Android(安卓)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();

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