4.View的工作原理
ViewRoot对应于ViewRootImpl类，它是连接WindowManager和DecorView的纽带，View的三大流程均是通过ViewRoot来完成的。在ActivityThread中，当Activity对象被创建完毕后，会将DecorView添加到Window中，同时会创建ViewRootImpl对象，并将ViewRootImpl对象和DecorView建立关联。
View的绘制流程是从ViewRoot的performTraversals方法开始的，它经过measure、layout和draw三个过程才能最终将一个View绘制出来，其中measure用来测量View的宽高，layout用来确定View在父容器中的放置位置，而draw则负责将View绘制在屏幕上。
measure过程决定了View的宽/高，measure完成以后，可以通过getMeasuredWidth和getMeasuredHeight方法来获取到View测量后的宽/高。layout过程决定了View的四个顶点的坐标和实际的View的宽/高，完成以后，可以通过getTop，getBottom，getLeft，getRight来拿到View的四个顶点的位置，并可以通过getWidth和getHeight方法来拿到View的最终宽/高。
MeasureSpec代表一个32位的int值，高2位代表SpecMode，低30位代表SpecSize，SpecMode是指测量模式，而SpecSize是指在某种测量模式下的规格大小。
SpecMode有三类，每一类都表示特殊的含义：
- UNSPECIFIED，父容器不对View有任何限制，要多大给多大，这种情况一般用于系统内部，表示一种测量的状态。
- EXACTLY，父容器已经检测出View所需要的精确大小，这个时候View的最终大小就是SpecSize所指定的值。它对应于LayoutParams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的大小不能大于这个值，具体是什么值要看不同View的具体实现。它对应于LayoutParams中的wrap_content。
/**
* Does the hard part of measureChildren: figuring out the MeasureSpec to
* pass to a particular child. This method figures out the right MeasureSpec
* for one dimension (height or width) of one child view.
*
* The goal is to combine information from our MeasureSpec with the
* LayoutParams of the child to get the best possible results. For example,
* if the this view knows its size (because its MeasureSpec has a mode of
* EXACTLY), and the child has indicated in its LayoutParams that it wants
* to be the same size as the parent, the parent should ask the child to
* layout given an exact size.
*
* @param spec The requirements for this view
* @param padding The padding of this view for the current dimension and
* margins, if applicable
* @param childDimension How big the child wants to be in the current
* dimension
* @return a MeasureSpec integer for the child
*/
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);
}
根据父容器的MeasureSpec同时结合View本身的LayoutParams来确定子元素的MeasureSpec，参数中padding是指父容器中已占用的空间大小，因此子元素可用的大小为父容器的尺寸减去padding。对于普通View，其MeasureSpec由父容器的MeasureSpec和自身的LayoutParams来共同决定。当View采用固定宽/高的时候，不管父容器的MeasureSpec是什么，View的MeasureSpec都是精确模式并且其大小遵循LayoutParams中的大小。当View的宽/高是match_parent时，如果父容器的模式是EXACTLY模式，那么View也是EXACTLY并且其大小是父容器的剩余空间；如果父容器是AT_MOST模式，那么View也是AT_MOST模式并且其大小不会超过父容器的剩余空间。当View的宽/高是wrap_content时，不管父容器的模式是EXACTLY还是AT_MOST，View的模式总是AT_MOST并且大小不能超过父容器的剩余空间。
/**
* <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 overridden 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));
}
/**
* 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;
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;
}
/**
* 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());
}
从getDefaultSize方法的实现来看，View的宽/高由specSize决定，所以，对于直接继承View的自定义控件需要重写onMeasure方法并设置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_parent。如果View在布局中使用wrap_content，那么它的specMode是AT_MOST模式，在这种模式下，它的宽/高等于specSize；这种情况下View的specSize是parentSize，而parentSize是父容器中目前可以使用的大小，也就是父容器当前剩余的空间大小。View的宽/高等于父容器当前剩余的空间大小，这种效果和在布局中使用2match_parent完全一致。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
if(widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(DEFAULT_WIDTH, DEFAULT_HEIGHT);
} else if (widthSpecMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(DEFAULT_WIDTH, heightSpecSize);
} else if (heightSpecMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(widthSpecSize, DEFAULT_HEIGHT);
}
}
获取某个View的宽/高：
- Activity/View#onWindowFocusChanged，onWindowFocusChanged这个方法的含义是：View已经初始化完毕了，宽/高已经准备好了，这个时候去获取宽高是没问题的。需要注意的是，onWindowFocusChanged会被调用多次，当Activity的窗口得到焦点和失去焦点时均会被调用一次。具体来说，当Activity继续执行和暂停执行时，onWindowFocusChanged均会被调用，如果频繁地进行onResume和onPause，那么onWindowFoucsChanged也会被频繁地调用。
- View#post(runnable)
- ViewTreeObserver，使用ViewTreeObserver的众多回调可以完成这个功能，比如使用OnGlobalLayoutListener这个接口，当View树的状态发生变化或者View树内部的View的可见性发生改变时，onGlobalLayout方法将被回调，Inc这是获取View的宽/高一个很好的时机。
- view#measure(widthMeasureSpec,heightMeasureSpec)
在View的默认实现中，View的测量宽/高和最终宽/高是相等的，只不过测量宽/高形成于View的measure过程，而最终宽/高形成于View的layout过程，即两者的赋值时机不同，测量宽高的赋值时机稍微早一些。
View的绘制过程遵循如下几步：
- 绘制背景 background.draw(canvas)
- 绘制自己 onDraw
- 绘制children dispatchDraw
- 绘制装饰 onDrawScrollBars
/**
* If this view doesn't do any drawing on its own, set this flag to
* allow further optimizations. By default, this flag is not set on
* View, but could be set on some View subclasses such as ViewGroup.
*
* Typically, if you override {@link #onDraw(android.graphics.Canvas)}
* you should clear this flag.
*
* @param willNotDraw whether or not this View draw on its own
*/
public void setWillNotDraw(boolean willNotDraw) {
setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
}
从setWillNotDraw方法的注释中可以看出，如果一个View不需要绘制任何内容，那么设置这个标志位为true以后，系统会进行相应的优化。默认情况下，View没有启用这个优化标志位，但是ViewGroup会默认启用这个优化标志位。这个标记位对实际开发的意义是：当我们的自定义控件继承于ViewGroup并且本身不具备绘制功能时，就可以开启这标记位从而便于系统进行后续的优化。当然，当明确知道一个ViewGroup需要通过onDraw来绘制内容时，需要显式地关闭WILL_NOT_DRAW这个标记位。
自定义View须知：
- 让View支持wrap_content
- 如果有必要，让View支持padding
- 尽量不要在View中使用Handler，没必要，View内部本身就提供了post系列的方法，完全可以替代Handler的作用
- View中如果有线程或者动画，需要及时停止，参考View#onDetachedFromWindow
- View带有滑动嵌套情形时，需要处理好滑动冲突。