知道如果写应用首页，我们会以主Activity+多个Fragment的方式去创建。
再管理这些Fragment的显示和隐藏：
FragmentTransaction fragmentTransaction = mFragmentManager.beginTransaction();
if(!mFragmentManager.getFragments().contains(fragment)){
fragmentTransaction.add(mId,fragment);
}
for(Fragment fragment1 :mFragmentManager.getFragments()){
fragmentTransaction.hide(fragment1);
}
fragmentTransaction.show(fragment);
fragmentTransaction.commit();
为什么不使用replace方法呢：
FragmentTransaction fragmentTransaction = mFragmentManager.beginTransaction();
fragmentTransaction.replace(mId,fragment);
fragmentTransaction.commit();
因为：
Fragment expandOps(ArrayList<Fragment> added, Fragment oldPrimaryNav) {
for (int opNum = 0; opNum < mOps.size(); opNum++) {
final Op op = mOps.get(opNum);
switch (op.mCmd) {
case OP_ADD:
case OP_ATTACH:
added.add(op.mFragment);
break;
case OP_REMOVE:
case OP_DETACH: {
added.remove(op.mFragment);
if (op.mFragment == oldPrimaryNav) {
mOps.add(opNum, new Op(OP_UNSET_PRIMARY_NAV, op.mFragment));
opNum++;
oldPrimaryNav = null;
}
}
break;
case OP_REPLACE: {
final Fragment f = op.mFragment;
final int containerId = f.mContainerId;
boolean alreadyAdded = false;
for (int i = added.size() - 1; i >= 0; i--) {
final Fragment old = added.get(i);
if (old.mContainerId == containerId) {
if (old == f) {
alreadyAdded = true;
} else {
// This is duplicated from above since we only make
// a single pass for expanding ops. Unset any outgoing primary nav.
if (old == oldPrimaryNav) {
mOps.add(opNum, new Op(OP_UNSET_PRIMARY_NAV, old));
opNum++;
oldPrimaryNav = null;
}
final Op removeOp = new Op(OP_REMOVE, old);
removeOp.mEnterAnim = op.mEnterAnim;
removeOp.mPopEnterAnim = op.mPopEnterAnim;
removeOp.mExitAnim = op.mExitAnim;
removeOp.mPopExitAnim = op.mPopExitAnim;
mOps.add(opNum, removeOp);
added.remove(old);
opNum++;
}
}
}
if (alreadyAdded) {
mOps.remove(opNum);
opNum--;
} else {
op.mCmd = OP_ADD;
added.add(f);
}
}
break;
case OP_SET_PRIMARY_NAV: {
// It's ok if this is null, that means we will restore to no active
// primary navigation fragment on a pop.
mOps.add(opNum, new Op(OP_UNSET_PRIMARY_NAV, oldPrimaryNav));
opNum++;
// Will be set by the OP_SET_PRIMARY_NAV we inserted before when run
oldPrimaryNav = op.mFragment;
}
break;
}
}
return oldPrimaryNav;
}
其中关于Replace操作的执行是：
case OP_REPLACE: {
final Fragment f = op.mFragment;
final int containerId = f.mContainerId;
boolean alreadyAdded = false;
for (int i = added.size() - 1; i >= 0; i--) {
final Fragment old = added.get(i);
if (old.mContainerId == containerId) {
if (old == f) {
alreadyAdded = true;
} else {
// This is duplicated from above since we only make
// a single pass for expanding ops. Unset any outgoing primary nav.
if (old == oldPrimaryNav) {
mOps.add(opNum, new Op(OP_UNSET_PRIMARY_NAV, old));
opNum++;
oldPrimaryNav = null;
}
final Op removeOp = new Op(OP_REMOVE, old);
removeOp.mEnterAnim = op.mEnterAnim;
removeOp.mPopEnterAnim = op.mPopEnterAnim;
removeOp.mExitAnim = op.mExitAnim;
removeOp.mPopExitAnim = op.mPopExitAnim;
mOps.add(opNum, removeOp);
added.remove(old);
opNum++;
}
}
}
if (alreadyAdded) {
mOps.remove(opNum);
opNum--;
} else {
op.mCmd = OP_ADD;
added.add(f);
}
}
break;
如果已经添加了这个Fragment，就不做操作，如果没有添加，就执行add，无论添加还是没有添加都会遍历FragmentManager中已经有的Fragments（不包含当前要替换的，从Fragments中删除，且执行删除操作：
mOps.add(opNum, removeOp);
added.remove(old);
opNum++;
因此还是要使用Hide和show方法来实现：
那么我们是怎么看源码的呢？怎么追踪到这个的呢：
首先，查看FragmentTransaction中的replace：
@NonNull
public FragmentTransaction replace(@IdRes int containerViewId, @NonNull Fragment fragment) {
return replace(containerViewId, fragment, null);
}
进入replace：
public FragmentTransaction replace(@IdRes int containerViewId, @NonNull Fragment fragment,
@Nullable String tag) {
if (containerViewId == 0) {
throw new IllegalArgumentException("Must use non-zero containerViewId");
}
doAddOp(containerViewId, fragment, tag, OP_REPLACE);
return this;
}
查看doAddOp：
void doAddOp(int containerViewId, Fragment fragment, @Nullable String tag, int opcmd) {
........
addOp(new Op(opcmd, fragment));
}
void addOp(Op op) {
mOps.add(op);
op.mEnterAnim = mEnterAnim;
op.mExitAnim = mExitAnim;
op.mPopEnterAnim = mPopEnterAnim;
op.mPopExitAnim = mPopExitAnim;
}
replace()并没有做什么操作，那么主要的操作就是在commit中进行的：
public abstract int commit();
查找FragmentTransaction的实现类,在FragmentManager中：
public FragmentTransaction beginTransaction() {
return new BackStackRecord(this);
}
查看BackStackRecord的commit()进行了那些操作：
@Override
public int commit() {
return commitInternal(false);
}
int commitInternal(boolean allowStateLoss) {
if (mCommitted) throw new IllegalStateException("commit already called");
if (FragmentManager.isLoggingEnabled(Log.VERBOSE)) {
Log.v(TAG, "Commit: " + this);
LogWriter logw = new LogWriter(TAG);
PrintWriter pw = new PrintWriter(logw);
dump(" ", pw);
pw.close();
}
mCommitted = true;
if (mAddToBackStack) {
mIndex = mManager.allocBackStackIndex();
} else {
mIndex = -1;
}
mManager.enqueueAction(this, allowStateLoss);
return mIndex;
}
根据：mManager.enqueueAction(this, allowStateLoss);定位：
void enqueueAction(@NonNull OpGenerator action, boolean allowStateLoss) {
if (!allowStateLoss) {
if (mHost == null) {
if (mDestroyed) {
throw new IllegalStateException("FragmentManager has been destroyed");
} else {
throw new IllegalStateException("FragmentManager has not been attached to a "
+ "host.");
}
}
checkStateLoss();
}
synchronized (mPendingActions) {
if (mHost == null) {
if (allowStateLoss) {
// This FragmentManager isn't attached, so drop the entire transaction.
return;
}
throw new IllegalStateException("Activity has been destroyed");
}
mPendingActions.add(action);
scheduleCommit();
}
}
定位到scheduleCommit:
void scheduleCommit() {
synchronized (mPendingActions) {
boolean postponeReady =
mPostponedTransactions != null && !mPostponedTransactions.isEmpty();
boolean pendingReady = mPendingActions.size() == 1;
if (postponeReady || pendingReady) {
mHost.getHandler().removeCallbacks(mExecCommit);
mHost.getHandler().post(mExecCommit);
updateOnBackPressedCallbackEnabled();
}
}
}
定位到mExecCommit：
private Runnable mExecCommit = new Runnable() {
@Override
public void run() {
execPendingActions(true);
}
};
定位到execPendingActions:
boolean execPendingActions(boolean allowStateLoss) {
ensureExecReady(allowStateLoss);
boolean didSomething = false;
while (generateOpsForPendingActions(mTmpRecords, mTmpIsPop)) {
mExecutingActions = true;
try {
removeRedundantOperationsAndExecute(mTmpRecords, mTmpIsPop);
} finally {
cleanupExec();
}
didSomething = true;
}
updateOnBackPressedCallbackEnabled();
doPendingDeferredStart();
mFragmentStore.burpActive();
return didSomething;
}
定位到removeRedundantOperationsAndExecute：
private void removeRedundantOperationsAndExecute(@NonNull ArrayList<BackStackRecord> records,
@NonNull ArrayList<Boolean> isRecordPop) {
if (records.isEmpty()) {
return;
}
if (records.size() != isRecordPop.size()) {
throw new IllegalStateException("Internal error with the back stack records");
}
// Force start of any postponed transactions that interact with scheduled transactions:
executePostponedTransaction(records, isRecordPop);
final int numRecords = records.size();
int startIndex = 0;
for (int recordNum = 0; recordNum < numRecords; recordNum++) {
final boolean canReorder = records.get(recordNum).mReorderingAllowed;
if (!canReorder) {
// execute all previous transactions
if (startIndex != recordNum) {
executeOpsTogether(records, isRecordPop, startIndex, recordNum);
}
// execute all pop operations that don't allow reordering together or
// one add operation
int reorderingEnd = recordNum + 1;
if (isRecordPop.get(recordNum)) {
while (reorderingEnd < numRecords
&& isRecordPop.get(reorderingEnd)
&& !records.get(reorderingEnd).mReorderingAllowed) {
reorderingEnd++;
}
}
executeOpsTogether(records, isRecordPop, recordNum, reorderingEnd);
startIndex = reorderingEnd;
recordNum = reorderingEnd - 1;
}
}
if (startIndex != numRecords) {
executeOpsTogether(records, isRecordPop, startIndex, numRecords);
}
}
定位到executeOpsTogether(records, isRecordPop, recordNum, reorderingEnd);：
private void executeOpsTogether(@NonNull ArrayList<BackStackRecord> records,
@NonNull ArrayList<Boolean> isRecordPop, int startIndex, int endIndex) {
final boolean allowReordering = records.get(startIndex).mReorderingAllowed;
boolean addToBackStack = false;
if (mTmpAddedFragments == null) {
mTmpAddedFragments = new ArrayList<>();
} else {
mTmpAddedFragments.clear();
}
mTmpAddedFragments.addAll(mFragmentStore.getFragments());
Fragment oldPrimaryNav = getPrimaryNavigationFragment();
for (int recordNum = startIndex; recordNum < endIndex; recordNum++) {
final BackStackRecord record = records.get(recordNum);
final boolean isPop = isRecordPop.get(recordNum);
if (!isPop) {
oldPrimaryNav = record.expandOps(mTmpAddedFragments, oldPrimaryNav);
} else {
oldPrimaryNav = record.trackAddedFragmentsInPop(mTmpAddedFragments, oldPrimaryNav);
}
addToBackStack = addToBackStack || record.mAddToBackStack;
}
mTmpAddedFragments.clear();
if (!allowReordering) {
FragmentTransition.startTransitions(this, records, isRecordPop, startIndex, endIndex,
false, mFragmentTransitionCallback);
}
executeOps(records, isRecordPop, startIndex, endIndex);
int postponeIndex = endIndex;
if (allowReordering) {
ArraySet<Fragment> addedFragments = new ArraySet<>();
addAddedFragments(addedFragments);
postponeIndex = postponePostponableTransactions(records, isRecordPop,
startIndex, endIndex, addedFragments);
makeRemovedFragmentsInvisible(addedFragments);
}
if (postponeIndex != startIndex && allowReordering) {
// need to run something now
FragmentTransition.startTransitions(this, records, isRecordPop, startIndex,
postponeIndex, true, mFragmentTransitionCallback);
moveToState(mCurState, true);
}
for (int recordNum = startIndex; recordNum < endIndex; recordNum++) {
final BackStackRecord record = records.get(recordNum);
final boolean isPop = isRecordPop.get(recordNum);
if (isPop && record.mIndex >= 0) {
record.mIndex = -1;
}
record.runOnCommitRunnables();
}
if (addToBackStack) {
reportBackStackChanged();
}
}
定位到``:
executeOps(records, isRecordPop, startIndex, endIndex);
.......
if (!isPop) {
oldPrimaryNav = record.expandOps(mTmpAddedFragments, oldPrimaryNav);
} else {
oldPrimaryNav = record.trackAddedFragmentsInPop(mTmpAddedFragments, oldPrimaryNav);
}
这样就可以查看到我们想要的代码逻辑了
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