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，待變一個控件。

ViewGroup

extends View
implements ViewManager ViewParent

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

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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內部包含一組View，ViewGroup也繼承自View，這就說明View本身就可以使單個控件或多個控件組成的一組控件。

UI界面架構圖：

每個Activity都包含一個Window對象，Window對象通常由PhoneWindow來實現。PhoneWindow將一個DecorWindow設置為整個窗口的根View。DecView里有所有View的監聽事件，通過WindowManagerService進行接收，并通過Activity對象回調相應的onClickListener。顯示界面上，將屏幕分為兩部分，分別為TitleView和ContentView。ContentView是一個ID為content的FrameLayout，布局文件activity_main.xml就是設置在這樣一個FrameLayout中。

2、

View的位置參數：

View根據上圖得到View的寬高和坐標（相對于ViewGroup，而不是原點）的關系：

width = right - left;

height = bottom - top;

獲取這幾個參數的方法如下：

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

Android3.0開始View增加了幾個參數:x,y,translationX,translationY,其中x,y是View左上角的坐標，translationX,translationY是View左上角相對于父容器的偏移量：

x = left + translationX;

y = top + translationY;

需要注意：View在平移過程中，top/left表示的是原始左上角的位置信息，其值在移動期間不會發生改變，變化的是x,y,translationX,translationY.

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

View的繪制流程從ViewRoot的performTraversals方法開始，ViewRoot對應于ViewRootImpl類，是連接WindowManager和DecorView的紐帶，View的三大流程均是通過ViewRoot來完成。在ActivityThread中，當Activity對象被創建完畢后，會將DecorView添加到Window中，同時會創建ViewRootImpl對象，并將ViewRootImpl對象與DecorView相關聯：

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

View的繪制流程從ViewRoot的performTraversals方法開始，經過measure,layout,draw三個過程將一個View繪制出來。

performTraversals會依次調用performMeasure,performLayout,performDraw三個方法，分別完成頂級View的measure,layout,draw三大流程，其中performMeasure中會調用measure方法，在measure方法中又調用onMeasure方法，在inMeasure方法中會對所有子元素進行measure過程，此時measure流程就從父元素傳到子元素了，這樣完成了一次measure過程。接著子元素會重復父容器的measure過程，如此反復完成整個View樹的遍歷。performLayout與performDraw與之類似，不同的是performDraw的傳遞過程是在draw方法中通過dispatchDraw來實現。

測量過程在onMeasure()方法中進行。

Android提供了一個功能強大的類--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32位的int值，高2位為測量模式（SpecMode），低30位是測量大小（SpecSize）.

View默認的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()方法來指定wrap_content大小（進一步說明，由源碼得出，wrap_content下的SpecMode是AT_MOST，此時的specSize是parentSize，即如果不指定wrap_content，在布局中使用wrap_content就相當于使用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的默認寬高（mWidth/mHeight），并在wrap_content時設置此寬高。
Demo：

MeasureView.java

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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}@Overrideprotected 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);}}}

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activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"xmlns:tools="http://schemas.android.com/tools"android:layout_width="match_parent"android:layout_height="match_parent"android:paddingBottom="@dimen/activity_vertical_margin"android:paddingLeft="@dimen/activity_horizontal_margin"android:paddingRight="@dimen/activity_horizontal_margin"android:paddingTop="@dimen/activity_vertical_margin"tools:context="sunny.example.ahthreeviewmeasure.MainActivity" ><sunny.example.ahthreeviewmeasure.MeasureViewandroid:id="@+id/measureView"android:layout_width="match_parent"android:layout_height="wrap_content"android:background="#09acda" /></RelativeLayout>

MainActivity.java

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package sunny.example.ahthreeviewmeasure;import android.support.v7.app.ActionBarActivity; import android.os.Bundle;public class MainActivity extends ActionBarActivity {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);//MeasureView mView = (MeasureView)findViewById(R.id.measureView);}}

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