Android FragmentManager之beginTransaction（）

  使用： 

    private Fragment fragment;
    private FragmentManager fm;
    fm = getSupportFragmentManager();
    fragment = fm.findFragmentById(R.id.fragment_container);
    fm.beginTransaction()
                    .add(R.id.fragment_container, fragment)
                    .commit();

beginTransaction源码：

 /**
     * Start a series of edit operations on the Fragments associated with
     * this FragmentManager.
     *
     * 
Note: A fragment transaction can only be created/committed prior
     * to an activity saving its state.  If you try to commit a transaction
     * after {@link FragmentActivity#onSaveInstanceState FragmentActivity.onSaveInstanceState()}
     * (and prior to a following {@link FragmentActivity#onStart FragmentActivity.onStart}
     * or {@link FragmentActivity#onResume FragmentActivity.onResume()}, you will get an error.
     * This is because the framework takes care of saving your current fragments
     * in the state, and if changes are made after the state is saved then they
     * will be lost.
     */
    public abstract FragmentTransaction beginTransaction();

add源码：

/**
     * Calls {@link #add(int, Fragment, String)} with a null tag.
     */
    public abstract FragmentTransaction add(@IdRes int containerViewId, Fragment fragment);

commit源码：

 /**
     * Schedules a commit of this transaction.  The commit does
     * not happen immediately; it will be scheduled as work on the main thread
     * to be done the next time that thread is ready.
     *
     * 
A transaction can only be committed with this method
     * prior to its containing activity saving its state.  If the commit is
     * attempted after that point, an exception will be thrown.  This is
     * because the state after the commit can be lost if the activity needs to
     * be restored from its state.  See {@link #commitAllowingStateLoss()} for
     * situations where it may be okay to lose the commit.
     * 
     * @return Returns the identifier of this transaction's back stack entry,
     * if {@link #addToBackStack(String)} had been called.  Otherwise, returns
     * a negative number.
     */
    public abstract int commit();