[Android]Fragment源代码分析 状态

Posted 2020-06-27 Brenda

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。