Android View体系（八）从源码解析View的layout和draw流程

本文首发于微信公众号「后厂技术官」

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