Android startActivity流程

startActivity(new Intent(this,MainActivity.class));

    /**
     * Same as {@link #startActivity(Intent, Bundle)} with no options
     * specified.
     *
     * @param intent The intent to start.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see #startActivity(Intent, Bundle)
     * @see #startActivityForResult
     */
    @Override
    public void startActivity(Intent intent) {
        this.startActivity(intent, null);
    }

    /**
     * Launch a new activity.  You will not receive any information about when
     * the activity exits.  This implementation overrides the base version,
     * providing information about
     * the activity performing the launch.  Because of this additional
     * information, the {@link Intent#FLAG_ACTIVITY_NEW_TASK} launch flag is not
     * required; if not specified, the new activity will be added to the
     * task of the caller.
     *
     * <p>This method throws {@link android.content.ActivityNotFoundException}
     * if there was no Activity found to run the given Intent.
     *
     * @param intent The intent to start.
     * @param options Additional options for how the Activity should be started.
     * See {@link android.content.Context#startActivity(Intent, Bundle)}
     * Context.startActivity(Intent, Bundle)} for more details.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see #startActivity(Intent)
     * @see #startActivityForResult
     */
    @Override
    public void startActivity(Intent intent, @Nullable Bundle options) {
        getAutofillClientController().onStartActivity(intent, mIntent);
        if (options != null) {
            startActivityForResult(intent, -1, options);
        } else {
            // Note we want to go through this call for compatibility with
            // applications that may have overridden the method.
            startActivityForResult(intent, -1);
        }
    }

    /**
     * Same as calling {@link #startActivityForResult(Intent, int, Bundle)}
     * with no options.
     *
     * @param intent The intent to start.
     * @param requestCode If >= 0, this code will be returned in
     *                    onActivityResult() when the activity exits.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see #startActivity
     */
    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {
        startActivityForResult(intent, requestCode, null);
    }

    /**
     * Launch an activity for which you would like a result when it finished.
     * When this activity exits, your
     * onActivityResult() method will be called with the given requestCode.
     * Using a negative requestCode is the same as calling
     * {@link #startActivity} (the activity is not launched as a sub-activity).
     *
     * <p>Note that this method should only be used with Intent protocols
     * that are defined to return a result.  In other protocols (such as
     * {@link Intent#ACTION_MAIN} or {@link Intent#ACTION_VIEW}), you may
     * not get the result when you expect.  For example, if the activity you
     * are launching uses {@link Intent#FLAG_ACTIVITY_NEW_TASK}, it will not
     * run in your task and thus you will immediately receive a cancel result.
     *
     * <p>As a special case, if you call startActivityForResult() with a requestCode
     * >= 0 during the initial onCreate(Bundle savedInstanceState)/onResume() of your
     * activity, then your window will not be displayed until a result is
     * returned back from the started activity.  This is to avoid visible
     * flickering when redirecting to another activity.
     *
     * <p>This method throws {@link android.content.ActivityNotFoundException}
     * if there was no Activity found to run the given Intent.
     *
     * @param intent The intent to start.
     * @param requestCode If >= 0, this code will be returned in
     *                    onActivityResult() when the activity exits.
     * @param options Additional options for how the Activity should be started.
     * See {@link android.content.Context#startActivity(Intent, Bundle)}
     * Context.startActivity(Intent, Bundle)} for more details.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see #startActivity
     */
    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);
            if (ar != null) {
                mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
            }
            if (requestCode >= 0) {
                // If this start is requesting a result, we can avoid making
                // the activity visible until the result is received.  Setting
                // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
                // activity hidden during this time, to avoid flickering.
                // This can only be done when a result is requested because
                // that guarantees we will get information back when the
                // activity is finished, no matter what happens to it.
                mStartedActivity = true;
            }
 
            cancelInputsAndStartExitTransition(options);
            // TODO Consider clearing/flushing other event sources and events for child windows.
        } else {
            if (options != null) {
                mParent.startActivityFromChild(this, intent, requestCode, options);
            } else {
                // Note we want to go through this method for compatibility with
                // existing applications that may have overridden it.
                mParent.startActivityFromChild(this, intent, requestCode);
            }
        }
    }

    /**
     * Execute a startActivity call made by the application.  The default 
     * implementation takes care of updating any active {@link ActivityMonitor}
     * objects and dispatches this call to the system activity manager; you can
     * override this to watch for the application to start an activity, and 
     * modify what happens when it does. 
     *
     * <p>This method returns an {@link ActivityResult} object, which you can 
     * use when intercepting application calls to avoid performing the start 
     * activity action but still return the result the application is 
     * expecting.  To do this, override this method to catch the call to start 
     * activity so that it returns a new ActivityResult containing the results 
     * you would like the application to see, and don't call up to the super 
     * class.  Note that an application is only expecting a result if 
     * <var>requestCode</var> is &gt;= 0.
     *
     * <p>This method throws {@link android.content.ActivityNotFoundException}
     * if there was no Activity found to run the given Intent.
     *
     * @param who The Context from which the activity is being started.
     * @param contextThread The main thread of the Context from which the activity
     *                      is being started.
     * @param token Internal token identifying to the system who is starting 
     *              the activity; may be null.
     * @param target Which activity is performing the start (and thus receiving 
     *               any result); may be null if this call is not being made
     *               from an activity.
     * @param intent The actual Intent to start.
     * @param requestCode Identifier for this request's result; less than zero 
     *                    if the caller is not expecting a result.
     * @param options Addition options.
     *
     * @return To force the return of a particular result, return an 
     *         ActivityResult object containing the desired data; otherwise
     *         return null.  The default implementation always returns null.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see Activity#startActivity(Intent)
     * @see Activity#startActivityForResult(Intent, int)
     *
     * {@hide}
     */
    @UnsupportedAppUsage
    public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        Uri referrer = target != null ? target.onProvideReferrer() : null;
        if (referrer != null) {
            intent.putExtra(Intent.EXTRA_REFERRER, referrer);
        }
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    final ActivityMonitor am = mActivityMonitors.get(i);
                    ActivityResult result = null;
                    if (am.ignoreMatchingSpecificIntents()) {
                        if (options == null) {
                            options = ActivityOptions.makeBasic().toBundle();
                        }
                        result = am.onStartActivity(who, intent, options);
                    }
                    if (result != null) {
                        am.mHits++;
                        return result;
                    } else if (am.match(who, null, intent)) {
                        am.mHits++;
                        if (am.isBlocking()) {
                            return requestCode >= 0 ? am.getResult() : null;
                        }
                        break;
                    }
                }
            }
        }
        try {
            intent.migrateExtraStreamToClipData(who);
            intent.prepareToLeaveProcess(who);
            int result = ActivityTaskManager.getService().startActivity(whoThread,
                    who.getOpPackageName(), who.getAttributionTag(), intent,
                    intent.resolveTypeIfNeeded(who.getContentResolver()), token,
                    target != null ? target.mEmbeddedID : null, requestCode, 0, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }

    /**
     * This is called when a child activity of this one calls its
     * {@link #startActivity} or {@link #startActivityForResult} method.
     *
     * <p>This method throws {@link android.content.ActivityNotFoundException}
     * if there was no Activity found to run the given Intent.
     *
     * @param child The activity making the call.
     * @param intent The intent to start.
     * @param requestCode Reply request code.  < 0 if reply is not requested.
     * @param options Additional options for how the Activity should be started.
     * See {@link android.content.Context#startActivity(Intent, Bundle)}
     * Context.startActivity(Intent, Bundle)} for more details.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see #startActivity
     * @see #startActivityForResult
     * @deprecated Use {@code androidx.fragment.app.FragmentActivity#startActivityFromFragment(
     * androidx.fragment.app.Fragment,Intent,int,Bundle)}
     */
    @Deprecated
    public void startActivityFromChild(@NonNull Activity child, @RequiresPermission Intent intent,
            int requestCode, @Nullable Bundle options) {
        options = transferSpringboardActivityOptions(options);
        Instrumentation.ActivityResult ar =
            mInstrumentation.execStartActivity(
                this, mMainThread.getApplicationThread(), mToken, child,
                intent, requestCode, options);
        if (ar != null) {
            mMainThread.sendActivityResult(
                mToken, child.mEmbeddedID, requestCode,
                ar.getResultCode(), ar.getResultData());
        }
        cancelInputsAndStartExitTransition(options);
    }

@Override
    public final int startActivity(IApplicationThread caller, String callingPackage,
            String callingFeatureId, Intent intent, String resolvedType, IBinder resultTo,
            String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo,
            Bundle bOptions) {
        return startActivityAsUser(caller, callingPackage, callingFeatureId, intent, resolvedType,
                resultTo, resultWho, requestCode, startFlags, profilerInfo, bOptions,
                UserHandle.getCallingUserId());
    }

@Override
    public int startActivityAsUser(IApplicationThread caller, String callingPackage,
            String callingFeatureId, Intent intent, String resolvedType, IBinder resultTo,
            String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo,
            Bundle bOptions, int userId) {
        return startActivityAsUser(caller, callingPackage, callingFeatureId, intent, resolvedType,
                resultTo, resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
                true /*validateIncomingUser*/);
    }

private int startActivityAsUser(IApplicationThread caller, String callingPackage,
            @Nullable String callingFeatureId, Intent intent, String resolvedType,
            IBinder resultTo, String resultWho, int requestCode, int startFlags,
            ProfilerInfo profilerInfo, Bundle bOptions, int userId, boolean validateIncomingUser) {
        assertPackageMatchesCallingUid(callingPackage);
        enforceNotIsolatedCaller("startActivityAsUser");
 
        userId = getActivityStartController().checkTargetUser(userId, validateIncomingUser,
                Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");
 
        // TODO: Switch to user app stacks here.
        return getActivityStartController().obtainStarter(intent, "startActivityAsUser")
                .setCaller(caller)
                .setCallingPackage(callingPackage)
                .setCallingFeatureId(callingFeatureId)
                .setResolvedType(resolvedType)
                .setResultTo(resultTo)
                .setResultWho(resultWho)
                .setRequestCode(requestCode)
                .setStartFlags(startFlags)
                .setProfilerInfo(profilerInfo)
                .setActivityOptions(bOptions)
                .setUserId(userId)
                .execute();
 
    }

ActivityStartController getActivityStartController() {
        return mActivityStartController;
    }

ActivityStarter obtainStarter(Intent intent, String reason) {
        return mFactory.obtain().setIntent(intent).setReason(reason);
    }

 
    /**
     * Resolve necessary information according the request parameters provided earlier, and execute
     * the request which begin the journey of starting an activity.
     * @return The starter result.
     */
    int execute() {
        try {
            // Refuse possible leaked file descriptors
            if (mRequest.intent != null && mRequest.intent.hasFileDescriptors()) {
                throw new IllegalArgumentException("File descriptors passed in Intent");
            }
 
            final LaunchingState launchingState;
            synchronized (mService.mGlobalLock) {
                final ActivityRecord caller = ActivityRecord.forTokenLocked(mRequest.resultTo);
                final int callingUid = mRequest.realCallingUid == Request.DEFAULT_REAL_CALLING_UID
                        ?  Binder.getCallingUid() : mRequest.realCallingUid;
                launchingState = mSupervisor.getActivityMetricsLogger().notifyActivityLaunching(
                        mRequest.intent, caller, callingUid);
            }
 
            // If the caller hasn't already resolved the activity, we're willing
            // to do so here. If the caller is already holding the WM lock here,
            // and we need to check dynamic Uri permissions, then we're forced
            // to assume those permissions are denied to avoid deadlocking.
            if (mRequest.activityInfo == null) {
                mRequest.resolveActivity(mSupervisor);
            }
 
            // Add checkpoint for this shutdown or reboot attempt, so we can record the original
            // intent action and package name.
            if (mRequest.intent != null) {
                String intentAction = mRequest.intent.getAction();
                String callingPackage = mRequest.callingPackage;
                if (intentAction != null && callingPackage != null
                        && (Intent.ACTION_REQUEST_SHUTDOWN.equals(intentAction)
                                || Intent.ACTION_SHUTDOWN.equals(intentAction)
                                || Intent.ACTION_REBOOT.equals(intentAction))) {
                    ShutdownCheckPoints.recordCheckPoint(intentAction, callingPackage, null);
                }
            }
 
            int res;
            synchronized (mService.mGlobalLock) {
                final boolean globalConfigWillChange = mRequest.globalConfig != null
                        && mService.getGlobalConfiguration().diff(mRequest.globalConfig) != 0;
                final Task rootTask = mRootWindowContainer.getTopDisplayFocusedRootTask();
                if (rootTask != null) {
                    rootTask.mConfigWillChange = globalConfigWillChange;
                }
                ProtoLog.v(WM_DEBUG_CONFIGURATION, "Starting activity when config "
                        + "will change = %b", globalConfigWillChange);
 
                final long origId = Binder.clearCallingIdentity();
 
                res = resolveToHeavyWeightSwitcherIfNeeded();
                if (res != START_SUCCESS) {
                    return res;
                }
                res = executeRequest(mRequest);
 
                Binder.restoreCallingIdentity(origId);
 
                if (globalConfigWillChange) {
                    // If the caller also wants to switch to a new configuration, do so now.
                    // This allows a clean switch, as we are waiting for the current activity
                    // to pause (so we will not destroy it), and have not yet started the
                    // next activity.
                    mService.mAmInternal.enforceCallingPermission(
                            android.Manifest.permission.CHANGE_CONFIGURATION,
                            "updateConfiguration()");
                    if (rootTask != null) {
                        rootTask.mConfigWillChange = false;
                    }
                    ProtoLog.v(WM_DEBUG_CONFIGURATION,
                                "Updating to new configuration after starting activity.");
 
                    mService.updateConfigurationLocked(mRequest.globalConfig, null, false);
                }
 
                // The original options may have additional info about metrics. The mOptions is not
                // used here because it may be cleared in setTargetRootTaskIfNeeded.
                final ActivityOptions originalOptions = mRequest.activityOptions != null
                        ? mRequest.activityOptions.getOriginalOptions() : null;
                // If the new record is the one that started, a new activity has created.
                final boolean newActivityCreated = mStartActivity == mLastStartActivityRecord;
                // Notify ActivityMetricsLogger that the activity has launched.
                // ActivityMetricsLogger will then wait for the windows to be drawn and populate
                // WaitResult.
                mSupervisor.getActivityMetricsLogger().notifyActivityLaunched(launchingState, res,
                        newActivityCreated, mLastStartActivityRecord, originalOptions);
                if (mRequest.waitResult != null) {
                    mRequest.waitResult.result = res;
                    res = waitResultIfNeeded(mRequest.waitResult, mLastStartActivityRecord,
                            launchingState);
                }
                return getExternalResult(res);
            }
        } finally {
            onExecutionComplete();
        }
    }