Android View體系（八）從原始碼解析View的layout和draw流程

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Android View體系（五）從原始碼解析View的事件分發機制
Android View體系（六）從原始碼解析Activity的構成
Android View體系（七）從原始碼解析View的measure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，假設你理解了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;    }

傳進來裡面的四個參數各自是View的四個點的座標。它的座標不是相對螢幕的原點，並且相對於它的父布局來說的。

l 和 t 是子控制項左邊緣和上邊緣相對於父類控制項左邊緣和上邊緣的距離。
r 和 b是子控制項右邊緣和下邊緣相對於父類控制項左邊緣和上邊緣的距離。來看看setFrame()方法裡寫了什麼：

    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()方法裡主要是用來設定View的四個頂點的值，也就是mLeft 、mTop、mRight和 mBottom的值。在調用setFrame()方法後，調用onLayout()方法：

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

onLayout()方法沒有去做什麼，這個和onMeasure()方法相似，確定位置時依據不同的控制項有不同的實現，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做了什麼呢？

 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);            }        }    }

這種方法會遍曆子項目並調用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()方法中調用子項目的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，子項目是一個接一個排列的而不是重疊的。

2.View的draw流程

View的draw流程非常easy。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);        }...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);        }   }

從原始碼的凝視我們看到draw流程有六個步驟。當中第2步和第5步能夠跳過：

1. 假設有設定背景。則繪製背景
2. 儲存canvas層
3. 繪製自身內容
4. 假設有子項目則繪製子項目
5. 繪製效果
6. 繪製裝飾品(scrollbars)

好了，關於View的工作流程就說到這裡了，接下來會講到自己定義View。

Android View體系（八）從原始碼解析View的layout和draw流程