[Android]Fragment源代码分析(三) 事务

上一讲我们说到Fragment通过状态机的变更来生成内部的mView。当你使用的是非from layout.xml方式的时候,它会在Fragment.CREATED状态下搜索container相应的控件然后将mView增加到这个父控件中。

那么这个事务又在这里面承担什么样的作用呢？

我们先来看Manager.beginTransaction这种方法的返回值:

/**
     * Start a series of edit operations on the Fragments associated with this
     * FragmentManager.
     *
     * <p>
     * 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.
     * </p>
     */
    public abstract FragmentTransaction beginTransaction();

FragmentManagerImpl.java:
@Override
    public FragmentTransaction beginTransaction() {
        return new BackStackRecord(this);
    }

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 FragmentTransaction add(Fragment fragment, String tag) {
        doAddOp(0, fragment, tag, OP_ADD);
        return this;
    }

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

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

是採用"Builder"的方式来组织。

文章開始我已经提到了,Fragment的事务管理是比較出彩的代码,单纯的事务採用了至少三套模式来组织,并且组织起来丝毫没有感觉。

当然Fragment带给我们的惊喜还不仅限于此。我们总上面的代码片段能够看出,实际上,通过事务类BackStackRecord生成Op对象实际上在复制BackStackRecord的属性,所以当我们分析每个Op里面的数据的时候,能够直接用BackStackRecord中的属性映射。

    int mNumOp;//Op数量
    int mEnterAnim;//进入动画
    int mExitAnim;//退出动画
    int mPopEnterAnim;//弹出进入动画
    int mPopExitAnim;//弹出退出动画
    int mTransition;//转场动画
    int mTransitionStyle;
    boolean mAddToBackStack;//是否增加到BackStack中

    static final int OP_NULL = 0;
    static final int OP_ADD = 1;
    static final int OP_REPLACE = 2;
    static final int OP_REMOVE = 3;
    static final int OP_HIDE = 4;
    static final int OP_SHOW = 5;
    static final int OP_DETACH = 6;
    static final int OP_ATTACH = 7;

BackStackRecord.java：
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;
    }

FragmentManager.java:
 
public void enqueueAction(Runnable action, boolean allowStateLoss) {
        if (!allowStateLoss) {
            checkStateLoss();
        }
        synchronized (this) {
            if (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);
            }
        }
    }

BackStackRecord.java：
       Op op = mHead;
        while (op != null) {
             ...
        }

Op.java:

case OP_ADD: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.enterAnim;
                    mManager.addFragment(f, false);
                } break;

FragmentManager.java:
public void addFragment(Fragment fragment, boolean moveToStateNow) {
        if (mAdded == null) {
            mAdded = new ArrayList<Fragment>();
        }
        makeActive(fragment);
        if (!fragment.mDetached) {
            if (mAdded.contains(fragment)) {
                throw new IllegalStateException("Fragment already added: "
                        + fragment);
            }
            mAdded.add(fragment);
            fragment.mAdded = true;
            fragment.mRemoving = false;
            if (fragment.mHasMenu && fragment.mMenuVisible) {
                mNeedMenuInvalidate = true;
            }
            if (moveToStateNow) {
                moveToState(fragment);
            }
        }
    }