RecyclerView源码解析(一)开篇

@Override
protected void onMeasure(int widthSpec, int heightSpec) {
    if (mLayout == null) {
       ...
       return;
    }
    if (mLayout.isAutoMeasureEnabled()) {
        ...
    } else {
        ...
    }
}

if (mLayout == null) {
    defaultOnMeasure(widthSpec, heightSpec);
    return;
}

/**

* An implementation of {@link View#onMeasure(int, int)} to fall back to in various scenarios
* where this RecyclerView is otherwise lacking better information.
*/

void defaultOnMeasure(int widthSpec, int heightSpec) {
    // calling LayoutManager here is not pretty but that API is already public and it is better
    // than creating another method since this is internal.
    final int width = LayoutManager.chooseSize(widthSpec,
            getPaddingLeft() + getPaddingRight(),
            ViewCompat.getMinimumWidth(this));
    final int height = LayoutManager.chooseSize(heightSpec,
            getPaddingTop() + getPaddingBottom(),
            ViewCompat.getMinimumHeight(this));

    setMeasuredDimension(width, height);
}

if (mLayout.isAutoMeasureEnabled()) {
    final int widthMode = MeasureSpec.getMode(widthSpec);
    final int heightMode = MeasureSpec.getMode(heightSpec);
    
    mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);

    final boolean measureSpecModeIsExactly =
            widthMode == MeasureSpec.EXACTLY && heightMode == MeasureSpec.EXACTLY;
    if (measureSpecModeIsExactly || mAdapter == null) {
        return;
    }

    if (mState.mLayoutStep == State.STEP_START) {
        dispatchLayoutStep1();
    }
    // set dimensions in 2nd step. Pre-layout should happen with old dimensions for
    // consistency
    ...
    mState.mIsMeasuring = true;
    dispatchLayoutStep2();

    // now we can get the width and height from the children.
    mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
    
    // if RecyclerView has non-exact width and height and if there is at least one child
    // which also has non-exact width & height, we have to re-measure.
    if (mLayout.shouldMeasureTwice()) {
       ...
        dispatchLayoutStep2();
        // now we can get the width and height from the children.
        mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
    }
}

void setMeasuredDimensionFromChildren(int widthSpec, int heightSpec) {
    final int count = getChildCount();
    if (count == 0) {
        mRecyclerView.defaultOnMeasure(widthSpec, heightSpec);
        return;
    }
    int minX = Integer.MAX_VALUE;
    int minY = Integer.MAX_VALUE;
    int maxX = Integer.MIN_VALUE;
    int maxY = Integer.MIN_VALUE;

    for (int i = 0; i < count; i++) {
        View child = getChildAt(i);
        final Rect bounds = mRecyclerView.mTempRect;
        getDecoratedBoundsWithMargins(child, bounds);
        if (bounds.left < minX) {
            minX = bounds.left;
        }
        if (bounds.right > maxX) {
            maxX = bounds.right;
        }
        if (bounds.top < minY) {
            minY = bounds.top;
        }
        if (bounds.bottom > maxY) {
            maxY = bounds.bottom;
        }
    }
    mRecyclerView.mTempRect.set(minX, minY, maxX, maxY);
    setMeasuredDimension(mRecyclerView.mTempRect, widthSpec, heightSpec);
}

@Override
boolean shouldMeasureTwice() {
    return getHeightMode() != View.MeasureSpec.EXACTLY
            && getWidthMode() != View.MeasureSpec.EXACTLY
            && hasFlexibleChildInBothOrientations();
}

boolean hasFlexibleChildInBothOrientations() {
    final int childCount = getChildCount();
    for (int i = 0; i < childCount; i++) {
        final View child = getChildAt(i);
        final ViewGroup.LayoutParams lp = child.getLayoutParams();
        if (lp.width < 0 && lp.height < 0) {
            return true;
        }
    }
    return false;
}

else {
    if (mHasFixedSize) {
        mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
        return;
    }
    ...

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

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    ...
    dispatchLayout();
    ...
}

void dispatchLayout() {
    if (mAdapter == null) {
        Log.e(TAG, "No adapter attached; skipping layout");
        // leave the state in START
        return;
    }
    if (mLayout == null) {
        Log.e(TAG, "No layout manager attached; skipping layout");
        // leave the state in START
        return;
    }
    mState.mIsMeasuring = false;
    if (mState.mLayoutStep == State.STEP_START) {
        dispatchLayoutStep1();
        ...
        dispatchLayoutStep2();
    } else if (mAdapterHelper.hasUpdates() || mLayout.getWidth() != getWidth()
            || mLayout.getHeight() != getHeight()) {
        // First 2 steps are done in onMeasure but looks like we have to run again due to
        // changed size.
        mLayout.setExactMeasureSpecsFrom(this);
        dispatchLayoutStep2();
    }
    ...
    dispatchLayoutStep3();
}

/**
* The second layout step where we do the actual layout of the views for the final state.
* This step might be run multiple times if necessary (e.g. measure).
*/

private void dispatchLayoutStep2() {
    ...
    // Step 2: Run layout
    ...
    mLayout.onLayoutChildren(mRecycler, mState);
    ...
}

@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);
    }
}