getTop()	获取View自身顶边到其父布局顶边的距离
getLeft()	获取View自身左边到其父布局左边的距离
getRight()	获取View自身右边到其父布局左边的距离
getBottom()	获取View自身底边到其父布局顶边的距离

MotionEvent提供的方法

我们看上图那个深蓝色的点，假设就是我们触摸的点，我们知道无论是View还是ViewGroup，最终的点击事件都会由onTouchEvent(MotionEvent event)方法来处理，MotionEvent也提供了各种获取焦点坐标的方法：

getX()	获取点击事件距离控件左边的距离，即视图坐标
getY()	获取点击事件距离控件顶边的距离，即视图坐标
getRawX()	获取点击事件距离整个屏幕左边距离，即绝对坐标
getRawY()	取点击事件距离整个屏幕顶边的的距离，即绝对坐标

二 View的事件分发

点击事件分发：点击屏幕就会产生触摸事件，这个事件被封装成了一个类：MotionEvent。当这个MotionEvent产生后，系统就会将这个MotionEvent传递给View的各个层级，MotionEvent在View层级传递的过程，就是点击事件分发。

点击事件有三个重要的方法：

dispatchTouchEvent(MotionEvent ev)	用来进行事件的分发
onInterceptTouchEvent(MotionEvent ev)	用来进行事件的拦截，在dispatchTouchEvent()中调用，需要注意的是View没有提供该方法
onTouchEvent(MotionEvent ev)	用来处理点击事件，在dispatchTouchEvent()方法中进行调用

点击事件分发的这三个重要方法的关系，用伪代码来简单表示就是：


 public boolean dispatchTouchEvent(MotionEvent ev) {
 boolean result=false;
 if(onInterceptTouchEvent(ev)){
       result=super.onTouchEvent(ev);
  }else{
       result=child.dispatchTouchEvent(ev);
 }
 return result;

这个U型图就很好的解释了事件分发：

总结：dispatchTouchEvent代表分发事件，onInterceptTouchEvent()代表拦截事件，onTouchEvent()代表消耗事件，由自己处理。

三 View的工作流程

3.1 概述

measure：测量View的宽高；测量；

layout：用来确定View的位置；布局；

draw：则用来绘制View；绘制；

3.2 测量

3.2.1 预备知识MeasureSpec（重要）

MeasureSpec是View的静态内部类；View的测量就是测量View的宽高，而View的宽高被封装到了MeasureSpec里面。

测量的过程实际上就是如何确定View的MeasureSpec的过程，而MeasureSpec主要包含两个参数：模式+尺寸。

MeasureSpec是一个32位int值，前2位表示SpecMode即模式，后30位表示SpecSize即尺寸。

3.2.2 MeasureSpec的SpecMode分类（模式）

UNSPECIFIED

public static final int UNSPECIFIED = 0 << MODE_SHIFT; 00000000000000000000000000000000

父容器不对View做任何限制，系统内部使用

EXACTLY
public static final int EXACTLY = 1 << MODE_SHIFT; 01000000000000000000000000000000
父容器检测出View的大小，Vew的大小就是SpecSize LayoutPamras match_parent 固定大小

AT_MOST
public static final int AT_MOST = 2 << MODE_SHIFT; 10000000000000000000000000000000
父容器指定一个可用大小，View的大小不能超过这个值，LayoutPamras wrap_content

3.2.3 测绘过程

ViewGroup的测绘过程：

measure --> onMeasure(测量子控件的宽高)--> setMeasuredDimension -->setMeasuredDimensionRaw(保存自己的宽高)

View的测绘过程：

measure --> onMeasure --> setMeasuredDimension -->setMeasuredDimensionRaw(保存自己的宽高)

3.2.4 测绘源码分析，以DecorView为例

首先在ViewRootImpl中，执行performMeasure方法：


   private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
        if (mView == null) {
            return;
        }
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
        try {
            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

点击mView.measure方法，会进入View的measure方法（标注1）（view的该方法为final，不能被重写），如下：


 
    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int oWidth  = insets.left + insets.right;
            int oHeight = insets.top  + insets.bottom;
            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
        }
 
        // Suppress sign extension for the low bytes
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
 
        final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
 
        // Optimize layout by avoiding an extra EXACTLY pass when the view is
        // already measured as the correct size. In API 23 and below, this
        // extra pass is required to make LinearLayout re-distribute weight.
        final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
                || heightMeasureSpec != mOldHeightMeasureSpec;
        final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
                && MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
        final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
                && getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
        final boolean needsLayout = specChanged
                && (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
 
        if (forceLayout || needsLayout) {
            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
 
            resolveRtlPropertiesIfNeeded();
 
            int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
 
 
                //这里！重要的onMeasure方法，终于找到你！
                //这里！重要的onMeasure方法，终于找到你！
                //这里！重要的onMeasure方法，终于找到你！
 
                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;
            }
 
            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("View with id " + getId() + ": "
                        + getClass().getName() + "#onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }
 
            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }
 
        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;
 
        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }

代码有点多，不用全看。前半部分主要是取了缓存，然后执行了 onMeasure(widthMeasureSpec, heightMeasureSpec)方法。如果缓存没读到会进行缓存的一个设置。（关于缓存可以先忽略不用管，主要是对测量做一些优化）

紧接着上面的代码段中，点击主要的onMeasure方法一探究竟：


    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

会发现，onMeasure方法里最主要调用了两个方法：setMeasuredDimension方法和getDefaultSize方法。先看下getDefaultSize方法：


 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;
    }

注意，可以看到，这里无论是AT_MOST模式还是EXACTLY模式，都是默认的specSize，所以我们在自定义View的时候，一定要重写onMeasure方法，否则match_parent和wrap_content属性时，将不起作用。

我们再来看看setMeasuredDimension方法里是什么：


    protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int opticalWidth  = insets.left + insets.right;
            int opticalHeight = insets.top  + insets.bottom;
 
            measuredWidth  += optical ? opticalWidth  : -opticalWidth;
            measuredHeight += optical ? opticalHeight : -opticalHeight;
        }
        setMeasuredDimensionRaw(measuredWidth, measuredHeight);
    }

可以看到，setMeasuredDimension方法里继续调用了setMeasuredDimensionRaw(measuredWidth, measuredHeight)方法，继续查看这个方法的详细：


    private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;
 
        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
    }

谢天谢地，终于到头了！可以看到，该方法对宽高成员变量赋值以及设置了一个标记位。

不要因为走得太远而忘记了为何出发，我们是在ViewRootImpl中执行performMeasure方法中的mView.measure(childWidthMeasureSpec, childHeightMeasureSpec)不断追溯来到这边的，所以measure（测量）实质上就是确定控件的宽和高。那么控件的宽和高，他们的值又是如何确定的呢？现在我们就需要查看mView.measure(childWidthMeasureSpec, childHeightMeasureSpec)中的childWidthMeasureSpec, childHeightMeasureSpec这两个参数！

这边打个分割线便于大家理解，上方是mView.measure方法测量的源码过程解析，

接下来我们需要知道mView.measure方法的两个参数如何获取。

在ViewRootlmpl中，继续解读源码：


                    int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
                    int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
 
                    if (DEBUG_LAYOUT) Log.v(mTag, "Ooops, something changed!  mWidth="
                            + mWidth + " measuredWidth=" + host.getMeasuredWidth()
                            + " mHeight=" + mHeight
                            + " measuredHeight=" + host.getMeasuredHeight()
                            + " coveredInsetsChanged=" + contentInsetsChanged);
 
                     // Ask host how big it wants to be
                    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

可以看到，childWidthMeasureSpec,和childHeightMeasureSpec这两个参数由getRootMeasureSpec(mWidth, lp.width)方法和getRootMeasureSpec(mHeight, lp.height)得出。以第一个为例，mWidth表示的是窗口（PhoneWindow）的宽，第二个参数表示的是顶层View的本身的宽。参数有了，我们接下来再详细进入getRootMeasureSpec这个方法里一探究竟：


    /**
     * 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) {
 
        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

注释写的很详细了，可以看到，该方法里对测量的view就行了switch判断，根据view的match_parent属性/wrap_content属性设置了measureSpec的不同模式(EXACTLY/AT_MOST)。

又这个方法我们可以得出结论：DecorView的MeasureSpec方法由窗口大小和自己的LayoutParams决定。具体的决定规则逻辑在如上代码中，具象下来可以总结如下：

继续分析。刚刚我们已经确定了这两个参数的由来。实际上这两个参数确定了DccorView的模式和尺寸（注意子view的测量还没有，过程才刚开始）。

继续分割一下避免混乱，上面是mView.measure方法的两个参数由来，

接下来我们继续分析mView.measure方法。

mView.measure（也就是View的measure方法,这里的mView就是DecorView）中调用onMeasure方法。我们知道DecorView实际上是集成FrameLayout的，所以我们直接去FrameLayout中看看这个onMeasure方法的具体实现：


protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();
 
        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        mMatchParentChildren.clear();
 
        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) {
                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());
        }
 
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));
 
        count = mMatchParentChildren.size();
        if (count > 1) {
            for (int i = 0; i < count; i++) {
                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
 
                final int childWidthMeasureSpec;
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    final int width = Math.max(0, getMeasuredWidth()
                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                            - lp.leftMargin - lp.rightMargin);
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            width, MeasureSpec.EXACTLY);
                } else {
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                }
 
                final int childHeightMeasureSpec;
                if (lp.height == LayoutParams.MATCH_PARENT) {
                    final int height = Math.max(0, getMeasuredHeight()
                            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                            - lp.topMargin - lp.bottomMargin);
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            height, MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }
 
                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }

很符合我们心理预期，FrameLayout作为容器，肯定会包含很多的子View，可以看到上面的源码方法里，也对其所有的子View进行了遍历，不难想象，它将对它的所有子View进行测量。为了验证猜想，继续点击该方法中的measureChildWithMargins中一探究竟：


    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);
    }

该方法中增加了对padding的处理，很容易理解不赘述。

看child.measure(childWidthMeasureSpec, childHeightMeasureSpec)方法。验证了我们的猜想，从名字就可以看出，开始对FrameLayout中的子View进行宽高测量了。

这里有个非常重要的点：getChildMeasureSpec方法，即对子View的MeasureSpec的获取，点击看详细的getChildMeasureSpec方法实现：


    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;
 
        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } 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;
 
        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

这个方法有点长，不要怕。可以看到该方法的第一个参数表示父容器的MeasureSpec的值（父容器的模式和尺寸），第二个参数padding就不说了，第三个参数表示该view本身的尺寸。该方法就通过对父容器的模式的判断，确定该子View的模式和尺寸。具体的判断逻辑可看如上代码，不详细赘述了，总结如下：

在对所有的子控件测量完成后，FrameLayout开始需要最终确定自己的宽高。继续看FrameLayout中的onMeasure方法，看它的后半段，我再粘贴下关键代码：


      // 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());
        }
 
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));
 
 
count = mMatchParentChildren.size();
        if (count > 1) {
            for (int i = 0; i < count; i++) {
                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
 
                final int childWidthMeasureSpec;
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    final int width = Math.max(0, getMeasuredWidth()
                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                            - lp.leftMargin - lp.rightMargin);
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            width, MeasureSpec.EXACTLY);
                } else {
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                }
 
                final int childHeightMeasureSpec;
                if (lp.height == LayoutParams.MATCH_PARENT) {
                    final int height = Math.max(0, getMeasuredHeight()
                            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                            - lp.topMargin - lp.bottomMargin);
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            height, MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }
 
                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }

可以看到，FrameLayout找到所有子控件中，最底部子控件的高，和最右边子控件的高。由此我们最终确定了Framelayout的宽高！

3.3 布局

源码看起来总是费力些，先总结如下：

View ：

调用View.layout来确定自己的位置，即确定mLeft,mTop,mRight,mBottom的值，获得4个点的位置。

ViewGroup：

调用View.layout来确定自己的位置，再调用onLayout确定子View的位置。

3.3.1 布局源码分析，以FrameLayout为例

查看FrameLayout的onLayout方法如下：


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

点击查看 layoutChildren方法：


    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();
 
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
 
                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;
 
                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;
                }
 
                child.layout(childLeft, childTop, childLeft + width, childTop + height);
            }
        }
    }

可以看到， layoutChildren方法中对子View进行了遍历，对子控件进行位置的摆放。而对子控件的摆放，调用的是子控件的layout方法，也就是上面代码的最后一行。布局就是这么简单。

3.4 绘制

ViewGroup 绘制步骤：

绘制背景 drawBackground(canvas)

绘制自己onDraw(canvas)

绘制子View dispatchDraw(canvas)

绘制前景，滚动条等装饰onDrawForeground(canvas)

View绘制步骤：

绘制背景 drawBackground(canvas)

绘制自己onDraw(canvas)

绘制前景，滚动条等装饰onDrawForeground(canvas)

绘制自定义控件的步骤：

onMeasure --> onLayout(自定义的是容器时需要重写该方法，自定义View时可不重写该方法) --> onDraw（可选，若容器里都是系统控件，可不重写）

3.4.1 绘制源码分析，以FrameLayout为例

找到ViewRootlmpl中的performDraw方法：


private void performDraw() {
        if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
            return;
        } else if (mView == null) {
            return;
        }
 
        final boolean fullRedrawNeeded = mFullRedrawNeeded || mReportNextDraw;
        mFullRedrawNeeded = false;
 
        mIsDrawing = true;
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
 
        boolean usingAsyncReport = false;
        if (mReportNextDraw && mAttachInfo.mThreadedRenderer != null
                && mAttachInfo.mThreadedRenderer.isEnabled()) {
            usingAsyncReport = true;
            mAttachInfo.mThreadedRenderer.setFrameCompleteCallback((long frameNr) -> {
                // TODO: Use the frame number
                pendingDrawFinished();
            });
        }
 
        try {
            boolean canUseAsync = draw(fullRedrawNeeded);
            if (usingAsyncReport && !canUseAsync) {
                mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
                usingAsyncReport = false;
            }
        } finally {
            mIsDrawing = false;
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
 
        // For whatever reason we didn't create a HardwareRenderer, end any
        // hardware animations that are now dangling
        if (mAttachInfo.mPendingAnimatingRenderNodes != null) {
            final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();
            for (int i = 0; i < count; i++) {
                mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();
            }
            mAttachInfo.mPendingAnimatingRenderNodes.clear();
        }
 
        if (mReportNextDraw) {
            mReportNextDraw = false;
 
            // if we're using multi-thread renderer, wait for the window frame draws
            if (mWindowDrawCountDown != null) {
                try {
                    mWindowDrawCountDown.await();
                } catch (InterruptedException e) {
                    Log.e(mTag, "Window redraw count down interrupted!");
                }
                mWindowDrawCountDown = null;
            }
 
            if (mAttachInfo.mThreadedRenderer != null) {
                mAttachInfo.mThreadedRenderer.setStopped(mStopped);
            }
 
            if (LOCAL_LOGV) {
                Log.v(mTag, "FINISHED DRAWING: " + mWindowAttributes.getTitle());
            }
 
            if (mSurfaceHolder != null && mSurface.isValid()) {
                SurfaceCallbackHelper sch = new SurfaceCallbackHelper(this::postDrawFinished);
                SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();
 
                sch.dispatchSurfaceRedrawNeededAsync(mSurfaceHolder, callbacks);
            } else if (!usingAsyncReport) {
                if (mAttachInfo.mThreadedRenderer != null) {
                    mAttachInfo.mThreadedRenderer.fence();
                }
                pendingDrawFinished();
            }
        }
    }

代码很多不用都看，找到其中的draw方法，点进去，draw()方法里代码也很多，不粘贴了，找到draw()方法里的drawSoftware方法，然后是drawSoftware方法中的mView.draw（）方法。点进去即进入了View的draw()方法，如下：


   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)
...
...