canvas之save、restore详解

解释

Canvas的save()、restore()这两个方法字面意思就是保存、恢复，但为什么要保存和回复呢？不保存会怎么样？

如上图当使用 canvas.translate(50,50)方法后canvas的坐标中心就不是左上角，而是（50,50），如图左边数第一个黑色直角就是 第一次canvas.translate(50,50)后的坐标中心。我用了一个for循环对同一个canvas（注意是同一个canvas）执行了5次canvas.translate(50,50),从图中发现canvas每次都以当前坐标中心为基础移动（50,50）,如上图。但是如果遇到在for循环中对canvas执行translate后不想canvas改变坐标中心怎么办？那就在canvas translate前save，后再restore。如下注释代码。

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.Point;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;

/**
 * Creator: Jowney  (~._.~)
 * Date: 2019/3/22/0:43
 * Description:
 */
public class MainTest extends Activity {
    private static final String TAG = "MainTest";
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
    public static class MyView  extends View{
        public MyView(Context context, @Nullable AttributeSet attrs) {
            super(context, attrs);
        }

        public MyView(Context context,  @Nullable AttributeSet attrs, int defStyleAttr) {
            super(context, attrs, defStyleAttr);
        }

        @Override
        protected void onDraw(Canvas canvas) {
            super.onDraw(canvas);
            for (int i = 0; i < 5; i++) {
   //canvas.save()
                Path path = new Path();
                path.lineTo(100,0);
                path.moveTo(0,0);
                path.lineTo(0,100);
                Paint paint = new Paint();
                paint.setColor(Color.BLACK);
                paint.setStyle(Paint.Style.STROKE);
                paint.setStrokeWidth(10);
                canvas.translate(50,50);
                canvas.drawPath(path,paint);
   //canvas.restore()
            }

        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:animateLayoutChanges="true"
    tools:context=".MainActivity">

    <view
        class="jowney.com.animationtest.MainTest$MyView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@color/colorAccent" />

</LinearLayout>