android view绘制流程 面试

一、view树的绘制流程

measure--->layout--->draw

measure

1、ViewGroup.LayoutParams 指定部件的长宽

2、MeasureSpec 32位的int值 前两位代表模式 后30位测量规格的大小

layout

draw

invalidate()请求android系统 如果大小没有发生变化 就不会调用layout放置这个过程

requestLayout() 当布局发生变化时 希望重新测量尺寸大小 就会触发 measure和layout 但不会调用draw方法

View的绘制流程：OnMeasure()——>OnLayout()——>OnDraw()

各步骤的主要工作：

• OnMeasure()：测量视图大小。从顶层父View到子View递归调用measure方法，measure方法又回调OnMeasure。

• OnLayout()：确定View位置，进行页面布局。从顶层父View向子View的递归调用view.layout方法的过程，即父View根据上一步measure子View所得到的布局大小和布局参数，将子View放在合适的位置上。

• OnDraw()：绘制视图。ViewRoot创建一个Canvas对象，然后调用OnDraw()。六个步骤：①、绘制视图的背景；②、保存画布的图层（Layer）；③、绘制View的内容；④、绘制View子视图，如果没有就不用；⑤、还原图层（Layer）；⑥、绘制滚动条。

OnMeasure

View树的绘制是从ViewRoot的performTraversals()方法开始，这个方法的主要作用是判断是否重新measure、是否重新layout、是否重新draw。

  private void performTraversals() {
        ......
        int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
        int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
        ......
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        ......
        mView.layout(0, 0, mView.getMeasuredWidth(), mView.getMeasuredHeight());
        ......
        mView.draw(canvas);
        ......
    }

    private int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {
            case ViewGroup.LayoutParams.MATCH_PARENT:
                measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
                break;
            case ViewGroup.LayoutParams.WRAP_CONTENT:
                measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
                break;
            default:
                measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
                break;
        }
        return measureSpec;
    } 

    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {
            mPrivateFlags &= ~MEASURED_DIMENSION_SET;
            if (ViewDebug.TRACE_HIERARCHY) {
                ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_MEASURE);
            }
            onMeasure(widthMeasureSpec, heightMeasureSpec);
            if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }
            mPrivateFlags |= LAYOUT_REQUIRED;
        }
        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;

    //View的onMeasure默认实现方法，设置View的长和宽；如果写死，则外边变化时界面不变化
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

    public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);
        switch (specMode) {
            case MeasureSpec.UNSPECIFIED:
                result = size;
                break;
            case MeasureSpec.AT_MOST:
            case MeasureSpec.EXACTLY:
                result = specSize;
                break;
        }
        return result;
    }  

如果是ViewGroup，还应该进行嵌套测量

 protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
        final int size = mChildrenCount;
        final View[] children = mChildren;
        for (int i = 0; i < size; ++i) {
            final View child = children[i];
            if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
            }
        }
    }

    protected void measureChild(View child, int parentWidthMeasureSpec,
                                int parentHeightMeasureSpec) {
        final LayoutParams lp = child.getLayoutParams();
        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);
        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }  

    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);
        int size = Math.max(0, specSize - padding);
        int resultSize = 0;
        int resultMode = 0;
        switch (specMode) {
            case MeasureSpec.EXACTLY:
                if (childDimension >= 0) {
                    resultSize = childDimension;
                    resultMode = MeasureSpec.EXACTLY;
                } else if (childDimension == LayoutParams.MATCH_PARENT) {
                    resultSize = size;
                    resultMode = MeasureSpec.EXACTLY;
                } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                    // Child wants to determine its own size. It can't be
                    // bigger than us.
                    resultSize = size;
                    resultMode = MeasureSpec.AT_MOST;
                }
                break;
            ......
        }
        //将mode与size通过MeasureSpec方法整合为32位整数返回
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

MeasureSpec 的值由specSize和specMode共同组成的，高2位代表specMode，低30代表spceSize，其中specSize记录的是大小，specMode记录的是规格。

specMode一共有三种类型：

1. EXACTLY 
   表示父视图希望子视图的大小应该是由specSize的值来决定的，系统默认会按照这个规则来设置子视图的大小，开发人员当然也可以按照自己的意愿设置成任意的大小。

2. AT_MOST 
   表示子视图最多只能是specSize中指定的大小，开发人员应该尽可能小得去设置这个视图，并且保证不会超过specSize。系统默认会按照这个规则来设置子视图的大小，开发人员当然也可以按照自己的意愿设置成任意的大小。

3. UNSPECIFIED 
   表示开发人员可以将视图按照自己的意愿设置成任意的大小，没有任何限制。这种情况比较少见。

每个View控件的实际宽高都是由父视图和自身决定的。实际的测量是在onMeasure方法进行，所以在View的子类需要重写onMeasure方法，这是因为measure方法是final的，不允许重载，所以View子类只能通过重载onMeasure来实现自己的测量逻辑。

OnLayout

测量完各个组件的大小之后，就可以排列他们的位置了。

private void performTraversals() {
        ......
        int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
        int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
        ......
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        ......
        mView.layout(0, 0, mView.getMeasuredWidth(), mView.getMeasuredHeight());
        ......
        mView.draw(canvas);
        ......
    }

    public void layout(int l, int t, int r, int b) {
        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;
        boolean changed = setFrame(l, t, r, b);//视图大小是否发生变化
        if (changed || (mPrivateFlags & LAYOUT_REQUIRED) == LAYOUT_REQUIRED) {
            if (ViewDebug.TRACE_HIERARCHY) {
                ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_LAYOUT);
            }
            onLayout(changed, l, t, r, b);
            mPrivateFlags &= ~LAYOUT_REQUIRED;
            if (mOnLayoutChangeListeners != null) {
                ArrayList<OnLayoutChangeListener> listenersCopy =
                        (ArrayList<OnLayoutChangeListener>) 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 &= ~FORCE_LAYOUT;
    }  

View中的onLayout()方法就是一个空方法，因为onLayout()过程是为了确定视图在布局中所在的位置，而这个操作应该是由布局来完成的，即父视图决定子视图的显示位置。

@Override
/**
*ViewGroupd的onLayout方法
*/
protected abstract void onLayout(boolean changed, int l, int t, int r, int b);  

然而，ViewGroup中的onLayout()方法是一个抽象方法，这就意味着所有ViewGroup的子类都必须重写这个方法。自定义ViewGroup控件中，onLayout配合onMeasure方法一起使用可以实现自定义View的复杂布局。自定义View首先调用onMeasure进行测量，然后调用onLayout方法动态获取子View和子View的测量大小，然后进行layout布局。重载onLayout的目的就是安排其children在父View的具体位置，重载onLayout通常做法就是写一个for循环调用每一个子视图的layout(l, t, r, b)函数，传入不同的参数l, t, r, b来确定每个子视图在父视图中的显示位置。

下面代码以LinearLayout举例：

public class LinearLayout extends ViewGroup {
    @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);
        }
    }
}

void layoutVertical(int left, int top, int right, int bottom) {
        final int paddingLeft = mPaddingLeft;
        int childTop;
        int childLeft;
        final int width = right - left;
        int childRight = width - mPaddingRight;
        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:
                childTop = mPaddingTop + bottom - top - mTotalLength;
                break;
            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);
            }
        }
    }

到这里就不得不提getWidth()、getHeight()和getMeasuredWidth()、getMeasuredHeight()这两对方法之间的区别（上面分析measure过程已经说过getMeasuredWidth()、getMeasuredHeight()必须在onMeasure之后使用才有效）。可以看出来getWidth()与getHeight()方法必须在layout(int l, int t, int r, int b)执行之后才有效。那我们看下View源码中这些方法的实现吧，如下：

 public final int getMeasuredWidth() {
        return mMeasuredWidth & MEASURED_SIZE_MASK;
    }

    public final int getMeasuredHeight() {
        return mMeasuredHeight & MEASURED_SIZE_MASK;
    }

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

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

    public final int getLeft() {
        return mLeft;
    }

    public final int getRight() {
        return mRight;
    }

    public final int getTop() {
        return mTop;
    }

    public final int getBottom() {
        return mBottom;
    }

OnDraw

public void draw(Canvas canvas) {
    if (ViewDebug.TRACE_HIERARCHY) {
        ViewDebug.trace(this, ViewDebug.HierarchyTraceType.DRAW);
    }
    final int privateFlags = mPrivateFlags;
    final boolean dirtyOpaque = (privateFlags & DIRTY_MASK) == DIRTY_OPAQUE &&
            (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
    mPrivateFlags = (privateFlags & ~DIRTY_MASK) | DRAWN;
    // Step 1, draw the background, if needed
    int saveCount;
    if (!dirtyOpaque) {
        final Drawable background = mBGDrawable;
        if (background != null) {
            final int scrollX = mScrollX;
            final int scrollY = mScrollY;
            if (mBackgroundSizeChanged) {
                background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);
                mBackgroundSizeChanged = false;
            }
            if ((scrollX | scrollY) == 0) {
                background.draw(canvas);
            } else {
                canvas.translate(scrollX, scrollY);
                background.draw(canvas);
                canvas.translate(-scrollX, -scrollY);
            }
        }
    }
    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);
        // Step 6, draw decorations (scrollbars)
        onDrawScrollBars(canvas);
        // we're done...
        return;
    }
}