1、 public class View extends Object
implements Drawable.Callback KeyEvent.Callback AccessibilityEventSource java.lang.Object ???? android.view.View ? Class Overview

This class represents the basic building block for user interface components. A View occupies a rectangular area on the screen and is responsible for drawing and event handling. View is the base class forwidgets, which are used to create interactive UI components (buttons, text fields, etc.).

public abstract class ? View是Android中所有控件的基類，Button/TextView/RelativeLayout/ListView等，其共同基類都是View。View是界面層的控件的一種抽象，待變一個(gè)控件。 ViewGroup extends View
implements ViewManager ViewParent

java.lang.Object ???? android.view.View ? ???? android.view.ViewGroup ? Class Overview

A ViewGroup is a special view that can contain other views (called children.) The view group is the base class for layouts and views containers.

ViewGroup內(nèi)部包含一組View，ViewGroup也繼承自View，這就說(shuō)明View本身就可以使單個(gè)控件或多個(gè)控件組成的一組控件。

UI界面架構(gòu)圖：

每個(gè)Activity都包含一個(gè)Window對(duì)象，Window對(duì)象通常由PhoneWindow來(lái)實(shí)現(xiàn)。PhoneWindow將一個(gè)DecorWindow設(shè)置為整個(gè)窗口的根View。DecView里有所有View的監(jiān)聽(tīng)事件，通過(guò)WindowManagerService進(jìn)行接收，并通過(guò)Activity對(duì)象回調(diào)相應(yīng)的onClickListener。顯示界面上，將屏幕分為兩部分，分別為TitleView和ContentView。ContentView是一個(gè)ID為content的FrameLayout，布局文件activity_main.xml就是設(shè)置在這樣一個(gè)FrameLayout中。

2、

View的位置參數(shù)：

View根據(jù)上圖得到View的寬高和坐標(biāo)（相對(duì)于ViewGroup，而不是原點(diǎn)）的關(guān)系：

width = right - left;

height = bottom - top;

獲取這幾個(gè)參數(shù)的方法如下：

mLeft?=?getLeft();
mRight?=?getRight();
mTop?=?getTop();
mBottom?=?getBottpm();

Android3.0開(kāi)始View增加了幾個(gè)參數(shù):x,y,translationX,translationY,其中x,y是View左上角的坐標(biāo)，translationX,translationY是View左上角相對(duì)于父容器的偏移量：

x = left + translationX;

y = top + translationY;

需要注意：View在平移過(guò)程中，top/left表示的是原始左上角的位置信息，其值在移動(dòng)期間不會(huì)發(fā)生改變，變化的是x,y,translationX,translationY.

3、View的三大流程（measure,layout,draw）之View的測(cè)量

View的繪制流程從ViewRoot的performTraversals方法開(kāi)始，ViewRoot對(duì)應(yīng)于ViewRootImpl類，是連接WindowManager和DecorView的紐帶，View的三大流程均是通過(guò)ViewRoot來(lái)完成。在ActivityThread中，當(dāng)Activity對(duì)象被創(chuàng)建完畢后，會(huì)將DecorView添加到Window中，同時(shí)會(huì)創(chuàng)建ViewRootImpl對(duì)象，并將ViewRootImpl對(duì)象與DecorView相關(guān)聯(lián)：

root?=?new?ViewRootImpl(view.getContext(),display);
root.setView(view,wparams,panelParentView);

View的繪制流程從ViewRoot的performTraversals方法開(kāi)始，經(jīng)過(guò)measure,layout,draw三個(gè)過(guò)程將一個(gè)View繪制出來(lái)。

performTraversals會(huì)依次調(diào)用performMeasure,performLayout,performDraw三個(gè)方法，分別完成頂級(jí)View的measure,layout,draw三大流程，其中performMeasure中會(huì)調(diào)用measure方法，在measure方法中又調(diào)用onMeasure方法，在inMeasure方法中會(huì)對(duì)所有子元素進(jìn)行measure過(guò)程，此時(shí)measure流程就從父元素傳到子元素了，這樣完成了一次measure過(guò)程。接著子元素會(huì)重復(fù)父容器的measure過(guò)程，如此反復(fù)完成整個(gè)View樹(shù)的遍歷。performLayout與performDraw與之類似，不同的是performDraw的傳遞過(guò)程是在draw方法中通過(guò)dispatchDraw來(lái)實(shí)現(xiàn)。

測(cè)量過(guò)程在onMeasure()方法中進(jìn)行。

Android提供了一個(gè)功能強(qiáng)大的類--MeasureSpec類。

public static class View.MeasureSpec extends Object
java.lang.Object ???? android.view.View.MeasureSpec Summary: Constants |Ctors | Methods | Inherited Methods | [Expand All] Added in API level 1

public static class View.MeasureSpec extends Object
java.lang.Object ???? android.view.View.MeasureSpec Class Overview

A MeasureSpec encapsulates the layout requirements passed from parent to child. Each MeasureSpec represents a requirement for either the width or the height. A MeasureSpec is comprised of a size and a mode. There are three possible modes:

UNSPECIFIED The parent has not imposed any constraint on the child. It can be whatever size it wants.EXACTLY The parent has determined an exact size for the child. The child is going to be given those bounds regardless of how big it wants to be.AT_MOST The child can be as large as it wants up to the specified size.

MeasureSpec是一個(gè)32位的int值，高2位為測(cè)量模式（SpecMode），低30位是測(cè)量大?。⊿pecSize）.

View默認(rèn)的onMeasure()方法只支持EXACTLY模式，因此如果讓自定義View支持wrap_content屬性，就必須重寫onMeasure()方法來(lái)指定wrap_content大?。ㄟM(jìn)一步說(shuō)明，由源碼得出，wrap_content下的SpecMode是AT_MOST，此時(shí)的specSize是parentSize，即如果不指定wrap_content，在布局中使用wrap_content就相當(dāng)于使用match_parent）.

解決方法：

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){
		setMeasureDimension(mWidth,mHeight);
	}else?if(widthSpecMode?==?MeasureSpec.AT_MOST){
		setMeasureDimension(mWidth,heightSpecSize);
	}else?if(heightSpecMode?==?MeasureSpec.AT_MOST){
		setMeasureDimension(widthSoecSize,mHeight)
	}
}

在上面代碼中指定View的默認(rèn)寬高（mWidth/mHeight），并在wrap_content時(shí)設(shè)置此寬高。
Demo：

MeasureView.java

?

package?sunny.example.ahthreeviewmeasure;

import?android.content.Context;
import?android.util.AttributeSet;
import?android.view.View;
import?android.widget.Button;
import?android.widget.TextView;

public?class?MeasureView?extends?View{

	public?MeasureView(Context?context)?{
		super(context);
		//?TODO?Auto-generated?constructor?stub
		?
	}
	
	public?MeasureView(Context?context,?AttributeSet?attrs)?{
		super(context,?attrs);
		//?TODO?Auto-generated?constructor?stub
	}
	
	public?MeasureView(Context?context,?AttributeSet?attrs,?int?defStyle)?{
		super(context,?attrs,?defStyle);
		//?TODO?Auto-generated?constructor?stub
	}

	@Override
	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(400,400);//指定寬高
		}else?if(widthSpecMode?==?MeasureSpec.AT_MOST){
			setMeasuredDimension(400,heightSpecSize);
		}else?if(heightSpecMode?==?MeasureSpec.AT_MOST){
			setMeasuredDimension(widthSpecSize,400);
		}
	}

	

}

?

activity_main.xml

MainActivity.java

?

package?sunny.example.ahthreeviewmeasure;

import?android.support.v7.app.ActionBarActivity;
import?android.os.Bundle;

public?class?MainActivity?extends?ActionBarActivity?{

	@Override
	protected?void?onCreate(Bundle?savedInstanceState)?{
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		//MeasureView?mView?=?(MeasureView)findViewById(R.id.measureView);
	}

	
}

?

?

?