• 如果 root 为 null，attachToRoot 将失去作用，设置任何值都没有意义。
• 如果 root 不为 null，attachToRoot 设为 true，则会给加载的布局文件的指定一个父布局，即 root。
• 如果 root 不为 null，attachToRoot 设为 false，则会将布局文件最外层的所有 layout 属性进行设置，当该 view 被添加到父 view 当中时，这些 layout 属性会自动生效。
• 在不设置 attachToRoot 参数的情况下，如果 root 不为 null，attachToRoot 参数默认为 true。

Android 视图绘制流程

view的层级结构

一. onMeasure()  决定View的大小；

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

  public class MyView extends View {  @Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {setMeasuredDimension(200, 200);}}

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

ViewGroup中的源码：@Overrideprotected abstract void onLayout(boolean changed,int l, int t, int r, int b);

  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;}}

自定义View示例：实现类似水波扩散效果一共分为六步

1.

public SpreadView(Context context) {this(context,null,0);
}public SpreadView(Context context, @Nullable AttributeSet attrs) {this(context, attrs,0);
}public SpreadView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);initPaints();
}private void initPaints() {}@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}@Override
protected void onDraw(Canvas canvas) {super.onDraw(canvas);
}

2.实现画笔paint类

本文一共两只画笔

private void initPaints() {//画笔1:centerPaint = new Paint();centerPaint.setColor(Color.YELLOW);centerPaint.setAntiAlias(true);//抗锯齿效果//最开始不透明且扩散距离为0alphas.add(255);spreadRadius.add(0);//画笔2:spreadPaint = new Paint();spreadPaint.setAntiAlias(true);spreadPaint.setAlpha(255);spreadPaint.setColor(Color.RED);
}

3.覆写onMeasure(…)方法

实现这个方法告诉了母容器如何放弃自定义View，可以通过提供的measureSpecs来决定你的View的高和宽，以下是一个正方形，确认它的宽和高是一样的。

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {int w = MeasureSpec.getSize(widthMeasureSpec);int h = MeasureSpec.getSize(heightMeasureSpec);int size = Math.min(w, h);setMeasuredDimension(size, size);
}

注意：

这个方法需要至少保证一个setMeasuredDimension(..)调用，否则会报IllegalStateException错误。

4.实现onSizeChanged(…)方法

这个方法是你获取View现在的宽和高. 这里我们计算的是中心和半径

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {super.onSizeChanged(w, h, oldw, oldh);//圆心位置centerX = w / 2;centerY = h / 2;
}

5.实现onDraw(…)方法

这个方法提供了如何绘制view，它提供的Canvas类可以进行绘制。

@Override
protected void onDraw(Canvas canvas) {super.onDraw(canvas);for (int i = 0; i < spreadRadius.size(); i++) {int alpha = alphas.get(i);spreadPaint.setAlpha(alpha);int width = spreadRadius.get(i);//绘制扩散的圆canvas.drawCircle(centerX, centerY, radius + width, spreadPaint);//每次扩散圆半径递增，圆透明度递减if (alpha > 0 && width < 300) {alpha = alpha - distance > 0 ? alpha - distance : 1;alphas.set(i, alpha);spreadRadius.set(i, width + distance);}}//当最外层扩散圆半径达到最大半径时添加新扩散圆if (spreadRadius.get(spreadRadius.size() - 1) > maxRadius) {spreadRadius.add(0);alphas.add(255);}//超过8个扩散圆，删除最先绘制的圆，即最外层的圆if (spreadRadius.size() >= 8) {alphas.remove(0);spreadRadius.remove(0);}//中间的圆canvas.drawCircle(centerX, centerY, radius, centerPaint);//TODO 可以在中间圆绘制文字或者图片//延迟更新，达到扩散视觉差效果postInvalidateDelayed(delayMilliseconds);
}

6.添加你的View

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"xmlns:app="http://schemas.android.com/apk/res-auto"android:layout_width="match_parent"android:layout_height="match_parent"android:gravity="center"android:orientation="vertical"><com.qinqu.spreadviewdemo.SpreadViewandroid:id="@+id/spreadView"android:layout_width="match_parent"android:layout_height="wrap_content"app:spread_center_color="@color/colorAccent"app:spread_delay_milliseconds="35"app:spread_distance="5"app:spread_max_radius="90"app:spread_radius="100"app:spread_spread_color="@color/colorAccent" /></LinearLayout>

问题

demo:https://download.csdn.net/download/haoxuhong/10457408