saveLayer() 与 restoreToCount() 和 save 及 restore() 大同小异，只是前者多了一些东西

save() 方法保存的是 整个 Canvas ，而 saveLayer() 则可以选择性的保存某个区域的状态

restoreToCount(int) 可以直接传入要恢复到的 Layer 层数，直接就跳到对应的那一层，同时会将该层上面所有的Layer踢出栈，让该层成为栈顶

saveLayer()

saveLayer() 有四个重载方法，这几个方法在 API 21+ 中被添加

int saveLayer(RectF bounds, Paint paint)

int saveLayerAlpha(RectF bounds, int alpha)

int saveLayer(float left, float top, float right, float bottom, Paint paint)

int saveLayerAlpha(float left, float top, float right, float bottom, int alpha)

还有几个在 API 26+ 中被废弃的

saveLayer(RectF bounds, Paint paint, int saveFlags)

int saveLayer(float left, float top, float right, float bottom, Paint paint, int saveFlags)

int saveLayerAlpha(float left, float top, float right, float bottom, int alpha, int saveFlags)

int saveLayerAlpha(RectF bounds, int alpha, int saveFlags)

参数都很好理解，我们重点讲解下 int saveFlags ，这个是设置改保存哪个对象的，可选的值有

还有几个在 API 26+ 中被废弃的

范例

我们写一个范例来演示一下 saveLayer()

1. 创建一个 空的 Android 项目 cn.twle.android.SaveLayer

2. 下载 /static/i/android/avatar_128x128.png，并放到 res/drawable 目录下

3. 自定义一个 View 类 MsView.java

   package cn.twle.android.savelayer;
   
   import android.content.Context;
   import android.os.Build;
   import android.graphics.Bitmap;
   import android.graphics.BitmapFactory;
   import android.graphics.Canvas;
   import android.graphics.Paint;
   import android.graphics.RectF;
   import android.support.v7.widget.AppCompatImageView;
   import android.util.AttributeSet;
   
   public class MsView extends AppCompatImageView {
   
       private Bitmap mBitmap;
       private Paint mPaint = new Paint();
   
       public MsView(Context context) {
           this(context,null);
       }
   
       public MsView(Context context, AttributeSet attrs) {
           super(context, attrs);
   
           mBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.avatar_128x128);
   
           mPaint.setStyle(Paint.Style.STROKE);
           mPaint.setFlags(Paint.ANTI_ALIAS_FLAG);
           mPaint.setAntiAlias(true);// 抗锯尺
       }
   
       @Override
       protected void onDraw(Canvas canvas) {
           super.onDraw(canvas);
           if(mBitmap == null)
           {
               return;
           }
   
           RectF bounds = new RectF(0, 0, 400, 400);
   
           if ( Build.VERSION.SDK_INT >= 21)
               canvas.saveLayer(bounds, mPaint);
           else
               canvas.saveLayer(bounds, mPaint, Canvas.ALL_SAVE_FLAG);
   
           canvas.drawColor(0xffff0000);
           canvas.drawBitmap(mBitmap, 200, 200, mPaint);
           canvas.restoreToCount(1);
           canvas.drawBitmap(mBitmap, 400, 400, mPaint);
   
           invalidate();
       }
   }
   

4. 修改 MainActivity.java 设置 setContentView(new MsView(MainActivity.this))

   package cn.twle.android.savelayer;
   
   import android.support.v7.app.AppCompatActivity;
   import android.os.Bundle;
   
   public class MainActivity extends AppCompatActivity {
   
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(new MsView(MainActivity.this));
       }
   }