View加载详解（二）

上篇我们讲到了ViewRootImpl的performTraversals方法performMeasure测量之前要通过getRootMeasureSpec方法获得顶层视图DecorView的测量规格，跟踪代码进入getRootMeasureSpec（）

/** * Figures out the measure spec for the root view in a window based on it's * layout params. * * @param windowSize * The available width or height of the window * * @param rootDimension * The layout params for one dimension (width or height) of the * window. * * @return The measure spec to use to measure the root view. */
    private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {
        //匹配父容器时，测量模式为MeasureSpec.EXACTLY，测量大小直接为屏幕的大小，也就是充满真个屏幕
        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        //包裹内容时，测量模式为MeasureSpec.AT_MOST，测量大小直接为屏幕大小，也就是充满真个屏幕
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        //其他情况时，测量模式为MeasureSpec.EXACTLY，测量大小为DecorView顶层视图布局设置的大小。
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

分析：该方法主要作用是在整个窗口的基础上计算出root view（顶层视图DecorView）的测量规格，该方法的两个参数分别表示：
windowSize：当前手机窗口的有效宽和高，一般都是除了通知栏的屏幕宽和高
rootDimension 根布局DecorView请求的宽和高，由前面的博客我们知道是MATCH_PARENT
由 《从setContentView方法分析Android加载布局流程》可知，我们的DecorView根布局宽和高都是MATCH_PARENT，因此DecorView根布局的测量模式就是MeasureSpec.EXACTLY，测量大小一般都是整个屏幕大小，所以一般我们的Activity
窗口都是全屏的。因此上面代码走第一个分支，通过调用MeasureSpec.makeMeasureSpec方法将
DecorView的测量模式和测量大小封装成DecorView的测量规格。
由于performMeasure（）方法调用了 View中measure（）方法俩进行测量，并且DecorView（继承自FrameLayout）的父类是ViewGroup，祖父类是View。因此我们从View的成员函数measure开始分析整个测量过程。

虽然说上面的可能有点枯燥，大家感觉没有卵用，但下面重头戏开始了

int mOldWidthMeasureSpec = Integer.MIN_VALUE;

    int mOldHeightMeasureSpec = Integer.MIN_VALUE;

 public final void measure(int widthMeasureSpec, int heightMeasureSpec) {

        ..................
        //如果上一次的测量规格和这次不一样，则条件满足，重新测量视图View的大小
        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

            resolveRtlPropertiesIfNeeded();

            int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
                    mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
                onMeasure(widthMeasureSpec, heightMeasureSpec);
                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            } else {
                long value = mMeasureCache.valueAt(cacheIndex);
                // Casting a long to int drops the high 32 bits, no mask needed
                setMeasuredDimensionRaw((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }

            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;

    }

 if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

判断当前视图View是否需要重新测量，当上一次视图View测量的规格和本次视图View测量规格不一样时，就说明视图View的大小有改变，因此需要重新测量
然后调用了onMeasure方法进行测量，说明View主要的测量逻辑是在该方法中实现

/**
     * <p>
     * Measure the view and its content to determine the measured width and the
     * measured height. This method is invoked by {@link #measure(int, int)} and
     * should be overriden by subclasses to provide accurate and efficient
     * measurement of their contents.
     * </p>
     *
     * <p>
     * <strong>CONTRACT:</strong> When overriding this method, you
     * <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the
     * measured width and height of this view. Failure to do so will trigger an
     * <code>IllegalStateException</code>, thrown by
     * {@link #measure(int, int)}. Calling the superclass'
     * {@link #onMeasure(int, int)} is a valid use.
     * </p>
     *
     * <p>
     * The base class implementation of measure defaults to the background size,
     * unless a larger size is allowed by the MeasureSpec. Subclasses should
     * override {@link #onMeasure(int, int)} to provide better measurements of
     * their content.
     * </p>
     *
     * <p>
     * If this method is overridden, it is the subclass's responsibility to make * sure the measured height and width are at least the view's minimum height
     * and width ({@link #getSuggestedMinimumHeight()} and
     * {@link #getSuggestedMinimumWidth()}).
     * </p>
     *
     * @param widthMeasureSpec horizontal space requirements as imposed by the parent.
     *                         The requirements are encoded with
     *                         {@link android.view.View.MeasureSpec}.
     * @param heightMeasureSpec vertical space requirements as imposed by the parent.
     *                         The requirements are encoded with
     *                         {@link android.view.View.MeasureSpec}.
     *
     * @see #getMeasuredWidth()
     * @see #getMeasuredHeight()
     * @see #setMeasuredDimension(int, int)
     * @see #getSuggestedMinimumHeight()
     * @see #getSuggestedMinimumWidth()
     * @see android.view.View.MeasureSpec#getMode(int)
     * @see android.view.View.MeasureSpec#getSize(int)
     */
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

该方法的实现也很简单，直接调用setMeasuredDimension方法完成视图View的测量。我们知道，Android中所有的视图组件都是继承自View实现的。因此该方法提供了一个默认测量视图View大小的实现。言外之意，如果你不想你自己的View使用默认实现来测量View的宽高的话，你可以在子类中重写onMeasure方法来自定义测量方法。我们先来看看默认测量宽高的实现。跟踪代码进入getDefaultSize方法

 /** * Utility to return a default size. Uses the supplied size if the * MeasureSpec imposed no constraints. Will get larger if allowed * by the MeasureSpec. * * @param size Default size for this view * @param measureSpec Constraints imposed by the parent * @return The size this view should be. */
    public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        //获得测量模式
        int specMode = MeasureSpec.getMode(measureSpec);
        //获得父亲容器留给子视图View的大小
        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;
    }

分析：该方法的作用是根据View布局设置的宽高和父View传递的测量规格重新计算View的测量宽高。由此可以知道，我们布局的子View最终的大小是由布局大小和父容器的测量规格共同决定的。如果自定义View你没有重写onMeasure使用系统默认方法的话，测量模式MeasureSpec.AT_MOST和MeasureSpec.EXACTLY下的测量大小是一样的。我们来总结一下测量模式的种类：
MeasureSpec.EXACTLY：确定模式，父容器希望子视图View的大小是固定，也就是specSize大小。
MeasureSpec.AT_MOST：最大模式，父容器希望子视图View的大小不超过父容器希望的大小，也就是不超过specSize大小。
MeasureSpec.UNSPECIFIED: 不确定模式，子视图View请求多大就是多大，父容器不限制其大小范围，也就是size大小。
从上面代码可以看出，当测量模式是MeasureSpec.UNSPECIFIED时，View的测量值为size，当测量模式为MeasureSpec.AT_MOST或者case MeasureSpec.EXACTLY时，View的测量值为specSize。我们知道，specSize是由父容器决定，那么size是怎么计算出来的呢？getDefaultSize方法的第一个参数是调用getSuggestedMinimumWidth方法获得。进入getSuggestedMinimumWidth方法看看实现：

/**
     * Returns the suggested minimum width that the view should use. This
     * returns the maximum of the view's minimum width)
     * and the background's minimum width
     *  ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).
     * <p>
     * When being used in {@link #onMeasure(int, int)}, the caller should still
     * ensure the returned width is within the requirements of the parent.
     *
     * @return The suggested minimum width of the view.
     */
    protected int getSuggestedMinimumWidth() {
 return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
    }

原来size大小是获取View属性当中的最小值，也就是 android:minWidth和 android:minHeight的值，前提是View没有设置背景属性。否则就在最小值和背景的最小值中间取最大值。sizeSpec大小是有父容器决定的，我们知道父容器DecorView的测量模式是MeasureSpec.EXACTLY，测量大小sizeSpec是整个屏幕的大小。而DecorView是继承自FrameLayout的，那么我们来看看FrameLayout类中的onMeasure方法的实现

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();
        ..............
        int maxHeight = 0;
        int maxWidth = 0;
        int childState = 0;

        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                //测量FrameLayout下每个子视图View的宽和高
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState, child.getMeasuredState());
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
            }
        }

        // Account for padding too
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        // Check against our foreground's minimum height and width
        final Drawable drawable = getForeground();
        if (drawable != null) {
            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
        }
        //设置当前FrameLayout测量结果，此方法的调用表示当前View测量的结束。
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));
}

分析：由以上代码发现，ViewGroup测量结果都是带边距的，前面部分就是遍历测量FrameLayout下子视图View的大小了。最后调用setMeasuredDimension方法设置当前View的测量结果，此方法的调用表示当前View测量结束。那么我们来分析下代码第12行measureChildWithMargins方法测FrameLayout下的子视图View的大小，跟踪源码：
由于FrameLayout父类是ViewGroup，measureChildWithMargins方法在ViewGroup下

/**
     * Ask one of the children of this view to measure itself, taking into
     * account both the MeasureSpec requirements for this view and its padding
     * and margins. The child must have MarginLayoutParams The heavy lifting is
     * done in getChildMeasureSpec.
     *
     * @param child The child to measure
     * @param parentWidthMeasureSpec The width requirements for this view
     * @param widthUsed Extra space that has been used up by the parent
     *        horizontally (possibly by other children of the parent)
     * @param parentHeightMeasureSpec The height requirements for this view
     * @param heightUsed Extra space that has been used up by the parent
     *        vertically (possibly by other children of the parent)
     */
    protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                        + heightUsed, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

该方法中调用getChildMeasureSpec方法来获得ViewGroup下的子视图View的测量规格。然后将测量规格最为参数传递给View的measure方法，最终完成所有子视图View的测量。来看看这里是怎么获得子视图View的测量规格的，进入getChildMeasureSpec方法：

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) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                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;

       ...........

        }
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

分析：节我们知道根布局DecorView的测量规格中的测量模式是MeasureSpec.EXACTLY，测量大小是整个窗口大小。因此上面代码分支走MeasureSpec.EXACTLY。子视图View的测量规格由其宽和高参数决定。
当DecorView根布局的子视图View宽高为一个确定值childDimension时，该View的测量模式为MeasureSpec.EXACTLY，测量大小就是childDimension。
当子视图View宽高为MATCH_PARENT时，该View的测量模式为MeasureSpec.EXACTLY，测量大小是父容器DecorView规定的大小，为整个屏幕大小MATCH_PARENT。
当子视图View宽高为WRAP_CONTENT时，该View的测量模式为MeasureSpec.AT_MOST，测量大小是父容器DecorView规定的大小，为整个屏幕大小MATCH_PARENT。
这里我们来验证一下以上的结论，目的是进一步理解 View的几种测量模式和View的测量规格。

<com.xjp.layoutdemo.MyView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Button"
    android:gravity="start"/>

这个布局很简单，直接将自定义的MyView作为Activity的内容布局。
2.自定义MyView代码如下：

public class MyView extends View {

    private static final String TAG = "MyCustomView";
    private String titleText = "Hello world";

    private int titleColor = Color.BLACK;
    private int titleBackgroundColor = Color.RED;
    private int titleSize = 16;

    private Paint mPaint;
    private Rect mBound;

    public MyView(Context context) {
        this(context, null);
    }

    public MyView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public MyView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int specMode = MeasureSpec.getMode(widthMeasureSpec);
        int specSize = MeasureSpec.getSize(widthMeasureSpec);

        switch (specMode) {
            case MeasureSpec.UNSPECIFIED:
                Log.e(TAG, "UNSPECIFIED.....");
                break;
            case MeasureSpec.AT_MOST:
                Log.e(TAG, "AT_MOST.....");
                break;
            case MeasureSpec.EXACTLY:
                Log.e(TAG, "EXACTLY.....");
                break;
        }

        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

    /** * 初始化 */
    private void init() {
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        mPaint.setTextSize(titleSize);
        /** * 得到自定义View的titleText内容的宽和高 */
        mBound = new Rect();
        mPaint.getTextBounds(titleText, 0, titleText.length(), mBound);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        mPaint.setColor(titleBackgroundColor);
        canvas.drawCircle(getWidth() / 2f, getWidth() / 2f, getWidth() / 2f, mPaint);
        mPaint.setColor(titleColor);
        canvas.drawText(titleText, getWidth() / 2 - mBound.width() / 2, getHeight() / 2 + mBound.height() / 2, mPaint);
    }
}

自定义的MyView也很简单，仅仅重写了onDraw方法，onMeasure方法调用父类方法。代码运行之后你会发现，

1.布局中设置的MyView大小是wrap_content包裹内容的，但是View视图却充满整个屏幕。看打印发现当前的测量模式是MeasureSpec.AT_MOST。
2.当MyView大小是match_parent填满父容器时，View视图也是充满整个屏幕，看打印发现测量模式是MeasureSpec.EXACTLY。
3.当MyView大小是固定值，比如是1200dp和1200dp时，View视图是超出整个屏幕的。

原因是此处的Activity内容布局的父容器也是一个id为content的FrameLayout布局。这里就不解释以上三种情况的原因了，前面的很详细了。
至此，整个View树型结构的布局测量流程可以归纳如下：

measure总结

1、View的measure方法是final类型的，子类不可以重写，子类可以通过重写onMeasure方法来测量自己的大小，当然也可以不重写onMeasure方法使用系统默认测量大小。
2、View测量结束的标志是调用了View类中的setMeasuredDimension成员方法，言外之意是，如果你需要在自定义的View中重写onMeasure方法，在你测量结束之前你必须调用setMeasuredDimension方法测量才有效。
3、在Activity生命周期onCreate和onResume方法中调用View.getWidth()和View.getMeasuredHeight()返回值为0的，是因为当前View的测量还没有开始，这里关系到Activity启动过程，文章开头说了当ActivityThread类中的performResumeActivity方法执行之后才将DecorView添加到PhoneWindow窗口上，开始测量。在Activity生命周期onCreate在中performResumeActivity还为执行，因此调用View.getMeasuredHeight(）返回值为0。

4、子视图View的大小是由父容器View和子视图View布局共同决定的。
布局Layout
View视图绘制流程中的布局layout是由ViewRootImpl中的performLayout成员方法开始的，看源码：

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
            int desiredWindowHeight) {

        //标记当前开始布局
        mInLayout = true;
        //mView就是DecorView
        final View host = mView;

        //DecorView请求布局
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
        //标记布局结束
        mInLayout = false;

}

DecorView的四个位置左=0，顶=0，右=屏幕宽，底=屏幕宽，说明DecorView布局的位置是从屏幕最左最顶端开始布局，到屏幕最低最右结束。因此DecorView根布局是充满整个屏幕的。

该方法主要调用了View类的layout方法，跟踪代码进入View类的layout方法瞧瞧吧

/** * Assign a size and position to a view and all of its * descendants * * <p>This is the second phase of the layout mechanism. * (The first is measuring). In this phase, each parent calls * layout on all of its children to position them. * This is typically done using the child measurements * that were stored in the measure pass().</p> * * <p>Derived classes should not override this method. * Derived classes with children should override * onLayout. In that method, they should * call layout on each of their children.</p> * * @param l Left position, relative to parent * @param t Top position, relative to parent * @param r Right position, relative to parent * @param b Bottom position, relative to parent */
    @SuppressWarnings({"unchecked"})
    public void layout(int l, int t, int r, int b) {
        //判断是否需要重新测量
        if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }
        //保存上一次View的四个位置
        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;
        //设置当前视图View的左，顶，右，底的位置，并且判断布局是否有改变
        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
        //如果布局有改变，条件成立，则视图View重新布局
            if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            //调用onLayout，将具体布局逻辑留给子类实现
            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;
    }

前面部分保存布局的四个位置，用于布局变化的监听事件，如果用户设置了布局变化的监听事件，则代码后面就会执行设置监听事件。

后面设置当前View的布局位置，也就是当调用了setFrame(l, t, r, b)方法之后，当前View布局基本完成，这里来分析一下setFrame是怎么设置当前View的布局位置的。

/** * Assign a size and position to this view. * * This is called from layout. * * @param left Left position, relative to parent * @param top Top position, relative to parent * @param right Right position, relative to parent * @param bottom Bottom position, relative to parent * @return true if the new size and position are different than the * previous ones * {@hide} */
    protected boolean setFrame(int left, int top, int right, int bottom) {
        boolean changed = false;
        //当上，下，左，右四个位置有一个和上次的值不一样都会重新布局
        if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
            changed = true;

            // Remember our drawn bit
            int drawn = mPrivateFlags & PFLAG_DRAWN;
            //得到本次和上次的宽和高
            int oldWidth = mRight - mLeft;
            int oldHeight = mBottom - mTop;
            int newWidth = right - left;
            int newHeight = bottom - top;
            //判断本次View的宽高和上次View的宽高是否相等
            boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);

            // Invalidate our old position
            //清楚上次布局的位置
            invalidate(sizeChanged);
            //保存当前View的最新位置
            mLeft = left;
            mTop = top;
            mRight = right;
            mBottom = bottom;
            mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);

            mPrivateFlags |= PFLAG_HAS_BOUNDS;

            //如果当前View的尺寸有所变化
            if (sizeChanged) {
                sizeChange(newWidth, newHeight, oldWidth, oldHeight);
            }

            ...............
        return changed;
    }

分析：
如果当前View视图的最新位置和上一次不一样时，则View会重新布局。

保存当前View的最新位置，到此当前View的布局基本结束。从这里我们可以看到，四个全局变量 mLeft，mTop，mRight，mBottom在此刻赋值，联想我们平时使用的View.getWidth（）方法获得View的宽高，你可以发现，其实View.getWidth()方法的实现如下：

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

也就是说，以上两个方法是获得View布局时候的宽高，因此，我们只有在View 布局完之后调用getWidth才能真正获取到大于0的值。

细心的你会发现，前面调用了setFrame方法，也就是当前View的布局结束了，那么View中的onLayout方法又是干嘛的呢？进入onLayout方法：

/**
     * Called from layout when this view should
     * assign a size and position to each of its children.
     *
     * Derived classes with children should override
     * this method and call layout on each of
     * their children.
     * @param changed This is a new size or position for this view
     * @param left Left position, relative to parent
     * @param top Top position, relative to parent
     * @param right Right position, relative to parent
     * @param bottom Bottom position, relative to parent
     */
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    }

分析：原来这是一个空方法，既然是空方法，那么该方法的实现应该在子类中。前面分析过，DecorView是继承自FrameLayout的，那么进入FarmeLayout类中看看 onLayout方法的实现吧：

* {@inheritDoc}
     */
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        layoutChildren(left, top, right, bottom, false /* no force left gravity */);
    }

    void layoutChildren(int left, int top, int right, int bottom,
                                  boolean forceLeftGravity) {
        final int count = getChildCount();

        final int parentLeft = getPaddingLeftWithForeground();
        final int parentRight = right - left - getPaddingRightWithForeground();

        final int parentTop = getPaddingTopWithForeground();
        final int parentBottom = bottom - top - getPaddingBottomWithForeground();

        mForegroundBoundsChanged = true;
        //遍历当前FrameLayout下的子View
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            //当子视图View可见度设置为GONE时，不进行当前子视图View的布局，这就是为什么当你布局中使用Visibility=GONE时，该view是不占据空间的。
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                //获得子视图View的宽高
                final int width = child.getMeasuredWidth();
                final int height = child.getMeasuredHeight();

                int childLeft;
                int childTop;

                int gravity = lp.gravity;
                if (gravity == -1) {
                    gravity = DEFAULT_CHILD_GRAVITY;
                }

                final int layoutDirection = getLayoutDirection();
                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
                //一下代码获得子视图View的四个位置，用于子视图View布局。
                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                        lp.leftMargin - lp.rightMargin;
                        break;
                    case Gravity.RIGHT:
                        if (!forceLeftGravity) {
                            childLeft = parentRight - width - lp.rightMargin;
                            break;
                        }
                    case Gravity.LEFT:
                    default:
                        childLeft = parentLeft + lp.leftMargin;
                }

                switch (verticalGravity) {
                    case Gravity.TOP:
                        childTop = parentTop + lp.topMargin;
                        break;
                    case Gravity.CENTER_VERTICAL:
                        childTop = parentTop + (parentBottom - parentTop - height) / 2 +
                        lp.topMargin - lp.bottomMargin;
                        break;
                    case Gravity.BOTTOM:
                        childTop = parentBottom - height - lp.bottomMargin;
                        break;
                    default:
                        childTop = parentTop + lp.topMargin;
                }
                //子视图布局
                child.layout(childLeft, childTop, childLeft + width, childTop + height);
            }
        }
    }

分析：在FrameLayout中的onLayout方法中仅仅是调用了layoutChildren方法，从该方法名称我们不难看出，原来该方法的作用是

给子视图View进行布局的。也就是说FrameLayout布局其实在View类中的layout方法中已经实现，布局的逻辑实现是在父视图中

实现的，不像View视图的measure测量，通过子类实现onMeasure方法来实现测量逻辑。

1.，遍历获得FrameLayout的子视图View的四个位置，然后调用child.layout对子视图View进行布局操作。

2.，对每个子视图View的可见度进行了判断，如果当前子视图View可见度类型为GONE,则当前子视图View不进行布局，这也就是为什么可见度GONE类型时是不占据屏幕空间的，而其他两种VISIBLE和INVISIBLE是占据屏幕空间的。
由于FrameLayout类是继承自ViewGroup类的，那么我们进入ViewGroup类去窥探一下onLayout方法具体做了什么？

/** * {@inheritDoc} */
    @Override
    protected abstract void onLayout(boolean changed,
            int l, int t, int r, int b);
```![这里写图片描述](http://img.blog.csdn.net/20160406195336062)


layout布局总结

1.视图View的布局逻辑是由父View，也就是ViewGroup容器布局来实现的。因此，我们如果自定义View一般都无需重写onMeasure方法，但是如果自定义一个ViewGroup容器的话，就必须实现onLayout方法，因为该方法在ViewGroup是抽象的，所有ViewGroup的所有子类必须实现onLayout方法。

2.当我们的视图View在布局中使用 android:visibility=”gone” 属性时，是不占据屏幕空间的，因为在布局时ViewGroup会遍历每个子视图View，判断当前子视图View是否设置了 Visibility==GONE,如果设置了，当前子视图View就会添加到父容器上，因此也就不占据屏幕空间。

3.必须在View布局完之后调用getHeight（）和getWidth（）方法获取到的View的宽高才大于0。具体可


View的绘制Draw

View视图绘制流程中的最后一步绘制draw是由ViewRootImpl中的performDraw成员方法开始的，跟踪代码，最后会在ViewRootImpl类中的drawSoftware方法绘制View：





<div class="se-preview-section-delimiter"></div>

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {

    // Draw with software renderer.
    final Canvas canvas;
    try {
        //从surface对象中获得canvas变量
        canvas = mSurface.lockCanvas(dirty);

        // If this bitmap's format includes an alpha channel, we
        // need to clear it before drawing so that the child will
        // properly re-composite its drawing on a transparent
        // background. This automatically respects the clip/dirty region
        // or
        // If we are applying an offset, we need to clear the area
        // where the offset doesn't appear to avoid having garbage
        // left in the blank areas.
        if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
            canvas.drawColor(0, PorterDuff.Mode.CLEAR);
        }

       ......................

        try {
            //调整画布的位置
            canvas.translate(-xoff, -yoff);
            if (mTranslator != null) {
                mTranslator.translateCanvas(canvas);
            }
            canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
            attachInfo.mSetIgnoreDirtyState = false;
            //调用View类中的成员方法draw开始绘制View视图
            mView.draw(canvas);
        } 


    return true;
}

“`

 private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
            boolean scalingRequired, Rect dirty) {

        // Draw with software renderer.
        final Canvas canvas;
        try {
            //从surface对象中获得canvas变量
            canvas = mSurface.lockCanvas(dirty);

            // If this bitmap's format includes an alpha channel, we
            // need to clear it before drawing so that the child will
            // properly re-composite its drawing on a transparent
            // background. This automatically respects the clip/dirty region
            // or
            // If we are applying an offset, we need to clear the area
            // where the offset doesn't appear to avoid having garbage
            // left in the blank areas.
            if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
                canvas.drawColor(0, PorterDuff.Mode.CLEAR);
            }

           ......................

            try {
                //调整画布的位置
                canvas.translate(-xoff, -yoff);
                if (mTranslator != null) {
                    mTranslator.translateCanvas(canvas);
                }
                canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
                attachInfo.mSetIgnoreDirtyState = false;
                //调用View类中的成员方法draw开始绘制View视图
                mView.draw(canvas);
            } 


        return true;
    }

分析：代码第8行，从mSurface对象中获得canvas画布，然后将变量canvas变量作为参数传递给第33行代码中的draw方法。由此

可知，我们的视图View最终是绘制到Surface中去的，关于Surface相关的知识，可以参考这篇大神的博客：

Android应用程序窗口（Activity）的绘图表面（Surface）的创建过程分析

跟踪代码，进入View的draw方法分析源码：

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;

        /* * Draw traversal performs several drawing steps which must be executed * in the appropriate order: * * 1. Draw the background * 2. If necessary, save the canvas' layers to prepare for fading * 3. Draw view's content * 4. Draw children * 5. If necessary, draw the fading edges and restore layers * 6. Draw decorations (scrollbars for instance) */

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            drawBackground(canvas);
        }

        // skip step 2 & 5 if possible (common case)
        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);

            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // we're done...
            return;
        }

        /* * Here we do the full fledged routine... * (this is an uncommon case where speed matters less, * this is why we repeat some of the tests that have been * done above) */

        boolean drawTop = false;
        boolean drawBottom = false;
        boolean drawLeft = false;
        boolean drawRight = false;

        float topFadeStrength = 0.0f;
        float bottomFadeStrength = 0.0f;
        float leftFadeStrength = 0.0f;
        float rightFadeStrength = 0.0f;

        // Step 2, save the canvas' layers
        int paddingLeft = mPaddingLeft;

        final boolean offsetRequired = isPaddingOffsetRequired();
        if (offsetRequired) {
            paddingLeft += getLeftPaddingOffset();
        }

        int left = mScrollX + paddingLeft;
        int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
        int top = mScrollY + getFadeTop(offsetRequired);
        int bottom = top + getFadeHeight(offsetRequired);

        if (offsetRequired) {
            right += getRightPaddingOffset();
            bottom += getBottomPaddingOffset();
        }

        final ScrollabilityCache scrollabilityCache = mScrollCache;
        final float fadeHeight = scrollabilityCache.fadingEdgeLength;
        int length = (int) fadeHeight;

        // clip the fade length if top and bottom fades overlap
        // overlapping fades produce odd-looking artifacts
        if (verticalEdges && (top + length > bottom - length)) {
            length = (bottom - top) / 2;
        }

        // also clip horizontal fades if necessary
        if (horizontalEdges && (left + length > right - length)) {
            length = (right - left) / 2;
        }

        if (verticalEdges) {
            topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
            drawTop = topFadeStrength * fadeHeight > 1.0f;
            bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
            drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
        }

        if (horizontalEdges) {
            leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
            drawLeft = leftFadeStrength * fadeHeight > 1.0f;
            rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
            drawRight = rightFadeStrength * fadeHeight > 1.0f;
        }

        saveCount = canvas.getSaveCount();

        int solidColor = getSolidColor();
        if (solidColor == 0) {
            final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

            if (drawTop) {
                canvas.saveLayer(left, top, right, top + length, null, flags);
            }

            if (drawBottom) {
                canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
            }

            if (drawLeft) {
                canvas.saveLayer(left, top, left + length, bottom, null, flags);
            }

            if (drawRight) {
                canvas.saveLayer(right - length, top, right, bottom, null, flags);
            }
        } else {
            scrollabilityCache.setFadeColor(solidColor);
        }

        // 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;

        if (drawTop) {
            matrix.setScale(1, fadeHeight * topFadeStrength);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, right, top + length, p);
        }

        if (drawBottom) {
            matrix.setScale(1, fadeHeight * bottomFadeStrength);
            matrix.postRotate(180);
            matrix.postTranslate(left, bottom);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, bottom - length, right, bottom, p);
        }

        if (drawLeft) {
            matrix.setScale(1, fadeHeight * leftFadeStrength);
            matrix.postRotate(-90);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, left + length, bottom, p);
        }

        if (drawRight) {
            matrix.setScale(1, fadeHeight * rightFadeStrength);
            matrix.postRotate(90);
            matrix.postTranslate(right, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(right - length, top, right, bottom, p);
        }

        canvas.restoreToCount(saveCount);

        // Step 6, draw decorations (scrollbars)
        onDrawScrollBars(canvas);

        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas);
        }
    }

绘制当前视图的背景。

保存当前画布的堆栈状态，并且在在当前画布上创建额外的图层，以便接下来可以用来绘制当前视图在滑动时的边框渐变效果。

绘制当前视图的内容。

绘制当前视图的子视图的内容。

绘制当前视图在滑动时的边框渐变效果。

绘制当前视图的滚动条。
由于篇幅的原因绘制放在下个章节讲谢谢