用于返回当前任务栈顶端的ActivityRecord对象，如果栈是空的，就返回null，ActivityRecord用于代表在历史任务栈中的一个Activity对象的信息。
/**
* Returns the top activity in any existing task matching the given
* Intent. Returns null if no such task is found.
*/
private ActivityRecord findTaskLocked(Intent intent, ActivityInfo info) {
ComponentName cls = intent.getComponent();
if (info.targetActivity != null) {
cls = new ComponentName(info.packageName, info.targetActivity);
}
TaskRecord cp = null;
final int N = mHistory.size();
for (int i=(N-1); i>=0; i--) {
ActivityRecord r = (ActivityRecord)mHistory.get(i);
if (!r.finishing && r.task != cp
&& r.launchMode != ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
cp = r.task;
//Slog.i(TAG, "Comparing existing cls=" + r.task.intent.getComponent().flattenToShortString()
// + "/aff=" + r.task.affinity + " to new cls="
// + intent.getComponent().flattenToShortString() + "/aff=" + taskAffinity);
if (r.task.affinity != null) {
if (r.task.affinity.equals(info.taskAffinity)) {
//Slog.i(TAG, "Found matching affinity!");
return r;
}
} else if (r.task.intent != null
&& r.task.intent.getComponent().equals(cls)) {
//Slog.i(TAG, "Found matching class!");
//dump();
//Slog.i(TAG, "For Intent " + intent + " bringing to top: " + r.intent);
return r;
} else if (r.task.affinityIntent != null
&& r.task.affinityIntent.getComponent().equals(cls)) {
//Slog.i(TAG, "Found matching class!");
//dump();
//Slog.i(TAG, "For Intent " + intent + " bringing to top: " + r.intent);
return r;
}
}
}
return null;
贴一下ActivityRecord类的信息：
/**
* An entry in the history stack, representing an activity.
*/
class ActivityRecord extends IApplicationToken.Stub {
final ActivityManagerService service; // owner
final ActivityStack stack; // owner
final ActivityInfo info; // all about me
final int launchedFromUid; // always the uid who started the activity.
final Intent intent; // the original intent that generated us
final ComponentName realActivity; // the intent component, or target of an alias.
final String shortComponentName; // the short component name of the intent
final String resolvedType; // as per original caller;
final String packageName; // the package implementing intent's component
final String processName; // process where this component wants to run
final String taskAffinity; // as per ActivityInfo.taskAffinity
final boolean stateNotNeeded; // As per ActivityInfo.flags
final boolean fullscreen; // covers the full screen?
final boolean componentSpecified; // did caller specifiy an explicit component?
final boolean isHomeActivity; // do we consider this to be a home activity?
final String baseDir; // where activity source (resources etc) located
final String resDir; // where public activity source (public resources etc) located
final String dataDir; // where activity data should go
CharSequence nonLocalizedLabel; // the label information from the package mgr.
int labelRes; // the label information from the package mgr.
int icon; // resource identifier of activity's icon.
int theme; // resource identifier of activity's theme.
TaskRecord task; // the task this is in.
long launchTime; // when we starting launching this activity
long startTime; // last time this activity was started
long cpuTimeAtResume; // the cpu time of host process at the time of resuming activity
Configuration configuration; // configuration activity was last running in
ActivityRecord resultTo; // who started this entry, so will get our reply
final String resultWho; // additional identifier for use by resultTo.
final int requestCode; // code given by requester (resultTo)
ArrayList results; // pending ActivityResult objs we have received
HashSet<WeakReference<PendingIntentRecord>> pendingResults; // all pending intents for this act
ArrayList newIntents; // any pending new intents for single-top mode
HashSet<ConnectionRecord> connections; // All ConnectionRecord we hold
UriPermissionOwner uriPermissions; // current special URI access perms.
ProcessRecord app; // if non-null, hosting application
Bitmap thumbnail; // icon representation of paused screen
CharSequence description; // textual description of paused screen
ActivityState state; // current state we are in
Bundle icicle; // last saved activity state
boolean frontOfTask; // is this the root activity of its task?
boolean launchFailed; // set if a launched failed, to abort on 2nd try
boolean haveState; // have we gotten the last activity state?
boolean stopped; // is activity pause finished?
boolean delayedResume; // not yet resumed because of stopped app switches?
boolean finishing; // activity in pending finish list?
boolean configDestroy; // need to destroy due to config change?
int configChangeFlags; // which config values have changed
boolean keysPaused; // has key dispatching been paused for it?
boolean inHistory; // are we in the history stack?
int launchMode; // the launch mode activity attribute.
boolean visible; // does this activity's window need to be shown?
boolean waitingVisible; // true if waiting for a new act to become vis
boolean nowVisible; // is this activity's window visible?
boolean thumbnailNeeded;// has someone requested a thumbnail?
boolean idle; // has the activity gone idle?
boolean hasBeenLaunched;// has this activity ever been launched?
boolean frozenBeforeDestroy;// has been frozen but not yet destroyed.
String stringName; // for caching of toString().
