View的工作原理

View的测量

MeasureSpec

UNSPECIFIED

父控件无法推荐值，如listView

EXACTLY

父容器已经检测出View所需要的精确大小，此时View的最终大小即为SpecSize所指定的值。

对应于LayoutParam中的match_parent和具体的数值这两种模式

AT_MOST

父容器指定了一个可用大小即SpecSize，View的大小不能大于该值，而具体是什么值还需要看不同View的不同实现。

对应于LayoutParam中的wrap_content

从以下代码可看出：

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);
}

getChildMeasureSpec在measureChild和measureChildWithMargins会被用到：

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);
}

由上可知父控件会根据其MeasureSpec和子控件的LayoutParam获得子控件的MeasureSpec，最后调用子控件的measure方法并将子控件的MeasureSpec传过去。

在measure方法中会调用onMeasure方法：

//View.java
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;
}

在onMeasure方法中设置了View的测量宽高，而宽高又通过getDefaultSize方法获得。

根据如下图可得出结论：

直接继承View的自定义控件需要重写onMeasure并设置wrap_content时的自身大小，否则在布局中使用wrap_content就相当于使用match_content

example：

public class MyVieWer extends View {
    private static final String TAG="MyVieWer";
    public MyVieWer(Context context) {super(context);}
    public MyVieWer(Context context,AttributeSet attrs) {
        super(context, attrs);
        Log.d(TAG,"MeasureSpec.AT_MOST:"+MeasureSpec.AT_MOST+" MeasureSpec.EXACTLY:"+MeasureSpec.EXACTLY+" MeasureSpec.UNSPECIFIED:"+MeasureSpec.UNSPECIFIED);
    }
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
      // get calculate mode of width and height
      int modeWidth = MeasureSpec.getMode(widthMeasureSpec);
      int modeHeight = MeasureSpec.getMode(heightMeasureSpec);

      // get recommend width and height
      int sizeWidth = MeasureSpec.getSize(widthMeasureSpec);
      int sizeHeight = MeasureSpec.getSize(heightMeasureSpec);
      // width: wrap_content
      if (modeWidth == MeasureSpec.AT_MOST) {
          sizeWidth = Math.min(100,sizeWidth);
          modeWidth = MeasureSpec.EXACTLY;
      }
      // height: wrap_content
      if (modeHeight == MeasureSpec.AT_MOST) {
          sizeHeight = Math.min(100,sizeHeight);
          modeHeight = MeasureSpec.EXACTLY;
      }
      widthMeasureSpec = MeasureSpec.makeMeasureSpec(sizeWidth, modeWidth);
      heightMeasureSpec = MeasureSpec.makeMeasureSpec(sizeHeight, modeHeight);
      super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }
}

<io.github.grooters.practicer.viewer.MyVieWer
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@android:color/holo_red_dark"/>

MyVieWer: MeasureSpec.AT_MOST:-2147483648 MeasureSpec.EXACTLY:1073741824 MeasureSpec.UNSPECIFIED:0

当layout_width和layout_height均为wrap_content：

MyVieWer: widthSize:1440 widthMode:-2147483648 heightSize:2096 heightMode:-2147483648


MyVieWer: widthMode==MeasureSpec.AT_MOST&&heightMode==MeasureSpec.AT_MOST

当layout_width为wrap_content,layout_height为match_parent：

MyVieWer: widthSize:1440 widthMode:-2147483648 heightSize:2096 heightMode:1073741824


MyVieWer: widthMode==MeasureSpec.AT_MOST

当layout_width为wrap_content,layout_height为具体值：

MyVieWer: widthSize:1440 widthMode:-2147483648 heightSize:2000 heightMode:1073741824


MyVieWer: widthMode==MeasureSpec.AT_MOST

由上可知当layout_width/height设置为wrap_content时mode为AT_MOST，并且默认为占满整个父容器。当设置为match_parent或具体值时mode将为EXACTLY。所以自定义View如果要使用wrap_content属性需要在onMeasure方法中作相应处理否则等同于match_parent

注意：由于View的测量过程与Activity的生命周期并不统一，所以在onCreate，onStart等方法无法通过getMeasuredHeight/Width方法获取View的高宽度，可通过以下三种方法获取：

onWindowFocusChanged

@Override
public void onWindowFocusChanged(boolean hasFocus) {
    super.onWindowFocusChanged(hasFocus);
    if(hasFocus){
        Log.d(TAG,"VieWerActivity_debug:"+myVieWer.getMeasuredHeight());
    }
}

该方法在Activity失去获取/失去焦点都会被调用

view.post

myVieWer.post(new Runnable() {
    @Override
    public void run() {
        Log.d(TAG,"VieWerActivity_debug:"+myVieWer.getMeasuredHeight());
    }
});

将获取款高度的操作post到View消息队列的尾部，在执行到该操作时View就已经初始化完成了

ViewTreeObserver

ViewTreeObserver observer=myVieWer.getViewTreeObserver();
observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
    @Override
    public void onGlobalLayout() {
        //解决onGlobalLayout被调用多次问题
        myVieWer.getViewTreeObserver().removeOnGlobalLayoutListener(this);
        Log.d(TAG,"VieWerActivity_debug:"+myVieWer.getMeasuredHeight());
    }
});

在View树的状态或可见性发生改变时会调用onGlobalLayout方法

measure

int widthSpec=View.MeasureSpec.makeMeasureSpec((1<<30)-1,View.MeasureSpec.AT_MOST);
int heightSpec=View.MeasureSpec.makeMeasureSpec(dp2px(200),View.MeasureSpec.EXACTLY);
myVieWer.measure(widthSpec,heightSpec);
Log.d(TAG,"VieWerActivity_debug:"+myVieWer.getMeasuredWidth());

1<<30：即2^30-1也就是在AT_MOST的mode下的最大size

View的Layout

ViewGroup中的onLayout计算出l,r,t,b(mLeft,mRight,mTop,mBottom)，然后调用View的layout可确定出View在父容器中的位置。

其中mLeft指子View左边框到父控件左边框的距离，mRight指子View右边框到父控件左边框的距离。

public class MyLayouTer extends ViewGroup {
    private static final String TAG="MyLayouTer";
    private int totalWidth,totalHeight;
    private int marginSpace;
    public MyLayouTer(Context context) {
        super(context);
    }
    public MyLayouTer(Context context, AttributeSet attrs) {
        super(context, attrs);
        marginSpace=50;
        totalWidth=0;
        totalHeight=0;
    }
    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        Log.d(TAG,"getChildCount():"+getChildCount());
        int j=1;
        for(int i=0;i<getChildCount();i++){
            View view=getChildAt(i);
            int width=view.getMeasuredWidth();
            int height=view.getMeasuredHeight();
            int mLeft=marginSpace;
            int mTop=marginSpace;
            totalHeight=totalHeight+height;
            totalWidth=totalWidth+width;
            if(getMeasuredWidth()<totalWidth){
                Log.d(TAG,"getMeasuredWidth()<totalWidth");
                mTop=mTop+j*height;
            }else {
                mLeft=mLeft+i*width;
            }
            Log.d(TAG,"width:"+width+"  height:"+height+"  mTop:"+mTop+"  mLeft:"+mLeft);
            view.layout(mLeft,mTop,mLeft+width,mTop+height);
        }
    }
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        Log.d(TAG,"getChildCount():"+getChildCount());
        for(int i=0;i<getChildCount();i++){
            View view=getChildAt(i);
            measureChild(view,widthMeasureSpec,heightMeasureSpec);
        }
    }
}

<io.github.grooters.practicer.viewer.MyLayouTer
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <io.github.grooters.practicer.viewer.MyVieWer
        android:id="@+id/view_my"
        android:layout_width="wrap_content"
        android:layout_height="100dp"
        android:background="@android:color/holo_red_dark"/>

    <io.github.grooters.practicer.viewer.MyVieWer
        android:id="@+id/view_my_Two"
        android:layout_width="wrap_content"
        android:layout_height="200dp"
        android:background="@android:color/black"/>

    <io.github.grooters.practicer.viewer.MyVieWer
        android:id="@+id/view_my_Three"
        android:layout_width="wrap_content"
        android:layout_height="200dp"
        android:background="@android:color/darker_gray"/>

</io.github.grooters.practicer.viewer.MyLayouTer>

以上代码编写了一个类似与水平线性布局的ViewGroup，在MyLayouTer类中的onMeasure方法先测量出所有子控件的测量宽高度，然后在onLayout中根据子控件宽高度计算出子控件在ViewGroup中的位置。

View的onDraw

在该方法中将View绘制到布局中：

//绘制一个圆到布局中
@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    int paddingStart=getPaddingStart();
    int paddingEnd=getPaddingEnd();
    int paddingTop=getPaddingTop();
    int paddingBottom=getPaddingBottom();
    int width=getWidth()-paddingStart-paddingEnd;
    int height=getHeight()-paddingTop-paddingBottom;
    int r=Math.min(width,height)/2;
    canvas.drawCircle(paddingStart+width/2,paddingTop+height/2,r,mPaint);
}