layout过程及draw过程

Layout

分析完measure过程后,layout和draw过程则相对简单一点。
在performTraversals过程中的代码片段如下:

        //layout布局要求标志位
        final boolean didLayout = layoutRequested && (!mStopped || mReportNextDraw);
        boolean triggerGlobalLayoutListener = didLayout
                || mAttachInfo.mRecomputeGlobalAttributes;
        if (didLayout) {
            //--------开始执行布局操作---------
            performLayout(lp, mWidth, mHeight);

            // By this point all views have been sized and positioned
            // We can compute the transparent area
            //计算透明区域
            ....
        }

    private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
            int desiredWindowHeight) {
        ....

        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
        try {
            host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

            mInLayout = false;
            int numViewsRequestingLayout = mLayoutRequesters.size();
            if (numViewsRequestingLayout > 0) {
                // requestLayout() was called during layout.
                // If no layout-request flags are set on the requesting views, there is no problem.
                // If some requests are still pending, then we need to clear those flags and do
                // a full request/measure/layout pass to handle this situation.

                //layout期间仍有layout请求未处理则需要重新进行三个流程,
                //完成三大流程仍未完成则等待下一帧
                ......

            }
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        mInLayout = false;
    }

可以看到host.layout的调用,host即是DectorView。layout之后处理仍未消除的layout标志(重新走流程),若仍未消除标志通过requestLayout等待下一帧。所以跟踪去View#layout

    public void layout(int l, int t, int r, int b) {
        if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }

        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;
        //确定frame的四个端点
        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);//onLayout父元素确定子元素位置

            ......

            mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnLayoutChangeListeners != null) {
                ArrayList<OnLayoutChangeListener> listenersCopy =
                        (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
                int numListeners = listenersCopy.size();
                for (int i = 0; i < numListeners; ++i) {
                    listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
                }
            }
        }

        mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
        mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;

        ......
    }

在rootView起调layout流程时,传入host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); 就不难理解为确定DectorView的位置为屏幕原点到DectorView测量大小。而layout过程中调用的onLayout则View/ViewGroup并没有实现,由子类实现对应方法。而ViewGroup主要为布局子view位置,对应的实现分析:

  • LinearLayout (vertical):确定children布局的上边界,遍历子view按顺序通过setChildFrame确定children位置。
  • RelativeLayout:子view在measure过程中两次measure已经确定了view的位置,这时只需要获取LayoutParams中位置信息即可
  • FrameLayout:子view根据自身测量高度、自身padding/margin和父布局padding/margin共同确定位置信息。

最后通过child.layout(Left, Top, Right, Bottom);确定了view的位置。在View#layout中setFrame将赋值四个边界的值。这是getWidth、getHeight则可以拿到正确的view的大小。而根据源码分析则可以发现getMeasuredWidth和getWidth的值是一样只是赋值的时机不同

    public final int getHeight() {
        return mBottom - mTop;
    }
    public final int getWidth() {
        return mRight - mLeft;
    }

但是重写layout方法还是可以实现getWidth与getMeasuredWidth对应值不一致,但是实质意义不大。

    public void layout(int l, int t, int r, int b) {
        super.layout(l, t, r + 50, b + 50);
    }

Draw

走完layout流程一般将会进行draw流程。在ViewRootImpl中performDraw

      //performTraversals
        if (!cancelDraw && !newSurface) {
            ....
            //-----开始执行绘制操作-------
            performDraw();
        }

    private void performDraw() {
        .......

        final boolean fullRedrawNeeded = mFullRedrawNeeded;
        mFullRedrawNeeded = false;

        mIsDrawing = true;
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
        try {
            draw(fullRedrawNeeded);  // ----绘制
        } finally {
            mIsDrawing = false;
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }

        ......
    }

    private void draw(boolean fullRedrawNeeded) {
        Surface surface = mSurface;
        if (!surface.isValid()) {
            return;
        }

        .....

        mAttachInfo.mTreeObserver.dispatchOnDraw();

        .....

        if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
            if (mAttachInfo.mThreadedRenderer != null && mAttachInfo.mThreadedRenderer.isEnabled()) {
                ......
                mAttachInfo.mThreadedRenderer.draw(mView, mAttachInfo, this);
            } else {
                // If we get here with a disabled & requested hardware renderer, something went
                // wrong (an invalidate posted right before we destroyed the hardware surface
                // for instance) so we should just bail out. Locking the surface with software
                // rendering at this point would lock it forever and prevent hardware renderer
                // from doing its job when it comes back.
                // Before we request a new frame we must however attempt to reinitiliaze the
                // hardware renderer if it's in requested state. This would happen after an
                // eglTerminate() for instance.
                if (mAttachInfo.mThreadedRenderer != null &&
                        !mAttachInfo.mThreadedRenderer.isEnabled() &&
                        mAttachInfo.mThreadedRenderer.isRequested()) {

                    ......
                
                    scheduleTraversals();
                    return;
                }

                if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
                    return;
                }
            }
        }

        if (animating) {
            mFullRedrawNeeded = true;
            scheduleTraversals();
        }
    }

则可以看到最终如果进行软件绘制的话则通过surface进行绘制流程。这里稍微说明下就是,每一个Surface中保存了一个Canvas对象。Surface是原始图像缓冲区(raw buffer)的一个句柄,而原始图像缓冲区是由屏幕图像合成器(screen compositor)管理的。通过操作Canvas将绘制图像保存到Canvas中的bitmap,然后通过Surface处理图像缓冲将canvas中bitmap内容绘制到屏幕。

    private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
            boolean scalingRequired, Rect dirty) {

        // Draw with software renderer.
        final Canvas canvas;
        try {
            .....

            canvas = mSurface.lockCanvas(dirty);

            .....
        } catch (Surface.OutOfResourcesException e) {
            handleOutOfResourcesException(e);
            return false;
        } catch (IllegalArgumentException e) {
            ....
        }

        try {
            .....
            try {
                canvas.translate(-xoff, -yoff);

                ....

                mView.draw(canvas);

                drawAccessibilityFocusedDrawableIfNeeded(canvas);
            } finally {
                .......
            }
        } finally {
            try {
                surface.unlockCanvasAndPost(canvas);
            } catch (IllegalArgumentException e) {
                ......
            }

            .......
        }
        return true;
    }

drawSoftware方法通过Surface.lockCanvas获取到对应canvas,操作完canvas通过surface.unlockCanvasAndPost(canvas)释放canvas。
在使用SurfaceView时我们需要自己手动调用这两个方法,而View中则是由ViewRootImpl系统类自行调用。
而从上面的代码中看到我们最终的绘制流程的调用mView.draw(canvas);其中mView即DectorView。

    public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

        /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            drawBackground(canvas);
        }

        // skip step 2 & 5 if possible (common case)
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);

            // Step 4, draw the children
            dispatchDraw(canvas);

            drawAutofilledHighlight(canvas);

            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(canvas);

            // Step 7, draw the default focus highlight
            drawDefaultFocusHighlight(canvas);

            if (debugDraw()) {
                debugDrawFocus(canvas);
            }

            // we're done...
            return;
        }

        ......
    }

注释中说的很清楚了,绘制的过程主要分为4个步骤(本来6个步骤,一般情况下跳过2和5):

  1. 绘制背景drawBackground(canvas) -> background.draw(canvas);
  2. 绘制自己onDraw(canvas);
  3. 绘制子ViewdispatchDraw(canvas);,该方法在VIewGroup中有实现,即遍历调用子view的draw方法
  4. 绘制前景(前景、scrollbars等等)onDrawForeground(canvas);

view中有个特殊的方法setWillNotDraw。View不需要绘制时可以通过setWillNotDraw(true)优化绘制流程。ViewGroup默认开启这个标志位,所以ViewGroup需要绘制功能需要将这个标志位置false。

    /**
     * If this view doesn't do any drawing on its own, set this flag to
     * allow further optimizations. By default, this flag is not set on
     * View, but could be set on some View subclasses such as ViewGroup.
     *
     * Typically, if you override {@link #onDraw(android.graphics.Canvas)}
     * you should clear this flag.
     *
     * @param willNotDraw whether or not this View draw on its own
     */
    public void setWillNotDraw(boolean willNotDraw) {
        setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
    }

dispatchDraw在ViewGroup中的实现:

    protected void dispatchDraw(Canvas canvas) {
        boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
        final int childrenCount = mChildrenCount;
        final View[] children = mChildren;
        int flags = mGroupFlags;

        .....

        int clipSaveCount = 0;
        final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
        //是否禁止绘制到padding区域
        if (clipToPadding) {
            clipSaveCount = canvas.save(Canvas.CLIP_SAVE_FLAG);
            canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
                    mScrollX + mRight - mLeft - mPaddingRight,
                    mScrollY + mBottom - mTop - mPaddingBottom);
        }

        .....

        // Draw any disappearing views that have animations
        if (mDisappearingChildren != null) {
            final ArrayList<View> disappearingChildren = mDisappearingChildren;
            final int disappearingCount = disappearingChildren.size() - 1;
            // Go backwards -- we may delete as animations finish
            for (int i = disappearingCount; i >= 0; i--) {
                final View child = disappearingChildren.get(i);
                more |= drawChild(canvas, child, drawingTime);
            }
        }
        
        ....

        if (clipToPadding) {
            canvas.restoreToCount(clipSaveCount);
        }

        // mGroupFlags might have been updated by drawChild()
        flags = mGroupFlags;

        if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {
            invalidate(true);
        }

        ......
    }

LinearLayout

    void layoutVertical(int left, int top, int right, int bottom) {
        final int paddingLeft = mPaddingLeft;

        int childTop;
        int childLeft;

        // Where right end of child should go
        final int width = right - left;
        int childRight = width - mPaddingRight;

        // Space available for child
        int childSpace = width - paddingLeft - mPaddingRight;

        final int count = getVirtualChildCount();

        final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
        final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;

        switch (majorGravity) {//根据gravity,计算children的上边界
           ......
        }

        for (int i = 0; i < count; i++) {
            final View child = getVirtualChildAt(i);
            if (child == null) {
                childTop += measureNullChild(i);
            } else if (child.getVisibility() != GONE) {
                final int childWidth = child.getMeasuredWidth();
                final int childHeight = child.getMeasuredHeight();

                final LinearLayout.LayoutParams lp =
                        (LinearLayout.LayoutParams) child.getLayoutParams();

                int gravity = lp.gravity;
                if (gravity < 0) {
                    gravity = minorGravity;
                }
                final int layoutDirection = getLayoutDirection();
                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                //根据水平方向的children的gravity确定children左边界
                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                    .....
                }

                if (hasDividerBeforeChildAt(i)) {
                    childTop += mDividerHeight;
                }

                childTop += lp.topMargin;
                //设置children边界
                setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                        childWidth, childHeight);
                childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

                i += getChildrenSkipCount(child, i);
            }
        }
    }

    private void setChildFrame(View child, int left, int top, int width, int height) {
        child.layout(left, top, left + width, top + height);
    }

RelativeLayout

    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        //  The layout has actually already been performed and the positions
        //  cached.  Apply the cached values to the children.
        final int count = getChildCount();

        for (int i = 0; i < count; i++) {
            View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                RelativeLayout.LayoutParams st =
                        (RelativeLayout.LayoutParams) child.getLayoutParams();
                child.layout(st.mLeft, st.mTop, st.mRight, st.mBottom);
            }
        }
    }

FrameLayout

    void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
       ......

        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();

                final int width = child.getMeasuredWidth();
                final int height = child.getMeasuredHeight();

                int childLeft;
                int childTop;

                int gravity = lp.gravity;
                if (gravity == -1) {
                    gravity = DEFAULT_CHILD_GRAVITY;
                }

                final int layoutDirection = getLayoutDirection();
                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
                //根据水平Gravity及padding、margin确定child的left位置
                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                    .....
                }
                //根据垂直Gravity及padding、margin确定child的Top位置
                switch (verticalGravity) {
                    .....
                }

                child.layout(childLeft, childTop, childLeft + width, childTop + height);
            }
        }
    }
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