Android View相关介绍

1- View的draw和onDraw的区别
大概扫一下源码就可以明白，draw()这个函数本身会做很多事情，

         *      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)
在第三步的时候，它就会调用onDraw()方法，来绘制view的内容。也就是draw会调用onDraw。

所以看需要，一般情况下，直接用onDraw绘制view的content就可以了，如果绘制多一点的内容，可以调用draw()，不过Android官方推荐用只用onDraw就可以了。“When implementing a view, do not override this method; instead,
you should implement onDraw”

代码片段：

<span style="font-family:SimSun;font-size:18px;"> // skip step 2 & 5 if possible (common case)
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);
</span>

二- View的刷新机制

在Android的布局体系中，父View负责刷新、布局显示子View；而当子View需要刷新时，则是通知父View来完成。

子view执行 invalidate函数， 会调用父View的invalidateChild函数，

有个Do while循环， 以此向上寻找到父view进行判断，每个父view调用自己的invalidateChildInParent

计算dirty区域

这个向上回溯的过程直到ViewRoot那里结束，由ViewRoot对这个最终的刷新区域做刷新。

<span style="font-family:SimSun;font-size:18px;">void invalidate(boolean invalidateCache) {
final AttachInfo ai = mAttachInfo;
final ViewParent p = mParent;
//noinspection PointlessBooleanExpression,ConstantConditions
if (!HardwareRenderer.RENDER_DIRTY_REGIONS) {
if (p != null && ai != null && ai.mHardwareAccelerated) {
// fast-track for GL-enabled applications; just invalidate the whole hierarchy
// with a null dirty rect, which tells the ViewAncestor to redraw everything
p.invalidateChild(this, null);
return;
}
}

if (p != null && ai != null) {
final Rect r = ai.mTmpInvalRect;
r.set(0, 0, mRight - mLeft, mBottom - mTop);
// Don't call invalidate -- we don't want to internally scroll
// our own bounds
p.invalidateChild(this, r);
}
}
}  </span>

<span style="font-family:SimSun;font-size:18px;">public final void invalidateChild(View child, final Rect dirty) {
ViewParent parent = this;

final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
final int[] location = attachInfo.mInvalidateChildLocation;
// 需要刷新的子View的位置
location[CHILD_LEFT_INDEX] = child.mLeft;
location[CHILD_TOP_INDEX] = child.mTop;

// If the child is drawing an animation, we want to copy this flag onto
// ourselves and the parent to make sure the invalidate request goes through
final boolean drawAnimation = (child.mPrivateFlags & DRAW_ANIMATION) == DRAW_ANIMATION;

// Check whether the child that requests the invalidate is fully opaque
final boolean isOpaque = child.isOpaque() && !drawAnimation && child.getAnimation() != null;
// Mark the child as dirty, using the appropriate flag
// Make sure we do not set both flags at the same time
final int opaqueFlag = isOpaque ? DIRTY_OPAQUE : DIRTY;

<span style="color:#ff0000;">do {  </span>
View view = null;
if (parent instanceof View) {
view = (View) parent;
}

if (drawAnimation) {
if (view != null) {
view.mPrivateFlags |= DRAW_ANIMATION;
} else if (parent instanceof ViewRoot) {
((ViewRoot) parent).mIsAnimating = true;
}
}

// If the parent is dirty opaque or not dirty, mark it dirty with the opaque
// flag coming from the child that initiated the invalidate
if (view != null && (view.mPrivateFlags & DIRTY_MASK) != DIRTY) {
view.mPrivateFlags = (view.mPrivateFlags & ~DIRTY_MASK) | opaqueFlag;
}

parent = parent.invalidateChildInParent(location, dirty);
<span style="color:#ff0000;"> } while (parent != null);  </span>
}
}

public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
if ((mPrivateFlags & DRAWN) == DRAWN) {
if ((mGroupFlags & (FLAG_OPTIMIZE_INVALIDATE | FLAG_ANIMATION_DONE)) !=
FLAG_OPTIMIZE_INVALIDATE) {
// 根据父View的位置，偏移刷新区域
dirty.offset(location[CHILD_LEFT_INDEX] - mScrollX, location[CHILD_TOP_INDEX] - mScrollY);

final int left = mLeft;
final int top = mTop;
//计算实际可刷新区域
if (dirty.intersect(0, 0, mRight - left, mBottom - top) ||
(mPrivateFlags & DRAW_ANIMATION) == DRAW_ANIMATION) {
mPrivateFlags &= ~DRAWING_CACHE_VALID;

location[CHILD
d73d
_LEFT_INDEX] = left;
location[CHILD_TOP_INDEX] = top;
return mParent;
}
} else {
mPrivateFlags &= ~DRAWN & ~DRAWING_CACHE_VALID;

location[CHILD_LEFT_INDEX] = mLeft;
location[CHILD_TOP_INDEX] = mTop;

dirty.set(0, 0, mRight - location[CHILD_LEFT_INDEX],
mBottom - location[CHILD_TOP_INDEX]);

return mParent;
}
}

return null;
}  </span>

三- Android中View的绘制过程

当Activity获得焦点时，它将被要求绘制自己的布局，Android framework将会处理绘制过程，Activity只需提供它的布局的根节点。
绘制过程从布局的根节点开始，从根节点开始测量和绘制整个layout tree。
每一个ViewGroup 负责要求它的每一个孩子被绘制，每一个View负责绘制自己。
因为整个树是按顺序遍历的，所以父节点会先被绘制，而兄弟节点会按照它们在树中出现的顺序被绘制。
绘制是一个两遍（two pass）的过程：一个measure pass和一个layout pass。
测量过程（measuring pass）是在measure(int,
int)中实现的，是从树的顶端由上到下进行的。
在这个递归过程中，每一个View会把自己的dimension specifications传递下去。
在measure pass的最后，每一个View都存储好了自己的measurements，即测量结果。
第二个是布局过程（layout pass），它发生在 layout(int,
int, int, int)中，仍然是从上到下进行（top-down）。
在这一遍中，每一个parent都会负责用测量过程中得到的尺寸，把自己的所有孩子放在正确的地方。

尺寸的父子关系处理

当一个View对象的 measure() 方法返回时，它的 

getMeasuredWidth()

 和 

getMeasuredHeight()

值应该被设置好了，并且它的所有子孙的值也应该一起被设置好了。
一个View对象的measured width 和measured height的值必须考虑到它的父容器给它的限制。
这样就保证了在measure pass的最后，所有的parent都接受了它的所有孩子的measurements结果。
注意：一个parent可能会不止一次地对它的孩子调用measure()方法。
比如，第一遍的时候，一个parent可能测量它的每一个孩子，并没有指定尺寸，parent只是为了发现它们想要多大；
如果第一遍之后得知，所有孩子的无限制的尺寸总和太大或者太小，parent会再次对它的孩子调用measure()方法，这时候parent会设定规则，介入这个过程，使用实际的值。
（即，让孩子自由发展不成，于是家长介入）。
一个自定义View的例子

<span style="font-family:SimSun;font-size:18px;">/*
* Copyright (C) 2007 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0 *
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package com.example.android.apis.view;

// Need the following import to get access to the app resources, since this
// class is in a sub-package.
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;

import com.example.android.apis.R;

/**
* Example of how to write a custom subclass of View. LabelView
* is used to draw simple text views. Note that it does not handle
* styled text or right-to-left writing systems.
*
*/
public class LabelView extends View {
private Paint mTextPaint;
private String mText;
private int mAscent;

/**
* Constructor. This version is only needed if you will be instantiating
* the object manually (not from a layout XML file).
* @param context
*/
public LabelView(Context context) {
super(context);
initLabelView();
}

/**
* Construct object, initializing with any attributes we understand from a
* layout file. These attributes are defined in
* SDK/assets/res/any/classes.xml.
*
* @see android.view.View#View(android.content.Context, android.util.AttributeSet)
*/
public LabelView(Context context, AttributeSet attrs) {
super(context, attrs);
initLabelView();

TypedArray a = context.obtainStyledAttributes(attrs,
R.styleable.LabelView);

CharSequence s = a.getString(R.styleable.LabelView_text);
if (s != null) {
setText(s.toString());
}

// Retrieve the color(s) to be used for this view and apply them.
// Note, if you only care about supporting a single color, that you
// can instead call a.getColor() and pass that to setTextColor().
setTextColor(a.getColor(R.styleable.LabelView_textColor, 0xFF000000));

int textSize = a.getDimensionPixelOffset(R.styleable.LabelView_textSize, 0);
if (textSize > 0) {
setTextSize(textSize);
}

a.recycle();
}

private final void initLabelView() {
mTextPaint = new Paint();
mTextPaint.setAntiAlias(true);
// Must manually scale the desired text size to match screen density
mTextPaint.setTextSize(16 * getResources().getDisplayMetrics().density);
mTextPaint.setColor(0xFF000000);
setPadding(3, 3, 3, 3);
}

/**
* Sets the text to display in this label
* @param text The text to display. This will be drawn as one line.
*/
public void setText(String text) {
mText = text;
requestLayout();
invalidate();
}

/**
* Sets the text size for this label
* @param size Font size
*/
public void setTextSize(int size) {
// This text size has been pre-scaled by the getDimensionPixelOffset method
mTextPaint.setTextSize(size);
requestLayout();
invalidate();
}

/**
* Sets the text color for this label.
* @param color ARGB value for the text
*/
public void setTextColor(int color) {
mTextPaint.setColor(color);
invalidate();
}

/**
* @see android.view.View#measure(int, int)
*/
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(measureWidth(widthMeasureSpec),
measureHeight(heightMeasureSpec));
}

/**
* Determines the width of this view
* @param measureSpec A measureSpec packed into an int
* @return The width of the view, honoring constraints from measureSpec
*/
private int measureWidth(int measureSpec) {
int result = 0;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);

if (specMode == MeasureSpec.EXACTLY) {
// We were told how big to be
result = specSize;
} else {
// Measure the text
result = (int) mTextPaint.measureText(mText) + getPaddingLeft()
+ getPaddingRight();
if (specMode == MeasureSpec.AT_MOST) {
// Respect AT_MOST value if that was what is called for by measureSpec
result = Math.min(result, specSize);
}
}

return result;
}

/**
* Determines the height of this view
* @param measureSpec A measureSpec packed into an int
* @return The height of the view, honoring constraints from measureSpec
*/
private int measureHeight(int measureSpec) {
int result = 0;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);

mAscent = (int) mTextPaint.ascent();
if (specMode == MeasureSpec.EXACTLY) {
// We were told how big to be
result = specSize;
} else {
// Measure the text (beware: ascent is a negative number)
result = (int) (-mAscent + mTextPaint.descent()) + getPaddingTop()
+ getPaddingBottom();
if (specMode == MeasureSpec.AT_MOST) {
// Respect AT_MOST value if that was what is called for by measureSpec
result = Math.min(result, specSize);
}
}
return result;
}

/**
* Render the text
*
* @see android.view.View#onDraw(android.graphics.Canvas)
*/
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.drawText(mText, getPaddingLeft(), getPaddingTop() - mAscent, mTextPaint);
}
}</span>

Android view中的requestLayout和invalidate方法：

requestLayout：当view确定自身已经不再适合现有的区域时，该view本身调用这个方法要求parent view重新调用他的onMeasure onLayout来对重新设置自己位置。

特别的当view的layoutparameter发生改变，并且它的值还没能应用到view上，这时候适合调用这个方法。

invalidate：View本身调用迫使view重画。