Fragment.java
public View onCreateView(LayoutInflater inflater,@Nullable ViewGroup container,@Nullable Bundle savedInstanceState) {
 return null;
}

View performCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {
 if (mChildFragmentManager != null) {
  mChildFragmentManager.noteStateNotSaved();
 }
 return onCreateView(inflater,container,savedInstanceState);
}

FragmentManager.java
void movetoState(Fragment f,int newState,int transit,int transitionStyle,boolean keepActive) {
 switch (f.mState) {
 case Fragment.INITIALIZING:
  if (f.mFromLayout) {
   f.mView = f.performCreateView(f.getLayoutInflater(
   f.mSavedFragmentState),null,f.mSavedFragmentState);
 }
 break;
case Fragment.CREATED:
 if (!f.mFromLayout) {
  f.mView = f.performCreateView(f.getLayoutInflater(
   f.mSavedFragmentState),f.mSavedFragmentState);
 }
break;
} 
}

FragmentActivity.java
//FragmentManagerImpl is subclass of FragmentManager
final FragmentManagerImpl mFragments = new FragmentManagerImpl();
public FragmentManager getSupportFragmentManager() {
 return mFragments;
}

FragmentManager.java
public abstract FragmentTransaction beginTransaction();
FragmentManagerImpl extends FragmentManager
@Override
public FragmentTransaction beginTransaction() {
 return new BackStackRecord(this);
}
/**
* Static library support version of the framework's {@link android.app.FragmentTransaction}.
* Used to write apps that run on platforms prior to Android 3.0. When running
* on Android 3.0 or above,this implementation is still used; it does not try
* to switch to the framework's implementation. See the framework SDK
* documentation for a class overview.
*/
public abstract class FragmentTransaction

BackStackRecord.java
final class BackStackRecord extends FragmentTransaction implements FragmentManager.BackStackEntry,Runnable 
final FragmentManagerImpl mManager;
public BackStackRecord(FragmentManagerImpl manager) {
 mManager = manager;
}

public FragmentTransaction add(Fragment fragment,String tag) {
 doAddOp(0,fragment,tag,OP_ADD);
 return this;
}
private void doAddOp(int containerViewId,Fragment fragment,String tag,int opcmd) {
 fragment.mFragmentManager = mManager;
 if (tag != null) {
  if (fragment.mTag != null && !tag.equals(fragment.mTag)) {
   throw new IllegalStateException("Can't change tag of fragment "
     + fragment + ": was " + fragment.mTag
     + " Now " + tag);
  }
  fragment.mTag = tag;
 }
 if (containerViewId != 0) {
  if (fragment.mFragmentId != 0 && fragment.mFragmentId != containerViewId) {
   throw new IllegalStateException("Can't change container ID of fragment "
     + fragment + ": was " + fragment.mFragmentId
     + " Now " + containerViewId);
  }
  fragment.mContainerId = fragment.mFragmentId = containerViewId;
 }
 Op op = new Op();
 op.cmd = opcmd;
 op.fragment = fragment;
 addOp(op);
}
void addOp(Op op) {
 if (mHead == null) {
  mHead = mTail = op;
 } else {
  op.prev = mTail;
  mTail.next = op;
  mTail = op;
 }
 op.enteranim = mEnteranim;
 op.exitAnim = mExitAnim;
 op.popEnteranim = mPopEnteranim;
 op.popExitAnim = mPopExitAnim;
 mNumOp++;
}

public int commit() {
 return commitInternal(false);
}
int commitInternal(boolean allowStateLoss) {
 if (mCommitted) throw new IllegalStateException("commit already called");
 mCommitted = true;
 if (mAddToBackStack) {
  mIndex = mManager.allocBackStackIndex(this);
 } else {
  mIndex = -1;
 }
 mManager.enqueueAction(this,allowStateLoss);
 return mIndex;
}

public void enqueueAction(Runnable action,boolean allowStateLoss) {
  if (!allowStateLoss) {
    checkStateLoss();
  }
  synchronized (this) {
    if (mDestroyed || mActivity == null) {
      throw new IllegalStateException("Activity has been destroyed");
    }
    if (mPendingActions == null) {
      mPendingActions = new ArrayList<Runnable>();
    }
    mPendingActions.add(action);
    if (mPendingActions.size() == 1) {
      mActivity.mHandler.removeCallbacks(mExecCommit);
      mActivity.mHandler.post(mExecCommit);
    }
  }
}

Runnable mExecCommit = new Runnable() {
  @Override
  public void run() {
    execPendingActions();
  }
};
mActivity.mHandler.post(mExecCommit); 说明它在主线程里执行了 mExecCommit 的 run 方法。别问我咋知道的。
execPendingActions() 方法稍微比较大，我把注释写在代码里。
public boolean execPendingActions() {
  if (mExecutingActions) {
    throw new IllegalStateException("Recursive entry to executePendingTransactions");
  }
  //如果不是在主线程，抛出一个异常。
  if (Looper.myLooper() != mActivity.mHandler.getLooper()) {
    throw new IllegalStateException("Must be called from main thread of process");
  }
  boolean didSomething = false;
  // 这里有一个 while true 循环。
  while (true) {
    int numActions;
    // 这里在一个同步语句块里，把上次 mPendingActions 里的元素转移到 mTmpActions 数组里。并且执行 run方法。执行谁的 run 方法呢？！就是 BackStackRecord ， 也就是 FragmentTransaction 。我在最后面贴了 BackStackRecord 的 run 方法。
    synchronized (this) {
      if (mPendingActions == null || mPendingActions.size() == 0) {
        break;
      }
      numActions = mPendingActions.size();
      if (mTmpActions == null || mTmpActions.length < numActions) {
        mTmpActions = new Runnable[numActions];
      }
      mPendingActions.toArray(mTmpActions);
      mPendingActions.clear();
      mActivity.mHandler.removeCallbacks(mExecCommit);
    }
    mExecutingActions = true;
    for (int i=0; i<numActions; i++) {
      mTmpActions[i].run();
      mTmpActions[i] = null;
    }
    mExecutingActions = false;
    didSomething = true;
  }
  // 这里有好几行代码，不知道干啥的，反正就是做了一些判断，最后可能会调用 startPendingDeferredFragments() 方法。
  if (mHavePendingDeferredStart) {
    boolean loadersRunning = false;
    for (int i=0; i<mActive.size(); i++) {
      Fragment f = mActive.get(i);
      if (f != null && f.mloaderManager != null) {
        loadersRunning |= f.mloaderManager.hasRunningLoaders();
      }
    }
    if (!loadersRunning) {
      mHavePendingDeferredStart = false;
      startPendingDeferredFragments();
    }
  }
  return didSomething;
}

void startPendingDeferredFragments() {
  if (mActive == null) return;
  for (int i=0; i<mActive.size(); i++) {
    Fragment f = mActive.get(i);
    if (f != null) {
      performPendingDeferredStart(f);
    }
  }
}

public void performPendingDeferredStart(Fragment f) {
  if (f.mDeferStart) {
    if (mExecutingActions) {
      // Wait until we're done executing our pending transactions
      mHavePendingDeferredStart = true;
      return;
    }
    f.mDeferStart = false;
    movetoState(f,mCurState,false);
  }
}

public void run() {
  if (FragmentManagerImpl.DEBUG) Log.v(TAG,"Run: " + this);
  if (mAddToBackStack) {
    if (mIndex < 0) {
      throw new IllegalStateException("addToBackStack() called after commit()");
    }
  }
  bumpBackStacknesting(1);
  TransitionState state = null;
  SparseArray<Fragment> firstOutFragments = null;
  SparseArray<Fragment> lastInFragments = null;
  if (SUPPORTS_TRANSITIONS) {
    firstOutFragments = new SparseArray<Fragment>();
    lastInFragments = new SparseArray<Fragment>();
    calculateFragments(firstOutFragments,lastInFragments);
    state = beginTransition(firstOutFragments,lastInFragments,false);
  }
  int transitionStyle = state != null ? 0 : mTransitionStyle;
  int transition = state != null ? 0 : mTransition;
  // 注意这里要开始 while 循环了，要遍历刚才咱们说的链表了。
  Op op = mHead;
  while (op != null) {
    int enteranim = state != null ? 0 : op.enteranim;
    int exitAnim = state != null ? 0 : op.exitAnim;
    switch (op.cmd) {
      // OP_ADD 很简单，mManager.addFragment(f,false); 其他的几个也类似，调用 mManager 相应的方法。
      case OP_ADD: {
        Fragment f = op.fragment;
        f.mNextAnim = enteranim;
        mManager.addFragment(f,false);
      } break;
      case OP_REPLACE: {
        Fragment f = op.fragment;
        if (mManager.mAdded != null) {
          for (int i=0; i<mManager.mAdded.size(); i++) {
            Fragment old = mManager.mAdded.get(i);
            if (FragmentManagerImpl.DEBUG) Log.v(TAG,"OP_REPLACE: adding=" + f + " old=" + old);
            if (f == null || old.mContainerId == f.mContainerId) {
              if (old == f) {
                op.fragment = f = null;
              } else {
                if (op.removed == null) {
                  op.removed = new ArrayList<Fragment>();
                }
                op.removed.add(old);
                old.mNextAnim = exitAnim;
                if (mAddToBackStack) {
                  old.mBackStacknesting += 1;
                  if (FragmentManagerImpl.DEBUG) Log.v(TAG,"Bump nesting of "
                      + old + " to " + old.mBackStacknesting);
                }
                mManager.removeFragment(old,transition,transitionStyle);
              }
            }
          }
        }
        if (f != null) {
          f.mNextAnim = enteranim;
          mManager.addFragment(f,false);
        }
      } break;
      case OP_REMOVE: {
        Fragment f = op.fragment;
        f.mNextAnim = exitAnim;
        mManager.removeFragment(f,transitionStyle);
      } break;
      case OP_HIDE: {
        Fragment f = op.fragment;
        f.mNextAnim = exitAnim;
        mManager.hideFragment(f,transitionStyle);
      } break;
      case OP_SHOW: {
        Fragment f = op.fragment;
        f.mNextAnim = enteranim;
        mManager.showFragment(f,transitionStyle);
      } break;
      case OP_DETACH: {
        Fragment f = op.fragment;
        f.mNextAnim = exitAnim;
        mManager.detachFragment(f,transitionStyle);
      } break;
      case OP_ATTACH: {
        Fragment f = op.fragment;
        f.mNextAnim = enteranim;
        mManager.attachFragment(f,transitionStyle);
      } break;
      default: {
        throw new IllegalArgumentException("UnkNown cmd: " + op.cmd);
      }
    }
    op = op.next;
  }
  // 最后还调用了movetoState() 这个方法。跟刚才的区别，看最后一个参数，一个true，一个false。
  // 而且注意，这行代码在 while 循环之后。
  mManager.movetoState(mManager.mCurState,transitionStyle,true);
  if (mAddToBackStack) {
    mManager.addBackStackState(this);
  }
}