androidstudio实例源码_从源码的角度去解析Android Fragment（二）

曾经终败给现在 2023-01-10 09:58 193阅读 0赞

![4f7ca1006cad4445d87fa16b86057635.png][]

### **1.概述** ###

上一篇博客已经简单的讲了一下Fragment的使用并写了一个基本的实例，接下来就将其整合到项目中。附视频地址：http://pan.baidu.com/s/1mhUus56

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**2．效果实现**

**2.1 整合上一个实例：**

列表和轮播条不做过多的解释就是访问接口获取数据而已，这个在Android Studio自定义模板和Android无限广告轮播都讲过了。我们直接整合进去这个时候我们发现一个奇怪的问题，就是切换之后会去重新加载数据很不正常。  
　　一般的思路我们会换实现方法，当然其他方式肯定也可以实现如ViewPager+Fragment但是我们需要预加载要不然也会出问题，一旦预加载就需要去访问网络，即使用户可能不切换Fragment就退出App了这个时候其实加载了所有Fragment的数据，而且主页一旦复杂有可能会崩溃或造成内存溢出的问题。  
　　我的签名就是，忘记不了铭记，坚持不了放弃，但只要活着... 既然这样我必须得**看看源码：**

**2.2 Fragment源码分析：**

把一个Fragment加到ViewGroup中就这么几行代码：**add(@IdRes int containerViewId, Fragment fragment)**和**commit()**，就这么两个方法：

@Override
        protected void initData() {
            FragmentManager fragmentManager = getSupportFragmentManager();
            FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
            mHomeFragment = new HomeFragment();
            fragmentTransaction.add(R.id.main_tab_fl, mHomeFragment);
            fragmentTransaction.commit();
        }

我们点击add方法发现是个抽象方法：

/**
         * Calls {@link #add(int, Fragment, String)} with a null tag.
         */
        public abstract FragmentTransaction add(@IdRes int containerViewId, Fragment fragment);

点击fragmentManager.beginTransaction()发现也是一个抽象方法：

/**
         * 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();

所以只能点击getSupportFragmentManager()方法这个方法在FragmentActivity中：

/**
         * Return the FragmentManager for interacting with fragments associated
         * with this activity.
         */
        public FragmentManager getSupportFragmentManager() {
            return mFragments.getSupportFragmentManager();
        }

一路摸索才找到这个add方法，发现并没有写注释这个google工程师有点打酱油节奏啊！只好自己来吧在需要的地方写一写。我们看下面贴出来的源码其实可以知道，add方法其实只是设置了一些必要参数，并没有做任何的处理，这也是说google为什么一定要我们不要忘记commit()的原因：

public FragmentTransaction add(int containerViewId, Fragment fragment) {
            doAddOp(containerViewId, fragment, null, OP_ADD);
            return this;
        }
    
        private void doAddOp(int containerViewId, Fragment fragment, String tag, int opcmd) {
            fragment.mFragmentManager = mManager;
    
            // tag可以说是唯一标识我们可以通过它从FragmentManager中找到对应的Fragment
            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);
                }
                // 把Fragment的ContainerId和FragmentId指定为我们传递过来的布局中的ViewGroup的id。
                fragment.mContainerId = fragment.mFragmentId = containerViewId;
            }
    
            // 见名思意 Op是什么？就当是一些基本参数吧
            Op op = new Op();
            op.cmd = opcmd;
            op.fragment = fragment;
            addOp(op);
        }
    
        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++;
        }

既然add方法只是设置了一些参数而已，那么肯定就在commit()中做了些什么，找啊找啊找啊找，找到这么个方法（有些代码我就省略）：

void moveToState(Fragment f, int newState, int transit, 
        int transitionStyle, boolean keepActive){
                    // ... 省略部分代码
                    f.onAttach(mHost.getContext());
                    // 这个方法一调用就会执行Fragment的onAttach(Activity activity)这个生命周期方法
                    if (f.mParentFragment == null) {
                        mHost.onAttachFragment(f);
                    }
    
                    if (!f.mRetaining) {
                        f.performCreate(f.mSavedFragmentState);
                        // 执行生命周期onCreate(savedInstanceState);
                    }
                    f.mRetaining = false;
                    if (f.mFromLayout) {
                        ViewGroup container = null;
                        if (f.mContainerId != 0) {
                             //从activity中找到我们需要存放Fragment的ViewGroup布局
                             container = (ViewGroup)mContainer.onFindViewById(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));
                            }
                        }
                        // 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);
                        // 这个方法过后会执行onCreateView()生命周期且f.mView就是我们自己覆盖Fragment返回的View
                        if (f.mView != null) {
                            f.mInnerView = f.mView;
                            // v4包兼容11以下的版本我还是没说错啊
                            if (Build.VERSION.SDK_INT >= 11) {
                                ViewCompat.setSaveFromParentEnabled(f.mView, false);
                            } else {
                                f.mView = NoSaveStateFrameLayout.wrap(f.mView);
                            }
    
                            if (container != null) {
                                Animation anim = loadAnimation(f, transit, true,
                                transitionStyle);
                                if (anim != null) {
                                      setHWLayerAnimListenerIfAlpha(f.mView, anim);
                                      f.mView.startAnimation(anim);
                                }
                                // 如果ViewGroup不等于null就把从onCreateView()生命周期中获得的View添加到该布局中
                                // 最主要的就是这个方法，其实我们可以把Fragment理解成一个自定义的类
                                // 通过onCreateView()获取的到View添加到一个FragmentActivity的一个ViewGroup中
                                // 只不过它有自己的生命周期而已......
                                container.addView(f.mView);
                            }
                            // 如果是隐藏那就设置为不可见
                            if (f.mHidden) f.mView.setVisibility(View.GONE);
                            // 执行onViewCreated()生命周期方法
                            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;
                    }
                    // 代码省略......
             }
    }
    // 后面的我们就不看了，这上面的代码我自己做了一些整合，把它连贯起来了
    // 因为我们把add方法写在了Activity中的onCreate()方法中所以做了一些处理......

到这里应该能够了解Fragment的工作流程了吧，接下来我们看replace方法中究竟做了？其实和add差不多只是把int opcmd变成了OP\_REPLACE替换操作：

public FragmentTransaction replace(int containerViewId, Fragment fragment, String tag) {
            if (containerViewId == 0) {
                throw new IllegalArgumentException("Must use non-zero containerViewId");
            }
    
            doAddOp(containerViewId, fragment, tag, OP_REPLACE);
            return this;
        }

这个时候去commit会调用mManager.removeFragment(old, transition, transitionStyle)方法把原来的移除，然后把当前的Fragment添加进去，那岂不是每点击一个上一就被销毁了，那之前华东到哪里来了做了写什么事都被干掉重新创建了。

if (mManager.mAdded != null) {
        for (int i = mManager.mAdded.size() - 1; i >= 0; i--) {
            Fragment old = mManager.mAdded.get(i);
            if (old.mContainerId == containerId) {
                 if (old == f) {
                     op.fragment = f = null;
                 } else {
                    if (op.removed == null) {
                        op.removed = new ArrayList<Fragment>();
                    }
                    op.removed.add(old);
                    old.mNextAnim = exitAnim;
                    if (mAddToBackStack) {
                        old.mBackStackNesting += 1;
                    }
                    mManager.removeFragment(old, transition, transitionStyle);
                }
            }
        }
    }
    if (f != null) {
        f.mNextAnim = enterAnim;
        mManager.addFragment(f, false);
    }

到这里源码就以完毕有兴趣的小伙伴可以自己仔细去看看源码，接下来我们就来解决问题，我们肯定在调用replace方法的时候希望它不要移除原来的，那怎么办改Android的底层源码吗？那就只能换方法了，思路就是如果该Fragment不存在FragmentManager中我们就去添加，否则我们把之前的隐藏而不是替换移除，把当前的显示即可，最后代码就是：

private void homeRbClick() {
            FragmentManager fragmentManager = getSupportFragmentManager();
            FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
            List<Fragment> fragments = fragmentManager.getFragments();
            for (Fragment fragment : fragments) {
                fragmentTransaction.hide(fragment);
            }
    
            fragmentTransaction.show(mHomeFragment);
            fragmentTransaction.commit();
        }
    
        @OnClick(R.id.find_rb)
        private void findRbClick() {
            FragmentManager fragmentManager = getSupportFragmentManager();
            FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
            List<Fragment> fragments = fragmentManager.getFragments();
            for (Fragment fragment : fragments) {
                fragmentTransaction.hide(fragment);
            }
    
            if(mFindFragment == null){
                mFindFragment = new FindFragment();
                fragmentTransaction.add(R.id.main_tab_fl,mFindFragment);
            }else {
                fragmentTransaction.show(mFindFragment);
            }
    
            fragmentTransaction.commit();
        }
    
        @OnClick(R.id.new_rb)
        private void newRbClick() {
            FragmentManager fragmentManager = getSupportFragmentManager();
            FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
            List<Fragment> fragments = fragmentManager.getFragments();
            for (Fragment fragment : fragments) {
                fragmentTransaction.hide(fragment);
            }
    
            if(mNewFragment == null){
                mNewFragment = new NewFragment();
                fragmentTransaction.add(R.id.main_tab_fl,mNewFragment);
            }else {
                fragmentTransaction.show(mNewFragment);
            }
    
            fragmentTransaction.commit();
        }
    
        @OnClick(R.id.message_rb)
        private void messageRbClick() {
            FragmentManager fragmentManager = getSupportFragmentManager();
            FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
            List<Fragment> fragments = fragmentManager.getFragments();
            for (Fragment fragment : fragments) {
                fragmentTransaction.hide(fragment);
            }
    
            if(mMessageFragment == null){
                mMessageFragment = new MessageFragment();
                fragmentTransaction.add(R.id.main_tab_fl,mMessageFragment);
            }else {
                fragmentTransaction.show(mMessageFragment);
            }
    
            fragmentTransaction.commit();
        }

这里已经写得太多了视频里还做了一个代码优化，具体附视频地址：http://pan.baidu.com/s/1mhUus56

![49f03bdca6879acef138eca9bc4146b4.png][]

[4f7ca1006cad4445d87fa16b86057635.png]: /images/20221119/8ee3e1643c2d4f8e81e51e8f006deb43.png
[49f03bdca6879acef138eca9bc4146b4.png]: /images/20221119/d9d8479a20294950bcbe394a2702fb6a.png