视图显示在屏幕上要经过三个过程，分别是measure、layout、draw，下面针对三个过程从源码角度解剖视图绘制的过程
一. onMeasure()
measure ，顾名思义，就是测量的意思，而onMeasure()就是测量视图的大小。View系统的绘制流程会从ViewRoot的performTraversals()方法中开始，在其内部调用View的measure()方法
ViewRoot实现类ViewRootImpl关键代码如下
...
if (mView == null) {
return;
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
...
View 的measure()方法是final修饰，不能被继承，内部调用onMeasure()，measure()关键代码
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
onMeasure(widthMeasureSpec, heightMeasureSpec);
...
}
View的onMeasure()有一个默认的实现
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
其中setMeasuredDimension()方法是设置视图的大小，getDefaultSize()是通过MeasureSpec获取宽高大小的默认方法，可以看看它的具体实现,
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;
}
我们自定义View时如果想怎么测量控件的大小时，直接覆盖实现onMeasure()方法，测量宽高大小，最后调用setMeasuredDimension()方法设置视图大小。
测量布局，ViewGroup中没有实现具体的OnMeasure()方法，因为不同的布局的测量方法不一样，所以要在ViewGroup子类中实现OnMeasure()方法。
一个布局ViewGroup一般包含多个子View，这时ViewGroup提供了measureChildren()来测量子View，代码如下
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
调用了measureChild来测量子View
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);
}
最终调用了子View的measure()方法测量子View。
所以自定义布局如果想自己测量布局的话，首先测量子View，再用setMeasuredDimension()方法测量布局本身
二.onLayout()
measure测量完成后，接下来就是layout，顾名思义就是给视图布局，也就是确定视图的位置。ViewRoot的performTraversals()方法在执行完measure()方法后，就会调用layout方法，关键代码如下
...
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
也就是调用了View的layout()方法,下面来看看layout()方法关键代码，参数l,t,r,b分别代表上,下,左,右坐标
public void layout(int l, int t, int r, int b) {
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
...
}
}
layout里面主要调用了onLayout()方法，进入看看
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
是一个空实现，是的，因为onLayout()过程是为了确定视图在布局中所在的位置，这个操作应该要布局视图来完成，即父布局决定了子视图的位置，既然这样，看看ViewGroup里面的onLayout()方法
@Override
protected abstract void onLayout(boolean changed,
int l, int t, int r, int b);
是一个抽象方法，意味着所有的ViewGroup子类都要实现onLayout()方法
下面看看ViewGroup的子类FrameLayout实现的onLayout()方法
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
layoutChildren()关键代码
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
final int count = getChildCount();
...
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);
}
}
}
代码实现的步骤是先算出上下左右的坐标，再通过child.layout()确认子View的位置。
三. onDraw()
layout确定视图位置之后，就要draw啦，顾名思义，就是真正的绘制啦，ViewRoot 执行要layout()之后，接着创建出一个Canvas对象，然后调用draw()方法要执行具体的绘制工作。draw具体的关键代码如下，第二步和第五步很少用到，这里省略了。
public void draw(Canvas canvas) {
/*
* 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
if (!dirtyOpaque) {
drawBackground(canvas);
}
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// Step 7, draw the default focus highlight
drawDefaultFocusHighlight(canvas);
if (debugDraw()) {
debugDrawFocus(canvas);
}
通过上面的注释大概也看懂什么意思
step 1 drawBackground()是绘制背景
step 3 onDraw()是绘制内容，点进去看
protected void onDraw(Canvas canvas) {
}
是一个空方法，具体的控件有具体的绘制方法
step 4 dispatchDraw()是绘制子视图，这个方法也是空实现，ViewGroup才会用到
step 6 绘制前景，滚动条
一般情况下我们只要实现onDraw()方法来实现我们绘制的内容
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