RecyclerView 源码分析

RecyclerView 整体用到得设计模式有一下：

通过桥接模式，使RecyclerView 将布局方式独立成LayoutManager，实现对布局的定制化。

通过组合模式，使RecycleView通过dispatchLayout对Item View进行布局绘制的。

通过适配器模式，ViewHolder将RecycleView与ItemView联系起来，使得RecycleView方便操作ItemView。

通过观察者模式，给ViewHolder注册观察者，当调用notifyDataSetChanged时，就能重新绘制。

 

RecycleView得源码数据量非常多先列出几个核心点

RecyclerViewDataObserver 数据观察器

Recycler View循环复用系统，核心部件

SavedState RecyclerView状态

AdapterHelper 适配器更新

ChildHelper 管理子View

ViewInfoStore 存储子VIEW的动画信息

Adapter 数据适配器

LayoutManager 负责子VIEW的布局，核心部件

ItemAnimator Item动画

ViewFlinger 快速滑动管理

NestedScrollingChildHelper 管理子VIEW嵌套滑动

一般使用RecycleView得时候如下：

recyclerView = (RecyclerView) findViewById(R.id.recyclerView);

LinearLayoutManager layoutManager = new LinearLayoutManager(this);

//设置布局管理器

recyclerView.setLayoutManager(layoutManager);

//设置为垂直布局，这也是默认的

layoutManager.setOrientation(OrientationHelper. VERTICAL);

//设置Adapter

recyclerView.setAdapter( recycleAdapter);

//设置分隔线

recyclerView.addItemDecoration( new DividerGridItemDecoration(this ));

//设置增加或删除条目的动画

recyclerView.setItemAnimator( new DefaultItemAnimator());

看下RecycleView得构造函数

   

public RecyclerView(Context context, @Nullable AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        setScrollContainer(true);
        setFocusableInTouchMode(true);
        final int version = Build.VERSION.SDK_INT;
        mPostUpdatesOnAnimation = version >= 16;

        final ViewConfiguration vc = ViewConfiguration.get(context);
        mTouchSlop = vc.getScaledTouchSlop();
        mMinFlingVelocity = vc.getScaledMinimumFlingVelocity();
        mMaxFlingVelocity = vc.getScaledMaximumFlingVelocity();
        setWillNotDraw(ViewCompat.getOverScrollMode(this) == ViewCompat.OVER_SCROLL_NEVER);

        mItemAnimator.setListener(mItemAnimatorListener);
        initAdapterManager();
        initChildrenHelper();
        // If not explicitly specified this view is important for accessibility.
        if (ViewCompat.getImportantForAccessibility(this)
                == ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
            ViewCompat.setImportantForAccessibility(this,
                    ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_YES);
        }
        mAccessibilityManager = (AccessibilityManager) getContext()
                .getSystemService(Context.ACCESSIBILITY_SERVICE);
        setAccessibilityDelegateCompat(new RecyclerViewAccessibilityDelegate(this));
        // Create the layoutManager if specified.
        if (attrs != null) {
            int defStyleRes = 0;
            TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.RecyclerView,
                    defStyle, defStyleRes);
            String layoutManagerName = a.getString(R.styleable.RecyclerView_layoutManager);
            a.recycle();
            createLayoutManager(context, layoutManagerName, attrs, defStyle, defStyleRes);
        }

        mScrollingChildHelper = new NestedScrollingChildHelper(this);
        setNestedScrollingEnabled(true);
    }

总体上进行了一系列得出事划后，createLayoutManager，创建了一个布局管理器，看下源码实现

/**
     * Instantiate and set a LayoutManager, if specified in the attributes.
     */
    private void createLayoutManager(Context context, String className, AttributeSet attrs,
            int defStyleAttr, int defStyleRes) {
        if (className != null) {
            className = className.trim();
            if (className.length() != 0) {  // Can't use isEmpty since it was added in API 9.
                className = getFullClassName(context, className);
                try {
                    ClassLoader classLoader;
                    if (isInEditMode()) {
                        // Stupid layoutlib cannot handle simple class loaders.
                        classLoader = this.getClass().getClassLoader();
                    } else {
                        classLoader = context.getClassLoader();
                    }
                    Class layoutManagerClass =
                            classLoader.loadClass(className).asSubclass(LayoutManager.class);
                    Constructor constructor;
                    Object[] constructorArgs = null;
                    try {
                        constructor = layoutManagerClass
                                .getConstructor(LAYOUT_MANAGER_CONSTRUCTOR_SIGNATURE);
                        constructorArgs = new Object[]{context, attrs, defStyleAttr, defStyleRes};
                    } catch (NoSuchMethodException e) {
                        try {
                            constructor = layoutManagerClass.getConstructor();
                        } catch (NoSuchMethodException e1) {
                            e1.initCause(e);
                            throw new IllegalStateException(attrs.getPositionDescription() +
                                    ": Error creating LayoutManager " + className, e1);
                        }
                    }
                    constructor.setAccessible(true);
                    setLayoutManager(constructor.newInstance(constructorArgs));
                } catch (ClassNotFoundException e) {
                    throw new IllegalStateException(attrs.getPositionDescription()
                            + ": Unable to find LayoutManager " + className, e);
                } catch (InvocationTargetException e) {
                    throw new IllegalStateException(attrs.getPositionDescription()
                            + ": Could not instantiate the LayoutManager: " + className, e);
                } catch (InstantiationException e) {
                    throw new IllegalStateException(attrs.getPositionDescription()
                            + ": Could not instantiate the LayoutManager: " + className, e);
                } catch (IllegalAccessException e) {
                    throw new IllegalStateException(attrs.getPositionDescription()
                            + ": Cannot access non-public constructor " + className, e);
                } catch (ClassCastException e) {
                    throw new IllegalStateException(attrs.getPositionDescription()
                            + ": Class is not a LayoutManager " + className, e);
                }
            }
        }
    }

如果在布局里面设置了布局管理器得类型，那么会通过反射得反射实例化出对应得布局管理器，最后将实例化出得布局管理器设置到当前得RecycleView,看下setLayoutManager得实现

 /**
     * Set the {@link LayoutManager} that this RecyclerView will use.
     *
     * 
In contrast to other adapter-backed views such as {@link android.widget.ListView}
     * or {@link android.widget.GridView}, RecyclerView allows client code to provide custom
     * layout arrangements for child views. These arrangements are controlled by the
     * {@link LayoutManager}. A LayoutManager must be provided for RecyclerView to function.
     *
     * 
Several default strategies are provided for common uses such as lists and grids.
     *
     * @param layout LayoutManager to use
     */
    public void setLayoutManager(LayoutManager layout) {
        if (layout == mLayout) {
            return;
        }
        // TODO We should do this switch a dispachLayout pass and animate children. There is a good
        // chance that LayoutManagers will re-use views.
        if (mLayout != null) {
            if (mIsAttached) {
                mLayout.dispatchDetachedFromWindow(this, mRecycler);
            }
            mLayout.setRecyclerView(null);
        }
        mRecycler.clear();
        mChildHelper.removeAllViewsUnfiltered();
        mLayout = layout;
        if (layout != null) {
            if (layout.mRecyclerView != null) {
                throw new IllegalArgumentException("LayoutManager " + layout +
                        " is already attached to a RecyclerView: " + layout.mRecyclerView);
            }
            mLayout.setRecyclerView(this);
            if (mIsAttached) {
                mLayout.dispatchAttachedToWindow(this);
            }
        }
        requestLayout();
    }

设置布局管理器之前，清空所有得缓存得VIEW，最后通过VIEW刷新，调用requestLayout 进行刷新，看下实现

     

   /**
         * Calls {@code RecyclerView#requestLayout} on the underlying RecyclerView
         */
        public void requestLayout() {
            if(mRecyclerView != null) {
                mRecyclerView.requestLayout();
            }
        }

 @Override
    public void requestLayout() {
        if (!mEatRequestLayout && !mLayoutFrozen) {
            super.requestLayout();
        } else {
            mLayoutRequestEaten = true;
        }
    }

调用的是ViewGroup得requestLayout，采用得是责任链模式，层层向上传递，最后传递到ViewRootImpl，然后重新调用了View得measure、layout、draw 方法来展示布局。看下源码。

public void requestLayout() {
    if (mMeasureCache != null) mMeasureCache.clear();
    if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
        // Only trigger request-during-layout logic if this is the view requesting it,
        // not the views in its parent hierarchy
        ViewRootImpl viewRoot = getViewRootImpl();
        if (viewRoot != null && viewRoot.isInLayout()) {
            if (!viewRoot.requestLayoutDuringLayout(this)) {
                return;
            }
        }
        mAttachInfo.mViewRequestingLayout = this;
    }
    //为当前view设置标记位 PFLAG_FORCE_LAYOUT
    mPrivateFlags |= PFLAG_FORCE_LAYOUT;
    mPrivateFlags |= PFLAG_INVALIDATED;
    if (mParent != null && !mParent.isLayoutRequested()) {
        //向父容器请求布局
        mParent.requestLayout();
    }
    if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
        mAttachInfo.mViewRequestingLayout = null;
    }
}

接着调用

@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }
}

先检查当前是否在主线程，否则抛出异常，然后调用scheduleTraversals，看下源码。

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

调用了mTraversalRunnable。

final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}
void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
        if (mProfile) {
            Debug.startMethodTracing("ViewAncestor");
        }
        performTraversals();
        if (mProfile) {
            Debug.stopMethodTracing();
            mProfile = false;
        }
    }
}

最后调用了ViewRootImpl得performTraversals方法，这个是View 工作得核心方法，measure、layout、draw得三大流程就在这里面。看下源码。

private void performTraversals() {
        ...
    if (!mStopped) {
        int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);  // 1
        int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);       
        }
    }
    if (didLayout) {
        performLayout(lp, desiredWindowWidth, desiredWindowHeight);
        ...
    }
    if (!cancelDraw && !newSurface) {
        if (!skipDraw || mReportNextDraw) {
            if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
                for (int i = 0; i < mPendingTransitions.size(); ++i) {
                    mPendingTransitions.get(i).startChangingAnimations();
                }
                mPendingTransitions.clear();
            }
            performDraw();
        }
    }
    ...
}

最终调用到了recyclerview的onMeature ，看下recyclerView.setLayoutManager(layoutManager)，这里用到了桥接模式。

 

 

  public LinearLayoutManager(Context context, int orientation, boolean reverseLayout) {
        setOrientation(orientation);
        setReverseLayout(reverseLayout);
    }


    /**
     * @param context Current context, will be used to access resources.
     * @param spanCount The number of columns or rows in the grid
     * @param orientation Layout orientation. Should be {@link #HORIZONTAL} or {@link
     *                      #VERTICAL}.
     * @param reverseLayout When set to true, layouts from end to start.
     */
    public GridLayoutManager(Context context, int spanCount, int orientation,
            boolean reverseLayout) {
        super(context, orientation, reverseLayout);
        setSpanCount(spanCount);
    }

GridLayoutManager继承LinearLayoutManager。看下onMeasure 是如何测量得。

 @SuppressLint("WrongCall")
    @Override
    protected void onMeasure(int widthSpec, int heightSpec) {
        if (mAdapterUpdateDuringMeasure) {
            eatRequestLayout();
            processAdapterUpdatesAndSetAnimationFlags();

            if (mState.mRunPredictiveAnimations) {
                // TODO: try to provide a better approach.
                // When RV decides to run predictive animations, we need to measure in pre-layout
                // state so that pre-layout pass results in correct layout.
                // On the other hand, this will prevent the layout manager from resizing properly.
                mState.mInPreLayout = true;
            } else {
                // consume remaining updates to provide a consistent state with the layout pass.
                mAdapterHelper.consumeUpdatesInOnePass();
                mState.mInPreLayout = false;
            }
            mAdapterUpdateDuringMeasure = false;
            resumeRequestLayout(false);
        }

        if (mAdapter != null) {
            mState.mItemCount = mAdapter.getItemCount();
        } else {
            mState.mItemCount = 0;
        }
        if (mLayout == null) {
            defaultOnMeasure(widthSpec, heightSpec);
        } else {
            mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
        }

        mState.mInPreLayout = false; // clear
    }

第一次会调用defaultOnMeasure来进行测量，否则会调用 mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);，看下defaultOnMeasure得实现。

  /**
     * Used when onMeasure is called before layout manager is set
     */
    private void defaultOnMeasure(int widthSpec, int heightSpec) {
        final int widthMode = MeasureSpec.getMode(widthSpec);
        final int heightMode = MeasureSpec.getMode(heightSpec);
        final int widthSize = MeasureSpec.getSize(widthSpec);
        final int heightSize = MeasureSpec.getSize(heightSpec);

        int width = 0;
        int height = 0;

        switch (widthMode) {
            case MeasureSpec.EXACTLY:
            case MeasureSpec.AT_MOST:
                width = widthSize;
                break;
            case MeasureSpec.UNSPECIFIED:
            default:
                width = ViewCompat.getMinimumWidth(this);
                break;
        }

        switch (heightMode) {
            case MeasureSpec.EXACTLY:
            case MeasureSpec.AT_MOST:
                height = heightSize;
                break;
            case MeasureSpec.UNSPECIFIED:
            default:
                height = ViewCompat.getMinimumHeight(this);
                break;
        }

        setMeasuredDimension(width, height);
    }

有三种测量模式： MeasureSpec.EXACTLY，  MeasureSpec.AT_MOST ， MeasureSpec.UNSPECIFIED，分别代表精确大小，不精确大小，最大值，通过 MeasureSpec.getMode 可以得到 mode 。mLayout.onMeasure 最终也是调用defaultOnMeasure来进行测量得。

接着看下onLayout 得源码。

    

@Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        eatRequestLayout();
        TraceCompat.beginSection(TRACE_ON_LAYOUT_TAG);
        dispatchLayout();
        TraceCompat.endSection();
        resumeRequestLayout(false);
        mFirstLayoutComplete = true;
    }

看下dispatchLayout 得实现

 

 

void dispatchLayout() {
 
   .......
   
   if (mState.mRunSimpleAnimations) {
            // Step 0: Find out where all non-removed items are, pre-layout
            mState.mPreLayoutHolderMap.clear();
            mState.mPostLayoutHolderMap.clear();
            int count = mChildHelper.getChildCount();
            for (int i = 0; i < count; ++i) {
                final ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i));
                if (holder.shouldIgnore() || (holder.isInvalid() && !mAdapter.hasStableIds())) {
                    continue;
                }
                final View view = holder.itemView;
                mState.mPreLayoutHolderMap.put(holder, new ItemHolderInfo(holder,
                        view.getLeft(), view.getTop(), view.getRight(), view.getBottom()));
            }
        }
        
        /Step 1
        
        if (mState.mRunPredictiveAnimations) {
        
             ......
             saveOldPositions();
             ......
             
        }
        ......
        // Step 2: Run layout
        mState.mInPreLayout = false;
        mLayout.onLayoutChildren(mRecycler, mState);

        mState.mStructureChanged = false;
        mPendingSavedState = null;
        
        ......
        
         resumeRequestLayout(false);
        mLayout.removeAndRecycleScrapInt(mRecycler);
        mState.mPreviousLayoutItemCount = mState.mItemCount;
        mDataSetHasChangedAfterLayout = false;
        mState.mRunSimpleAnimations = false;
        mState.mRunPredictiveAnimations = false;
        onExitLayoutOrScroll();
        mLayout.mRequestedSimpleAnimations = false;
        if (mRecycler.mChangedScrap != null) {
            mRecycler.mChangedScrap.clear();
        }
        mState.mOldChangedHolders = null;

        if (didChildRangeChange(mMinMaxLayoutPositions[0], mMinMaxLayoutPositions[1])) {
            dispatchOnScrolled(0, 0);
        }
        
 }

这里分为几个步骤，分别是记录Vew得状态，然后存在数组中，接着通过LayoutManager 去布局View，最后布局完成后，记录布局后得转态。dispatchLayout 太复杂了，看得有点懵了。

看下draw得实现

@Override
public void draw(Canvas c) {
    super.draw(c);
    final int count = mItemDecorations.size();
    for (int i = 0; i < count; i++) {
        mItemDecorations.get(i).onDrawOver(c, this, mState);
    }
    ...
}
@Override
public void onDraw(Canvas c) {
    super.onDraw(c);
    final int count = mItemDecorations.size();
    for (int i = 0; i < count; i++) {
        mItemDecorations.get(i).onDraw(c, this, mState);
    }

draw 比较简单，就是画出对应得布局在canvas上。