Android View體系（八）從源碼解析View的layout和draw流程
前言
上一篇文章我們講了View的measure的流程，接下來我們講下View的layout和draw流程，如果你理解了View的measure的流程，那這篇文章自然就不在話下了。
1.View的layout流程
先來看看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; }
傳進來里面的四個參數分別是View的四個點的坐標，它的坐標不是相對屏幕的原點，而且相對于它的父布局來說的。 l 和 t 是子控件左邊緣和上邊緣相對于父類控件左邊緣和上邊緣的距離；
r 和 b是子控件右邊緣和下邊緣相對于父類控件左邊緣和上邊緣的距離。來看看setFrame()方法里寫了什么：
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()方法里主要是用來設置View的四個頂點的值，也就是mLeft 、mTop、mRight和 mBottom的值。在調用setFrame()方法后，調用onLayout()方法：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) { }
onLayout()方法沒有去做什么，這個和onMeasure()方法類似，確定位置時根據不同的控件有不同的實現，所以在View和ViewGroup中均沒有實現onLayout()方法。既然這樣，我們就來看看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); } } }
這個方法會遍歷子元素并調用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()方法中調用子元素的layout()方法來確定自己的位置。我們看到childTop這個值是逐漸增大的，這是為了在垂直方向，子元素是一個接一個排列的而不是重疊的。
2.View的draw流程
View的draw流程很簡單，先來看看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); } }
從源碼的注釋我們看到draw流程有六個步驟，其中第2步和第5步可以跳過：
- 如果有設置背景，則繪制背景
- 保存canvas層
- 繪制自身內容
- 如果有子元素則繪制子元素
- 繪制效果
- 繪制裝飾品(scrollbars)
好了，關于View的工作流程就講到這里了，接下來會講到自定義View。
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