本文参考了《Android开发艺术探究》。
- measure流程
系统内部是通过MeasureSpec来进行View的测量,但是正常情况下我们使用View指定的MeasureSpec,尽管如此,但是我们可以给View设置LayoutParams。在View测量的时候,系统会将LayoutParams在父容器的约束下转换成对应的MeasureSpec,然后再根据这个MeasureSpec来确定View测量后的宽/高。需要注意的是,MeasureSpec不是唯一由LayoutParams决定的,LayoutParams需要和父容器一起才能决定View的MeasureSpec,从而进一步决定View的宽/高。另外,对于顶级View(即DecorView)和普通的View来说,MeasureSpec的转换过程略有不同。对于DecorView,其MeasureSpec由窗口的尺寸和其自身的LayoutParams来共同确定:对于普通View,其MeasureSpec有父容器的MeasureSpec和自身的LayoutParams来共同决定,MeasureSpec一旦确定后,onMeasure中就可以确定View的测量宽/高。
- layout流程(继承ViewGroup才需要实现)
Layout的作用是ViewGroup用来确定子元素的位置,当ViewGroup的位置被确定后,它在onLayout中会遍历所有的子元素并调用其layout方法,在layout方法中onLayout方法又会被调用。Layout过程和measure过程相比就简单多了,layout方法确定View本身的位置,而onLayout方法则会确定所有子元素的位置。 - draw流程
MainActivity类:
package com.example.test;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}
CircleView类:
package com.example.test;
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;
public class CircleView extends View {
private int mColor = Color.RED;
private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
public CircleView(Context context) {
super(context);
init();
}
public CircleView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public CircleView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.CircleView);
mColor = a.getColor(R.styleable.CircleView_circle_color, Color.RED);
a.recycle();
init();
}
private void init() {
mPaint.setColor(mColor);
}
@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(200, 200);
} else if (widthSpecMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(200, heightSpecSize);
} else if (heightSpecMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(widthSpecSize, 200);
}
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
final int paddingLeft = getPaddingLeft();
final int paddingRight = getPaddingRight();
final int paddingTop = getPaddingTop();
final int paddingBottom = getPaddingBottom();
int width = getWidth() - paddingLeft - paddingRight;
int height = getHeight() - paddingTop - paddingBottom;
int radius = Math.min(width, height) / 2;
canvas.drawCircle(paddingLeft + width / 2, paddingTop + height / 2,
radius, mPaint);
}
}
布局:
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#ffffff"
android:orientation="vertical"
<!--使用自定义属性,必须要有以下这句话-->
xmlns:app="http://schemas.android.com/apk/res/com.example.test">
<com.example.test.CircleView
android:id="@+id/circleView"
android:layout_width="wrap_content"
android:layout_height="100dp"
android:layout_margin="20dp"
android:padding="20dp"
app:circle_color="@color/light_green"
android:background="#000000"/>
</LinearLayout>
attrs文件:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<declare-styleable name="CircleView">
<attr name="circle_color" format="color" />
</declare-styleable>
</resources>
colors文件:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<color name="reveal_color">#1b000000</color>
<color name="light_green">#0ac39e</color>
</resources>
运行结果: