Google示例ElevationDrag的源码分析

/*
* Copyright (C) 2014 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*      http://www.apache.org/licenses/LICENSE-2.0 *
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package com.example.android.elevationdrag;

import android.content.Context;
import android.support.v4.widget.ViewDragHelper;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;
import android.widget.FrameLayout;

import java.util.ArrayList;
import java.util.List;

/**
* A {@link FrameLayout} that allows the user to drag and reposition child views.
* 一个帧布局允许用户去拖动和复位子view
*/
public class DragFrameLayout extends FrameLayout {

/**
* The list of {@link View}s that will be draggable.
* 允许被拖动的view列表，保存被拖动的view
*/
private List<View> mDragViews;

/**
* The {@link DragFrameLayoutController} that will be notify on drag.
* 拖动的时候被DragFrameLayoutController通知
*/
private DragFrameLayoutController mDragFrameLayoutController;

private ViewDragHelper mDragHelper;

public DragFrameLayout(Context context) {
this(context, null, 0, 0);
}

public DragFrameLayout(Context context, AttributeSet attrs) {
this(context, attrs, 0, 0);
}

public DragFrameLayout(Context context, AttributeSet attrs, int defStyleAttr) {
this(context, attrs, defStyleAttr, 0);
}

public DragFrameLayout(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
super(context, attrs, defStyleAttr, defStyleRes);
mDragViews = new ArrayList<>();

/**
* Create the {@link ViewDragHelper} and set its callback.
* 创建ViewDragHelper对象和设置回调，第二参数为灵敏度，如果值越大，相应的源码的helper.mTouchSlop越小，拖动开始前拖动的距离
* ViewDragHelper一个父类View用作交流的回调的渠道，
* 当重要的事件和一些访问方法期望从提供的ViewDragHelper上得到更多有关父类的View的状态信息时on*方法被调用，
* 回调也决定着子view的可拖动范围
*/
mDragHelper = ViewDragHelper.create(this, 1.0f, new ViewDragHelper.Callback() {
//tryCaptureView如何返回ture则表示可以捕获该view，你可以根据传入的第一个view参数决定哪些可以捕获
@Override
public boolean tryCaptureView(View child, int pointerId) {
Log.e("ViewDragHelper","pointerId = " + pointerId);
return mDragViews.contains(child);
}

@Override
public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
super.onViewPositionChanged(changedView, left, top, dx, dy);
}
//限制沿水平轴拖动子视图的动作，默认实现不允许水平动作，继承的类必须复写这个方法，并提供所需的值
//可以在该方法中对child移动的边界进行控制
@Override
public int clampViewPositionHorizontal(View child, int left, int dx) {
int rightBound = getMeasuredWidth() - child.getMeasuredWidth();
int leftBound = 0;
int newBound = Math.min(rightBound,Math.max(leftBound,left));
return newBound;
}

@Override
public int clampViewPositionVertical(View child, int top, int dy) {
int buttomBound = getMeasuredHeight() - child.getMeasuredHeight();
int topBound = 0;
int newBound = Math.min(buttomBound,Math.max(topBound,top));
return newBound;
}

@Override
public void onViewCaptured(View capturedChild, int activePointerId) {
super.onViewCaptured(capturedChild, activePointerId);
if (mDragFrameLayoutController != null) {
mDragFrameLayoutController.onDragDrop(true);
}
}

//当用户释放子view的时候调用
@Override
public void onViewReleased(View releasedChild, float xvel, float yvel) {
super.onViewReleased(releasedChild, xvel, yvel);
if (mDragFrameLayoutController != null) {
mDragFrameLayoutController.onDragDrop(false);
}
}

@Override
public void onViewDragStateChanged(int state) {
super.onViewDragStateChanged(state);
switch (state){
case ViewDragHelper.STATE_DRAGGING:
Log.e("ViewDragHelper","STATE_DRAGGING");
break;
case ViewDragHelper.STATE_IDLE:
Log.e("ViewDragHelper","STATE_IDLE");
break;
case ViewDragHelper.STATE_SETTLING:
Log.e("ViewDragHelper","STATE_SETTLING");
break;
}
}

//如果设置了setEdgeTrackingEnabled，从边界开始触摸的话将会调用该方法
@Override
public void onEdgeTouched(int edgeFlags, int pointerId) {
super.onEdgeTouched(edgeFlags, pointerId);
switch(edgeFlags){
case ViewDragHelper.EDGE_BOTTOM:
Log.e("ViewDragHelper","EDGE_BOTTOM");
break;
case ViewDragHelper.EDGE_LEFT:
Log.e("ViewDragHelper","EDGE_LEFT");
break;
case ViewDragHelper.EDGE_RIGHT:
Log.e("ViewDragHelper","EDGE_RIGHT");
break;
case ViewDragHelper.EDGE_TOP:
Log.e("ViewDragHelper","EDGE_TOP");
break;
}
}

@Override
public boolean onEdgeLock(int edgeFlags) {
//父类默认返回的是false
return super.onEdgeLock(edgeFlags);
}

//该方法在onEdgeTouched方法之后调用
@Override
public void onEdgeDragStarted(int edgeFlags, int pointerId) {
super.onEdgeDragStarted(edgeFlags, pointerId);
switch(edgeFlags){
case ViewDragHelper.EDGE_BOTTOM:
Log.e("ViewDragHelper","onEdgeDragStarted EDGE_BOTTOM");
break;
case ViewDragHelper.EDGE_LEFT:
Log.e("ViewDragHelper","onEdgeDragStarted EDGE_LEFT");
break;
case ViewDragHelper.EDGE_RIGHT:
Log.e("ViewDragHelper","onEdgeDragStarted EDGE_RIGHT");
break;
case ViewDragHelper.EDGE_TOP:
Log.e("ViewDragHelper","onEdgeDragStarted EDGE_TOP");
break;
}
}

@Override
public int getOrderedChildIndex(int index) {
Log.e("ViewDragHelper","index = " + index);
//父类就是直接返回index
return super.getOrderedChildIndex(index);
}

@Override
public int getViewHorizontalDragRange(View child) {
return getMeasuredWidth() - child.getMeasuredWidth();
}

//子类在垂直方向可拖动的幅度，view在垂直方向不可移动的话这个方法应该返回0
//当本view拦截触摸监听的话，就会调用本方法和getViewHorizontalDragRange方法，如果返回0的话，就会捕获不了
//只有这两个返回值大于0，才可以被捕获
@Override
public int getViewVerticalDragRange(View child) {
return getMeasuredHeight() - child.getMeasuredHeight();
}
});
//设置拖动的边界
mDragHelper.setEdgeTrackingEnabled(ViewDragHelper.EDGE_LEFT);
}

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
final int action = ev.getActionMasked();
if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
//对此方法的调用的结果等效于TouchEvent MotionEvent处理接收的ACTION_CANCEL事件
mDragHelper.cancel();
return false;
}
//传递中断事件给父view
return mDragHelper.shouldInterceptTouchEvent(ev);
}

@Override
public boolean onTouchEvent(MotionEvent ev) {
//Process a touch event received by the parent view. This method will dispatch callback events
// as needed before returning. The parent view's onTouchEvent implementation should call this.
//处理一个触摸事件被父类接收，这个方法在返回前将需要分发回调事件，父view的onTouchEvent实现应该在这里调用
mDragHelper.processTouchEvent(ev);
return true;
}

/**
* Adds a new {@link View} to the list of views that are draggable within the container.
* @param dragView the {@link View} to make draggable
* 增加一个新的view到view的列表中，增加的view可以在容器中被拖动
*/
public void addDragView(View dragView) {
mDragViews.add(dragView);
}

/**
* Sets the {@link DragFrameLayoutController} that will receive the drag events.
* @param dragFrameLayoutController a {@link DragFrameLayoutController}
*/
public void setDragFrameController(DragFrameLayoutController dragFrameLayoutController) {
mDragFrameLayoutController = dragFrameLayoutController;
}

/**
* A controller that will receive the drag events.
* 接受拖动事件的控制器
*/
public interface DragFrameLayoutController {

public void onDragDrop(boolean captured);
}
}