解决在硬件加速下WebView切换闪屏的问题

问题描述

在利用WebView进行开发时，遇到了这样一个问题，即在两个Fragment中分别嵌入两个WebView，切换Fragment时，页面会闪烁并伴随有黑块，用户体验非常糟糕。

解决历程

遇到问题，第一反应当然是百度了（现在国内的开发资料已经相当全面了，大多数的问题通过百度就可以解决，去Google比较麻烦）。通过关键词搜索，我得知这个bug是由于android的“硬件加速”引起的，何为硬件加速，即系统通过GPU来绘制图像，以牺牲内存为代价换取图像渲染的更加平滑，从Android 3.0开始支持使用硬件加速，Android 4.0开始默认开启。我这里只用一句话简单概括了一下，如果需要详细了解，可以查看这篇博文 Android——硬件加速（Hardware Acceleration）

既然问题的根源找到了，接下来就要知道怎么解决问题了。查询了很多资料博客，给出的解决方案都是一致的，手动关闭硬件加速。于是我照做，关闭后重新运行，Fragment切换果然顺畅了，没有出现闪屏，问题完美解决。如果你以为这样就结束了，那你就太年轻了，真正的难题才刚刚开始。闪烁的bug是解决了，但只要你细心，就会发现页面上下滚动的时候会卡顿，而之前开启硬件加速的时候是没有这个问题的。

这就难办了，硬件加速到底是应该开启还是关闭，这是个问题。我想，硬件加速是在Android 3.0之后新引入的，必然是Google对android系统做出的优化，而又在Android 4.0版本默认开启，充分说明了其重要性，所以肯定是不能关闭的。这样的话就又回到了起点的那个bug，怎么解决闪屏的问题。这里我反复搜索了很多信息，也有不少人遇到了同样的问题，但是都没有一个很好的解决方案，我甚至以为这是WebView的一个尚未解决的bug，不是我等区区几行代码就可以搞得定的。

后来我在一次偶然中发现，Fragment切换的时候，会走onAttach()和onDetach()等生命周期方法（这里我使用的是FragmentTabHost来控制Fragment的切换），为了验证我的猜测，我将代码进行重构，底部的导航按钮使用RadioButton，Activity初始化时加载所有Fragment，通过RadioButton来控制各个Fragment的hide()和show()，这样做的好处就是不会触发Fragment的任何生命周期方法。运行结果验证了我的猜想是正确的，之前使用FragmentTabHost来切换Fragment时会一次次地调用attach()和detach()方法，每一次切换，View都会进行一次重绘，这也就是在硬件加速下，导致WebView闪烁的罪魁祸首了。而改进过后的代码，虽然会一次性把所有的Fragment都加载进内存，加大了内存的消耗，但切换Fragment时不用对View进行重绘，恰好是解决这个bug的最佳方式。

既然了解了bug的产生原因，解决方案自然就不止一种。我们点开FragmentTabHost的源码，可以很明显地看到，doTabChanged()这个方法就是控制Fragment切换的核心逻辑。我们只要自定义一个MyFragmentTabHost，将源码复制进来，把所有的attach()和detach()方法改为show()和hide()就可以了。

自定义MyFragmentTabHost源码如下，可直接使用，

public class MyFragmentTabHost extends TabHost implements TabHost.OnTabChangeListener {
    private final ArrayList mTabs = new ArrayList();
    private FrameLayout mRealTabContent;
    private Context mContext;
    private FragmentManager mFragmentManager;
    private int mContainerId;
    private TabHost.OnTabChangeListener mOnTabChangeListener;
    private MyFragmentTabHost.TabInfo mLastTab;
    private boolean mAttached;

    static final class TabInfo {
        private final String tag;
        private final Class clss;
        private final Bundle args;
        private Fragment fragment;

        TabInfo(String _tag, Class _class, Bundle _args) {
            tag = _tag;
            clss = _class;
            args = _args;
        }
    }

    static class DummyTabFactory implements TabHost.TabContentFactory {
        private final Context mContext;

        public DummyTabFactory(Context context) {
            mContext = context;
        }

        @Override
        public View createTabContent(String tag) {
            View v = new View(mContext);
            v.setMinimumWidth(0);
            v.setMinimumHeight(0);
            return v;
        }
    }

    static class SavedState extends BaseSavedState {
        String curTab;

        SavedState(Parcelable superState) {
            super(superState);
        }

        private SavedState(Parcel in) {
            super(in);
            curTab = in.readString();
        }

        @Override
        public void writeToParcel(Parcel out, int flags) {
            super.writeToParcel(out, flags);
            out.writeString(curTab);
        }

        @Override
        public String toString() {
            return "FragmentTabHost.SavedState{"
                    + Integer.toHexString(System.identityHashCode(this))
                    + " curTab=" + curTab + "}";
        }

        public static final Parcelable.Creator CREATOR
                = new Parcelable.Creator() {
            public MyFragmentTabHost.SavedState createFromParcel(Parcel in) {
                return new MyFragmentTabHost.SavedState(in);
            }

            public MyFragmentTabHost.SavedState[] newArray(int size) {
                return new MyFragmentTabHost.SavedState[size];
            }
        };
    }

    public MyFragmentTabHost(Context context) {
        // Note that we call through to the version that takes an AttributeSet,
        // because the simple Context construct can result in a broken object!
        super(context, null);
        initFragmentTabHost(context, null);
    }

    public MyFragmentTabHost(Context context, AttributeSet attrs) {
        super(context, attrs);
        initFragmentTabHost(context, attrs);
    }

    private void initFragmentTabHost(Context context, AttributeSet attrs) {
        TypedArray a = context.obtainStyledAttributes(attrs,
                new int[] { android.R.attr.inflatedId }, 0, 0);
        mContainerId = a.getResourceId(0, 0);
        a.recycle();

        super.setOnTabChangedListener(this);
    }

    private void ensureHierarchy(Context context) {
        // If owner hasn't made its own view hierarchy, then as a convenience
        // we will construct a standard one here.
        if (findViewById(android.R.id.tabs) == null) {
            LinearLayout ll = new LinearLayout(context);
            ll.setOrientation(LinearLayout.VERTICAL);
            addView(ll, new FrameLayout.LayoutParams(
                    ViewGroup.LayoutParams.FILL_PARENT,
                    ViewGroup.LayoutParams.FILL_PARENT));

            TabWidget tw = new TabWidget(context);
            tw.setId(android.R.id.tabs);
            tw.setOrientation(TabWidget.HORIZONTAL);
            ll.addView(tw, new LinearLayout.LayoutParams(
                    ViewGroup.LayoutParams.FILL_PARENT,
                    ViewGroup.LayoutParams.WRAP_CONTENT, 0));

            FrameLayout fl = new FrameLayout(context);
            fl.setId(android.R.id.tabcontent);
            ll.addView(fl, new LinearLayout.LayoutParams(0, 0, 0));

            mRealTabContent = fl = new FrameLayout(context);
            mRealTabContent.setId(mContainerId);
            ll.addView(fl, new LinearLayout.LayoutParams(
                    LinearLayout.LayoutParams.FILL_PARENT, 0, 1));
        }
    }

    /**
     * @deprecated Don't call the original TabHost setup, you must instead
     * call {@link #setup(Context, FragmentManager)} or
     * {@link #setup(Context, FragmentManager, int)}.
     */
    @Override @Deprecated
    public void setup() {
        throw new IllegalStateException(
                "Must call setup() that takes a Context and FragmentManager");
    }

    public void setup(Context context, FragmentManager manager) {
        ensureHierarchy(context);  // Ensure views required by super.setup()
        super.setup();
        mContext = context;
        mFragmentManager = manager;
        ensureContent();
    }

    public void setup(Context context, FragmentManager manager, int containerId) {
        ensureHierarchy(context);  // Ensure views required by super.setup()
        super.setup();
        mContext = context;
        mFragmentManager = manager;
        mContainerId = containerId;
        ensureContent();
        mRealTabContent.setId(containerId);

        // We must have an ID to be able to save/restore our state.  If
        // the owner hasn't set one at this point, we will set it ourself.
        if (getId() == View.NO_ID) {
            setId(android.R.id.tabhost);
        }
    }

    private void ensureContent() {
        if (mRealTabContent == null) {
            mRealTabContent = (FrameLayout)findViewById(mContainerId);
            if (mRealTabContent == null) {
                throw new IllegalStateException(
                        "No tab content FrameLayout found for id " + mContainerId);
            }
        }
    }

    @Override
    public void setOnTabChangedListener(OnTabChangeListener l) {
        mOnTabChangeListener = l;
    }

    public void addTab(TabHost.TabSpec tabSpec, Class clss, Bundle args) {
        tabSpec.setContent(new MyFragmentTabHost.DummyTabFactory(mContext));
        String tag = tabSpec.getTag();

        MyFragmentTabHost.TabInfo info = new MyFragmentTabHost.TabInfo(tag, clss, args);

        if (mAttached) {
            // If we are already attached to the window, then check to make
            // sure this tab's fragment is inactive if it exists.  This shouldn't
            // normally happen.
            info.fragment = mFragmentManager.findFragmentByTag(tag);
            if (info.fragment != null && !info.fragment.isHidden()) {
                FragmentTransaction ft = mFragmentManager.beginTransaction();
                ft.hide(info.fragment);
                ft.commit();
            }
        }

        mTabs.add(info);
        addTab(tabSpec);
    }

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();

        String currentTab = getCurrentTabTag();

        // Go through all tabs and make sure their fragments match
        // the correct state.
        FragmentTransaction ft = null;
        for (int i=0; iif (tab.fragment != null && !tab.fragment.isHidden()) {
                if (tab.tag.equals(currentTab)) {
                    // The fragment for this tab is already there and
                    // active, and it is what we really want to have
                    // as the current tab.  Nothing to do.
                    mLastTab = tab;
                } else {
                    // This fragment was restored in the active state,
                    // but is not the current tab.  Deactivate it.
                    if (ft == null) {
                        ft = mFragmentManager.beginTransaction();
                    }
                    ft.hide(tab.fragment);
                }
            }
        }

        // We are now ready to go.  Make sure we are switched to the
        // correct tab.
        mAttached = true;
        ft = doTabChanged(currentTab, ft);
        if (ft != null) {
            ft.commit();
            mFragmentManager.executePendingTransactions();
        }
    }

    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        mAttached = false;
    }

    @Override
    protected Parcelable onSaveInstanceState() {
        Parcelable superState = super.onSaveInstanceState();
        MyFragmentTabHost.SavedState ss = new MyFragmentTabHost.SavedState(superState);
        ss.curTab = getCurrentTabTag();
        return ss;
    }

    @Override
    protected void onRestoreInstanceState(Parcelable state) {
        MyFragmentTabHost.SavedState ss = (MyFragmentTabHost.SavedState)state;
        super.onRestoreInstanceState(ss.getSuperState());
        setCurrentTabByTag(ss.curTab);
    }

    @Override
    public void onTabChanged(String tabId) {
        if (mAttached) {
            FragmentTransaction ft = doTabChanged(tabId, null);
            if (ft != null) {
                ft.commit();
            }
        }
        if (mOnTabChangeListener != null) {
            mOnTabChangeListener.onTabChanged(tabId);
        }
    }

    private FragmentTransaction doTabChanged(String tabId, FragmentTransaction ft) {
        MyFragmentTabHost.TabInfo newTab = null;
        for (int i=0; iif (tab.tag.equals(tabId)) {
                newTab = tab;
            }
        }
        if (newTab == null) {
            throw new IllegalStateException("No tab known for tag " + tabId);
        }
        if (mLastTab != newTab) {
            if (ft == null) {
                ft = mFragmentManager.beginTransaction();
            }
            if (mLastTab != null) {
                if (mLastTab.fragment != null) {
                    ft.hide(mLastTab.fragment);
                }
            }
            if (newTab != null) {
                if (newTab.fragment == null) {
                    newTab.fragment = Fragment.instantiate(mContext,
                            newTab.clss.getName(), newTab.args);
                    ft.add(mContainerId, newTab.fragment, newTab.tag);
                } else {
                    ft.show(newTab.fragment);
                }
            }

            mLastTab = newTab;
        }
        return ft;
    }
}