Android学习笔记——自定义View

首先来看自定义View的步骤：
1.自定义View的属性
2.在View的构造方法中获得我们自定义的属性
3.重写onMesure()方法
4.重写onDraw()

一、自定义View的属性

在res/values/目录下建立一个attrs.xml，在其中定义我们的属性和声明。

<?xml version="1.0" encoding="utf-8"?>
<resources>
    
    <attr name="titleText" format="string"/>
    <attr name="titleTextColor" format="color"/>
    <attr name="titleTextSize" format="dimension"/>
    
    <declare-styleable name="CustomTitleView">
        <attr name="titleText"/>
        <attr name="titleTextColor"/>
        <attr name="titleTextSize"/>
    </declare-styleable>
    
</resources>

我们定义了字体、字体颜色和字体大小，format是指该属性的取值类型，一共有以下几种：

string,color,demension,integer,enum,reference,float,boolean,fraction,flag
之后在布局中使用我们的自定义View

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:custom="http://schemas.android.com/apk/res/com.shieda.app.animation_demo"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.shieda.app.animation_demo.MainActivity">

    <Button
        android:id="@+id/btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <com.shieda.app.animation_demo.CustomTitleView
        custom:titleText="3712"
        custom:titleTextColor="#ff0000"
        custom:titleTextSize="40sp"
        android:layout_width="200dp"
        android:layout_height="100dp" />

</android.support.constraint.ConstraintLayout>

二、在View构造方法中获取我们自定义的样式

/** 
     * 文本 
     */  
    private String mTitleText;  
    /** 
     * 文本的颜色 
     */  
    private int mTitleTextColor;  
    /** 
     * 文本的大小 
     */  
    private int mTitleTextSize;  
  
    /** 
     * 绘制时控制文本绘制的范围 
     */  
    private Rect mBound;  
    private Paint mPaint;  
  
    public CustomTitleView(Context context, AttributeSet attrs)  
    {  
        this(context, attrs, 0);  
    }  
  
    public CustomTitleView(Context context)  
    {  
        this(context, null);  
    }  
  
    /** 
     * 获得我自定义的样式属性 
     *  
     * @param context 
     * @param attrs 
     * @param defStyle 
     */  
    public CustomTitleView(Context context, AttributeSet attrs, int defStyle)  
    {  
        super(context, attrs, defStyle);  
        /** 
         * 获得我们所定义的自定义样式属性 
         */  
        TypedArray a = context.getTheme().obtainStyledAttributes(attrs, R.styleable.CustomTitleView, defStyle, 0);  
        int n = a.getIndexCount();  
        for (int i = 0; i < n; i++)  
        {  
            int attr = a.getIndex(i);  
            switch (attr)  
            {  
            case R.styleable.CustomTitleView_titleText:  
                mTitleText = a.getString(attr);  
                break;  
            case R.styleable.CustomTitleView_titleTextColor:  
                // 默认颜色设置为黑色  
                mTitleTextColor = a.getColor(attr, Color.BLACK);  
                break;  
            case R.styleable.CustomTitleView_titleTextSize:  
                // 默认设置为16sp，TypeValue也可以把sp转化为px  
                mTitleTextSize = a.getDimensionPixelSize(attr, (int) TypedValue.applyDimension(  
                        TypedValue.COMPLEX_UNIT_SP, 16, getResources().getDisplayMetrics()));  
                break;  
  
            }  
  
        }  
        a.recycle();  
  
        /** 
         * 获得绘制文本的宽和高 
         */  
        mPaint = new Paint();  
        mPaint.setTextSize(mTitleTextSize);  
        // mPaint.setColor(mTitleTextColor);  
        mBound = new Rect();  
        mPaint.getTextBounds(mTitleText, 0, mTitleText.length(), mBound);  
  
    }  

三、重写onDraw、onMeasure方法

@Override  
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)  
    {  
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);  
    }  
  
    @Override  
    protected void onDraw(Canvas canvas)  
    {  
        mPaint.setColor(Color.YELLOW);  
        canvas.drawRect(0, 0, getMeasuredWidth(), getMeasuredHeight(), mPaint);  
  
        mPaint.setColor(mTitleTextColor);  
        canvas.drawText(mTitleText, getWidth() / 2 - mBound.width() / 2, getHeight() / 2 + mBound.height() / 2, mPaint);  
    }  

此时，我们实现了自定义View。但是我们把布局文件中宽高改为wrap_content，会发现填满了屏幕；这是因为系统帮我们测量的宽高都是Match_Parent；所以我们需要重写测量onMeasure方法。
MeasureSpec的specMode一共有三种类型：

• EXACTLY：一般是设置了明确的值或者MATCH_PARENT
• AT_MOST：表示子布局限制在一个最大值内，一般为WRAP_CONTENT
• UNSPECIFIED：表示子布局想要多大就多大，很少使用
  下面重写onMeasure()代码：

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
    {
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        int width;
        int height ;
        if (widthMode == MeasureSpec.EXACTLY)
        {
            width = widthSize;
        } else
        {
            mPaint.setTextSize(mTitleTextSize);
            mPaint.getTextBounds(mTitleText, 0, mTitleText.length(), mBound);
            float textWidth = mBound.width();
            int desired = (int) (getPaddingLeft() + textWidth + getPaddingRight());
            width = desired;
        }

        if (heightMode == MeasureSpec.EXACTLY)
        {
            height = heightSize;
        } else
        {
            mPaint.setTextSize(mTitleTextSize);
            mPaint.getTextBounds(mTitleText, 0, mTitleText.length(), mBound);
            float textHeight = mBound.height();
            int desired = (int) (getPaddingTop() + textHeight + getPaddingBottom());
            height = desired;
        }