Android View体系（8）从源码解析View的layout和draw流程

相关文章
Android View体系（一）视图坐标系
Android View体系（二）实现View滑动的六种方法
Android View体系（三）属性动画
Android View体系（四）从源码解析Scroller
Android View体系（五）从源码解析View的事件分发机制
Android View体系（六）从源码解析Activity的构成
Android View体系（七）从源码解析View的measure流程

前言

上一篇文章我们讲了View的measure的流程，接下来我们讲下View的layout和draw流程，如果你理解了View的measure的流程，那这篇文章自然就不在话下了。

1.View的layout流程

先来看看View的layout()方法：

 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;        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);            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;    }

传进来里面的四个参数分别是View的四个点的坐标，它的坐标不是相对屏幕的原点，而且相对于它的父布局来说的。 l 和 t 是子控件左边缘和上边缘相对于父类控件左边缘和上边缘的距离；
r 和 b是子控件右边缘和下边缘相对于父类控件左边缘和上边缘的距离。来看看setFrame()方法里写了什么：

    protected boolean setFrame(int left, int top, int right, int bottom) {        boolean changed = false;        if (DBG) {            Log.d("View", this + " View.setFrame(" + left + "," + top + ","                    + right + "," + bottom + ")");        }        if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {            changed = true;            // Remember our drawn bit            int drawn = mPrivateFlags & PFLAG_DRAWN;            int oldWidth = mRight - mLeft;            int oldHeight = mBottom - mTop;            int newWidth = right - left;            int newHeight = bottom - top;            boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);            // Invalidate our old position            invalidate(sizeChanged);            mLeft = left;            mTop = top;            mRight = right;            mBottom = bottom;            mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);        ...省略          }        return changed;    }

在setFrame()方法里主要是用来设置View的四个顶点的值，也就是mLeft 、mTop、mRight和 mBottom的值。在调用setFrame()方法后，调用onLayout()方法：

  protected void onLayout(boolean changed, int left, int top, int right, int bottom) {    }

onLayout()方法没有去做什么，这个和onMeasure()方法类似，确定位置时根据不同的控件有不同的实现，所以在View和ViewGroup中均没有实现onLayout()方法。既然这样，我们就来看看LinearLayout的onLayout()方法：

  @Override    protected void onLayout(boolean changed, int l, int t, int r, int b) {        if (mOrientation == VERTICAL) {            layoutVertical(l, t, r, b);        } else {            layoutHorizontal(l, t, r, b);        }    }

layoutVertical做了什么呢？

 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) {           case Gravity.BOTTOM:               // mTotalLength contains the padding already               childTop = mPaddingTop + bottom - top - mTotalLength;               break;               // mTotalLength contains the padding already           case Gravity.CENTER_VERTICAL:               childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;               break;           case Gravity.TOP:           default:               childTop = mPaddingTop;               break;        }        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);                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {                    case Gravity.CENTER_HORIZONTAL:                        childLeft = paddingLeft + ((childSpace - childWidth) / 2)                                + lp.leftMargin - lp.rightMargin;                        break;                    case Gravity.RIGHT:                        childLeft = childRight - childWidth - lp.rightMargin;                        break;                    case Gravity.LEFT:                    default:                        childLeft = paddingLeft + lp.leftMargin;                        break;                }                if (hasDividerBeforeChildAt(i)) {                    childTop += mDividerHeight;                }                childTop += lp.topMargin;                setChildFrame(child, childLeft, childTop + getLocationOffset(child),                        childWidth, childHeight);                childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);                i += getChildrenSkipCount(child, i);            }        }    }

这个方法会遍历子元素并调用setChildFrame()方法：

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

在setChildFrame()方法中调用子元素的layout()方法来确定自己的位置。我们看到childTop这个值是逐渐增大的，这是为了在垂直方向，子元素是一个接一个排列的而不是重叠的。

2.View的draw流程

View的draw流程很简单，先来看看View的draw()方法：

 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;        // Step 1, draw the background, if needed        int saveCount;        if (!dirtyOpaque) {            drawBackground(canvas);        }...省略   // Step 2, save the canvas' layers        int paddingLeft = mPaddingLeft;        final boolean offsetRequired = isPaddingOffsetRequired();        if (offsetRequired) {            paddingLeft += getLeftPaddingOffset();        }...省略  // Step 3, draw the content        if (!dirtyOpaque) onDraw(canvas);        // Step 4, draw the children        dispatchDraw(canvas);...省略   // Step 5, draw the fade effect and restore layers        final Paint p = scrollabilityCache.paint;        final Matrix matrix = scrollabilityCache.matrix;        final Shader fade = scrollabilityCache.shader;...省略  // Step 6, draw decorations (scrollbars)        onDrawScrollBars(canvas);        if (mOverlay != null && !mOverlay.isEmpty()) {            mOverlay.getOverlayView().dispatchDraw(canvas);        }   }

从源码的注释我们看到draw流程有六个步骤：

1. 如果有设置背景，则绘制背景
2. 保存canvas层
3. 绘制自身内容
4. 绘制子元素
5. 绘制效果
6. 绘制装饰品(scrollbars)

好了，关于View的工作流程就讲到这里了，接下来会讲到自定义View。