View的绘制过程

1.View的绘制

主要指onMeasure(),onLayout(),onDraw()的过程。measure()确定View的测量宽高，layout()确定View的最终宽高和四个顶点的位置。draw()负责把View绘制到屏幕上。

1.1 Measure过程

1.1.1 View的Measure过程

View通过调用measure()方法来完成测量，但是measure()是一个final方法，这意味着他不能被重写，但是它内部会调用onMeasure(),看下这个方法的实现

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


setMeasuredDimension会设置View的宽高测量值，具体实现方法是getDefaultSize():

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;
}


• 如果是specMode是UNSPECIFIED类型，那么宽高就都是传入的size，这个传入的size是通过getSuggestedMinimumHeight()计算出的，mMinHeight是在XML布局中设置的属性，mBackground是当前设置背景的Drawable对象，如果Drawable对象有初始值大小，那么就返回两者中大的，否则就返回mMinHeight，如果没有设置则默认是0。我们看一下源码：

protected int getSuggestedMinimumHeight() {
return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());
}


• 如果specMode是AT_MOST或者EXACTLY，返回的都是通过MeasureSpec.getSize()计算出的值。即View测量后的大小。

1.1.2 ViewGroup的Measure过程

ViewGroup是一个抽象类，没有重写onMeasure()，但是它有一个measureChildren()，在此除完成自己的measure过程外，还需要循环遍历子View的，并执行其measure().源码如下：

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
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);
}


• 即获取了当前子View 的LayoutParams以及父View 的MeasureSpec还有父View的padding，根据下标原理计算出当前子View的MeasureSpec，然后调用View的measure()。

1.1.3 Measure案例：

如何在Activity启动的时候获取某个组件的宽高：

• 在Activity的onWindowFocusChanged()中获取，此方法的含义是View已经初始化完毕了。在该方法中调用view.getMeasureHeight()来获取高度。


• View.post(Runnable)，当Looper调动此Runnable时，说明View也已经被初始化好了。


• ViewTreeObserver的众多回调可以完成这个功能,具体使用如下:

@Override
protected void onStart() {
super.onStart();
if (DEBUG) {
Log.d(TAG, "onStart: ");
}
final View view = new View(this);
ViewTreeObserver viewTreeObserver = view.getViewTreeObserver();
viewTreeObserver.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
int width = view.getMeasuredWidth();
int height = view.getMeasuredHeight();
}
});
}


1.2 layout过程

layout的作用父View来确定子View的位置，当父View的位置被确定后，他会遍历调用所有子View的layout(),而layout()又会调用onLayout()。

源码如下：

public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList listenersCopy =
(ArrayList)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
final boolean wasLayoutValid = isLayoutValid();
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
if (!wasLayoutValid && isFocused()) {
mPrivateFlags &= ~PFLAG_WANTS_FOCUS;
if (canTakeFocus()) {
// We have a robust focus, so parents should no longer be wanting focus.
clearParentsWantFocus();
} else if (getViewRootImpl() == null || !getViewRootImpl().isInLayout()) {
// This is a weird case. Most-likely the user, rather than ViewRootImpl, called
// layout. In this case, there's no guarantee that parent layouts will be evaluated
// and thus the safest action is to clear focus here.
clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
clearParentsWantFocus();
} else if (!hasParentWantsFocus()) {
// original requestFocus was likely on this view directly, so just clear focus
clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
}
// otherwise, we let parents handle re-assigning focus during their layout passes.
} else if ((mPrivateFlags & PFLAG_WANTS_FOCUS) != 0) {
mPrivateFlags &= ~PFLAG_WANTS_FOCUS;
View focused = findFocus();
if (focused != null) {
// Try to restore focus as close as possible to our starting focus.
if (!restoreDefaultFocus() && !hasParentWantsFocus()) {
// Give up and clear focus once we've reached the top-most parent which wants
// focus.
focused.clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
}
}
}
if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) {
mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;
notifyEnterOrExitForAutoFillIfNeeded(true);
}
}


• 通过setFrame()来设定View的四个顶点，顶点的位置确定，说明子View在父View中的位置确定。接着调用onLayout(),View与ViewGroup都没有真正的实现onLayout(),下面看一下实现onLayout()的LinearLayout 的该方法：

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical(l, t, r, b);
} else {
layoutHorizontal(l, t, r, b);
}
}
void layoutVertical(int left, int top, int right, int bottom) {
final int paddingLeft = mPaddingLeft;
int childTop;
int childLeft;
// Where right end of child should go
final int width = right - left;
int childRight = width - mPaddingRight;
// Space available for child
int childSpace = width - paddingLeft - mPaddingRight;
final int count = getVirtualChildCount();
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
switch (majorGravity) {
case Gravity.BOTTOM:
// mTotalLength contains the padding already
childTop = mPaddingTop + bottom - top - mTotalLength;
break;
// mTotalLength contains the padding already
case Gravity.CENTER_VERTICAL:
childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
break;
case Gravity.TOP:
default:
childTop = mPaddingTop;
break;
}
for (int i = 0; i final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
int gravity = lp.gravity;
if (gravity <0) {
gravity = minorGravity;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = paddingLeft + ((childSpace - childWidth) / 2)
+ lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
childLeft = childRight - childWidth - lp.rightMargin;
break;
case Gravity.LEFT:
default:
childLeft = paddingLeft + lp.leftMargin;
break;
}
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
childTop += lp.topMargin;
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}


• 此方法会调用所有的setChildFrame()为子元素指定位置，其中childTop会逐渐增大，每增加一个元素，就放在靠下的位置。setChildFrame()其实调用了子View的layout(),子元素又会根据onLayout()找准自己的位置。

1.3 draw过程

draw过程是将View绘制到屏幕上，遵循以下几步：

• 绘制背景：background.draw(canvas);


• 绘制自己：onDraw();


• 绘制children:(dispatchDraw())


• 绘制装饰:（onDrawScrollBar()）


• View的draw传递通过dispatchDraw()来实现，遍历所有子元素的draw()方法，那么draw事件就一层一层传递下去了。


• 当自定义View继承了View，但是没有绘制功能的时候，可以调用setWillNotDraw(true);设置当前View不需要绘制，以便于系统进行优化。