Fragment和FragmentManager已经Activity三者之间的研究

对Fragment进行添加、删除、替换就要使用FragmentTransaction来实现。

getFragmentManager().beginTransaction()就是获取相关Activity的 FragmentTransaction.
add(int containerViewId, Fragment fragment);用来添加Fragment到指定的容器内，这个容器可以是FrameLayout也可以是其他。
add(int containerViewId, Fragment fragment);用来替换Fragment到指定的容器内，这个容器可以是FrameLayout也可以是其他。
remove(Fragment fragment);)用来删除当前activity的Fragment
上面的过程到底是如何实现的呢？，我们来研究一下相关的具体的源代码。

关于Activity中的Fragmentmanager到底是什么？见Code：
Activity中定义了 FragmentManager的实例：

1	final FragmentManagerImpl mFragments = new FragmentManagerImpl ( ) ;

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final class FragmentManagerImpl extends FragmentManager {      static boolean DEBUG = false ;      static final String TAG = "FragmentManager" ;           static final String TARGET_REQUEST_CODE_STATE_TAG = "android:target_req_state" ;      static final String TARGET_STATE_TAG = "android:target_state" ;      static final String VIEW_STATE_TAG = "android:view_state" ;      static final String USER_VISIBLE_HINT_TAG = "android:user_visible_hint" ;        ArrayList < Runnable > mPendingActions ;      Runnable [ ] mTmpActions ;      boolean mExecutingActions ;           ArrayList < Fragment > mActive ;      ArrayList < Fragment > mAdded ;      ArrayList < Integer > mAvailIndices ;      ArrayList < BackStackRecord > mBackStack ;      ArrayList < Fragment > mCreatedMenus ;           // Must be accessed while locked.      ArrayList < BackStackRecord > mBackStackIndices ;      ArrayList < Integer > mAvailBackStackIndices ;

可以发现 FragmentManager其实就是一个容器用来存储Fragments。见：

1 2	ArrayList < Fragment > mActive ; ArrayList < Fragment > mAdded ;

存储和获取Fragment的code：

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@ Override      public void putFragment ( Bundle bundle , String key , Fragment fragment ) {          if ( fragment . mIndex < 0 ) {              throwException ( new IllegalStateException ( "Fragment " + fragment                      + " is not currently in the FragmentManager" ) ) ;          }          bundle . putInt ( key , fragment . mIndex ) ;      }        @ Override      public Fragment getFragment ( Bundle bundle , String key ) {          int index = bundle . getInt ( key , - 1 ) ;          if ( index == - 1 ) {              return null ;          }          if ( index >= mActive . size ( ) ) {              throwException ( new IllegalStateException ( "Fragement no longer exists for key "                      + key + ": index " + index ) ) ;          }          Fragment f = mActive . get ( index ) ;          if ( f == null ) {              throwException ( new IllegalStateException ( "Fragement no longer exists for key "                      + key + ": index " + index ) ) ;          }          return f ;      }

FragmentManger.beginFragmentTransaction()做了什么：

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@ Override      public FragmentTransaction beginTransaction ( ) {          return new BackStackRecord ( this ) ;      }

可以发现，这里的this就是Actvity中具体的FragmentManager,关于BackStackRecord 的实现：

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final class BackStackRecord extends FragmentTransaction implements FragmentManager . BackStackEntry , Runnable { static final String TAG = FragmentManagerImpl . TAG ; final FragmentManagerImpl mManager ;

当调用添加FragmentTransaction的add接口的时候，具体实现如下：

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public FragmentTransaction add ( int containerViewId , Fragment fragment ) { doAddOp ( containerViewId , fragment , null , OP_ADD ) ; return this ; }

doAddOp方法的具体实现：

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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 ) ; }

addOp方法的具体实现：

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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 ++ ; }

关于Op这个class的定义和结构：

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static final class Op { Op next ; Op prev ; int cmd ; Fragment fragment ; int enterAnim ; int exitAnim ; int popEnterAnim ; int popExitAnim ; ArrayList removed ; }

由上面的步骤就可以发现。FragmentManager 是始终贯穿在 Activity、FragementTransaction之间，用来存储Fragments。至于添加和删除的具体实现，大家去读读源代码吧。