How does activitygroup toggle animation work

**1 provides pseudo-code, only to illustrate their understanding, train of thought **/
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/ * * /
AActivity BActivity, CActivity, DActivity extends ActivityGroup.
A1Activity B1Activity, C1Activity, D1Activity extends Activity.
A2Activity B2Activity, C2Activity, D2Activity extends Activity

/*1, AActivity inherits Activity, and when startActivity() is called, contentView displays A1Activity*/ inside tab
AActivity.this.startActivity(A1Activity.class)
BActivity.this.startActivity(B1Activity.class)
CActivity.this.startActivity(C1Activity.class)
DActivity.this.startActivity(D1Activity.class)

/*2, A1Activity does not inherit ActivityGroup, so startActivity() is no different from a normal call, A2 will fill the window */
A1Activity.this.startActivity(A2Activity.class)
B1Activity.this.startActivity(B2Activity.class)
C1Activity.this.startActivity(C2Activity.class)
D1Activity.this.startActivity(D2Activity.class)

/ * 3, A1Activity. this. getParent AActivity () to obtain, is actually ActivityGroup; Although A1Activity does not inherit ActivityGroup, A2Activity is actually started by AActivity, the parent class of A1Activity, so A2Activity starts as much as 1 */
A1Activity.this.getParent().startActivity(A2Activity.class)
B1Activity.this.getParent().startActivity(B2Activity.class)
C1Activity.this.getParent().startActivity(C2Activity.class)
D1Activity.this.getParent().startActivity(D2Activity.class)