[Android]Fragment源代码分析(二) 状态

我们上一讲,抛出来一个问题,就是当Activity的onCreateView的时候,是怎样构造Fragment中的View參数。要回答这个问题我们先要了解Fragment的状态,这是Fragment管理中很重要的一环。我们先来看一下FragmentActivity提供的一些核心回调:

@Override
protected void onCreate(Bundle savedInstanceState) {
mFragments.attachActivity(this, mContainer, null);
// Old versions of the platform didn't do this!
if (getLayoutInflater().getFactory() == null) {
getLayoutInflater().setFactory(this);
}

super.onCreate(savedInstanceState);
....
mFragments.dispatchCreate();
}

我们跟入mFragments.dispatchCreate方法中:

public void dispatchCreate() {
mStateSaved = false;
moveToState(Fragment.CREATED, false);
}

我们看到,对于FragmentManager来说,做了一次状态转换。我上一篇说过FragmentManager是及其重要的类,它承担了Fragment管理最为核心的工作。它有它自身的状态机，而它的状态,能够理解为与Activity本身基本同步。

在Fm里面维护自己的一个状态,当你导入一个Fragment的时候,Fm的目的,就是为了让Fragment和自己的状态基本保持一致.

void moveToState(int newState, int transit, int transitStyle, boolean always) {
if (mActivity == null && newState != Fragment.INITIALIZING) {
throw new IllegalStateException("No activity");
}

if (!always && mCurState == newState) {
return;
}
mCurState = newState;
if (mActive != null) {
boolean loadersRunning = false;
for (int i = 0; i < mActive.size(); i++) {
Fragment f = mActive.get(i);
if (f != null) {
moveToState(f, newState, transit, transitStyle, false);
if (f.mLoaderManager != null) {
loadersRunning |= f.mLoaderManager.hasRunningLoaders();
}
}
}

...
}
}

我们看到,FragmentManager的每一次状态变更,都会引起mActive里面的Fragment的状态变更。而mActive是全部纳入FragmentManager管理的Fragment容器。我们来看一下Fragment的几个状态：

static final int INITIALIZING = 0;     // Not yet created.
static final int CREATED = 1;          // Created.
static final int ACTIVITY_CREATED = 2; // The activity has finished its creation.
static final int STOPPED = 3;          // Fully created, not started.
static final int STARTED = 4;          // Created and started, not resumed.
static final int RESUMED = 5;          // Created started and resumed.

能够看出实际上,你的状态越靠后你的状态值越大,实际上在Fm的管理中,也巧妙的用到了这一点。

if (f.mState < newState)
{
...
} else {
...
}

对于f.mState<newState能够理解为创造的过程。同一时候我们也能找到我们上一篇文章的问题的答案:

if (f.mFromLayout) {
// For fragments that are part of the content view
// layout, we need to instantiate the view immediately
// and the inflater will take care of adding it.
f.mView = f.performCreateView(
f.getLayoutInflater(f.mSavedFragmentState), null,
f.mSavedFragmentState);
if (f.mView != null) {
f.mInnerView = f.mView;
f.mView = NoSaveStateFrameLayout.wrap(f.mView);
if (f.mHidden)
f.mView.setVisibility(View.GONE);
f.onViewCreated(f.mView, f.mSavedFragmentState);
} else {
f.mInnerView = null;
}
}

f.mFromLayout代表的是你这个Fragment的生成是否是从layout.xml文件里生成的。而它的View的生成是调用performCreateView来生成的。

View performCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
if (mChildFragmentManager != null) {
mChildFragmentManager.noteStateNotSaved();
}
return onCreateView(inflater, container, savedInstanceState);
}

对,这里就是我们很熟悉的onCreateView回调的出处。

当然我们如今还是属于Fragment.INITIALIZING这个状态。但实际上,我们在调用Fragment的时候FragmentManageer已经进入了Create状态。也就是说newState參数应该是Create才对。

所以我们接着代码往下走:

case Fragment.CREATED:
if (newState > Fragment.CREATED) {
if (!f.mFromLayout) {
ViewGroup container = null;
if (f.mContainerId != 0) {
container = (ViewGroup) mContainer
.findViewById(f.mContainerId);
if (container == null && !f.mRestored) {
throwException(new IllegalArgumentException(
"No view found for id 0x"
+ Integer
.toHexString(f.mContainerId)
+ " ("
+ f.getResources()
.getResourceName(
f.mContainerId)
+ ") for fragment " + f));
}
}
f.mContainer = container;
f.mView = f.performCreateView(
f.getLayoutInflater(f.mSavedFragmentState),
container, f.mSavedFragmentState);
if (f.mView != null) {
f.mInnerView = f.mView;
f.mView = NoSaveStateFrameLayout.wrap(f.mView);
if (container != null) {
Animation anim = loadAnimation(f, transit,
true, transitionStyle);
if (anim != null) {
f.mView.startAnimation(anim);
}
container.addView(f.mView);
}
if (f.mHidden)
f.mView.setVisibility(View.GONE);
f.onViewCreated(f.mView, f.mSavedFragmentState);
} else {
f.mInnerView = null;
}
}

f.performActivityCreated(f.mSavedFragmentState);
if (f.mView != null) {
f.restoreViewState(f.mSavedFragmentState);
}
f.mSavedFragmentState = null;
}

我们看到实际上这段代码是对FragmentManager状态是Create以上状态且Fragment的导入并非採用layout.xml方式导入的处理。这是为什么呢？由于在onCreate之后,基本上你的控件已经在Create状态的时候生成的差点儿相同了,你所要做的就是在生成的控件中找到Fragment相应的容器,然后装入你的控件。

同一时候,我们也看到了对Fragment的动画处理:

if (f.mView != null) {
f.mInnerView = f.mView;
f.mView = NoSaveStateFrameLayout.wrap(f.mView);
if (container != null) {
Animation anim = loadAnimation(f, transit,
true, transitionStyle);
if (anim != null) {
f.mView.startAnimation(anim);
}
container.addView(f.mView);
}
if (f.mHidden)
f.mView.setVisibility(View.GONE);
f.onViewCreated(f.mView, f.mSavedFragmentState);
} else {
f.mInnerView = null;
}

而这样的动画的处理和參数的配置,我们留到后面讲到Fragment事务的时候再说。