Android自定义视图教程

发表于 5年以前 | 总阅读数：1009 次

Android的UI元素都是基于View(屏幕中单个元素)和ViewGroup(元素的集合),Android有许多自带的组件和布局，比如Button、TextView、RelativeLayout。在app开发过程中我们需要自定义视图组件来满足我们的需求。通过继承自View或者View的子类，覆写onDraw或者onTouchEvent等方法来覆盖视图的行为。

创建完全自定义的组件

创建自定义的组件主要围绕着以下五个方面：

绘图(Drawing)：控制视图的渲染，通常通过覆写onDraw方法来实现
交互(Interaction)：控制用户和视图的交互方式，比如OnTouchEvent,gestures
尺寸(Measurement)：控制视图内容的维度，通过覆写onMeasure方法
属性(Attributes)：在XML中定义视图的属性，使用TypedArray来获取属性值
持久化(Persistence)：配置发生改变时保存和恢复状态，通过onSaveInstanceState和onRestoreInstanceState

举个栗子，假设我们想创建一个图形允许用户点击的时候改变形状（方形、圆形、三角形）。如下所示：

定义视图类

我们创建一个ShapeSelectorView继承自View，实现必要的构造器，如下所示：

public class ShapeSelectorView extends View {
      // We must provide a constructor that takes a Context and an AttributeSet.
      // This constructor allows the UI to create and edit an instance of your view.
      public ShapeSelectorView(Context context, AttributeSet attrs) {
        super(context, attrs);
        }
    }

添加视图到布局中

<RelativeLayout 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:layout_width="match_parent"
        android:layout_height="match_parent" >
        <com.codepath.example.customviewdemo.ShapeSelectorView
        android:id="@+id/shapeSelector"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentRight="true"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true" />
    </RelativeLayout>

接下来我们定义一个命名空间app,这个命名空间允许Android自动解析而不需要指定具体的包名。

自定义属性

视图可以通过XML来配置属性和样式，你需要想清楚要添加那些自定义的属性，比如我们想让用户可以选择形状的颜色、是否显示形状的名称，比如我们想让视图可以像下面一样配置：

<com.codepath.example.customviewdemo.ShapeSelectorView
        app:shapeColor="#7f0000"
        app:displayShapeName="true"
        android:id="@+id/shapeSelector"
        ... />

为了能够定义shapeColor和displayShapeName,我们需要在res/values/attrs.xml中配置：

<?xml version="1.0" encoding="utf-8"?>
    <resources>
       <declare-styleable name="ShapeSelectorView">
           <attr name="shapeColor" format="color" />
           <attr name="displayShapeName" format="boolean" />
       </declare-styleable>
    </resources>

对于每个你想自定义的属性你需要定义attr节点，每个节点有name和format属性，format属性是我们期望的值的类型，比如color,dimension,boolean,integer,float等。一旦定义好了属性，你可以像使用自带属性一样使用他们，唯一的区别在于你的自定义属性属于一个不同的命名空间，你可以在根视图的layout里面定义命名空间，一般情况下你只需要这样子指定：http://schemas.android.com/apk/res/<package_name>,但是你可以使用http://schemas.android.com/apk/res-auto自动解析命名空间。

<?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >
        <com.codepath.example.customviewdemo.ShapeSelectorView
           app:shapeColor="#7f0000"
           app:displayShapeName="true"
           android:id="@+id/shapeSelector"
           android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:layout_above="@+id/btnSelect"
           android:layout_alignParentLeft="true"
           android:layout_below="@+id/tvPrompt" />
    </RelativeLayout>

应用自定义属性

在前面我们定义了shapeColor和displayShapeName两个属性值，我们需要提取这两个属性值来用在自定义的视图中，可以使用TypedArray和obtainStyledAttributes方法来完成，如下所示：

public class ShapeSelectorView extends View {
      private int shapeColor;
      private boolean displayShapeName;

      public ShapeSelectorView(Context context, AttributeSet attrs) {
        super(context, attrs);
        setupAttributes(attrs);
      }

      private void setupAttributes(AttributeSet attrs) {
        // Obtain a typed array of attributes
        TypedArray a = getContext().getTheme().obtainStyledAttributes(attrs, R.styleable.ShapeSelectorView, 0, 0);
        // Extract custom attributes into member variables
        try {
          shapeColor = a.getColor(R.styleable.ShapeSelectorView_shapeColor, Color.BLACK);
          displayShapeName = a.getBoolean(R.styleable.ShapeSelectorView_displayShapeName, false);
        } finally {
          // TypedArray objects are shared and must be recycled.
          a.recycle();
        }
      }
    }

接下来添加一些getter和setter方法：

public class ShapeSelectorView extends View {
      // ...
      public boolean isDisplayingShapeName() {
        return displayShapeName;
      }

      public void setDisplayingShapeName(boolean state) {
        this.displayShapeName = state;
        invalidate();
        requestLayout();
      }

      public int getShapeColor() {
        return shapeColor;
      }

      public void setShapeColor(int color) {
        this.shapeColor = color;
        invalidate();
        requestLayout();
      }
    }

当视图属性发生改变的时候可能需要重新绘图，你需要调用invalidate()和requestLayout()来刷新显示。

画图

假设我们要使用前面的属性画一个长方形，所有的绘图都是在onDraw方法里执行，使用Canvas对象来绘图，如下所示：

public class ShapeSelectorView extends View {
      // ...
      private int shapeWidth = 100;
      private int shapeHeight = 100;
      private int textXOffset = 0;
      private int textYOffset = 30;
      private Paint paintShape;

      // ...
      public ShapeSelectorView(Context context, AttributeSet attrs) {
        super(context, attrs);
        setupAttributes(attrs);
        setupPaint();
      }

      @Override
      protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawRect(0, 0, shapeWidth, shapeHeight, paintShape);
        if (displayShapeName) {
          canvas.drawText("Square", shapeWidth + textXOffset, shapeHeight + textXOffset, paintShape);
        }
      }

      private void setupPaint() { 
          paintShape = new Paint();
          paintShape.setStyle(Style.FILL);
          paintShape.setColor(shapeColor);
          paintShape.setTextSize(30);
       }
    }

这段代码就会根据XML里设置的shapeColor来画图，根据displayShapeName属性来决定是否显示图形的名称，结果如下图：

更多画图的教程可以参考这里 Custom 2D Drawing Tutorial

计算尺寸

为了更好的理解自定义视图的宽度和高度，我们需要定义onMeasure方法，这个方法根据视图的内容来决定它的宽度和高度，在这里宽度和高度是由形状和下面的文本决定的，如下所示：

public class ShapeSelectorView extends View {
      @Override
      protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // Defines the extra padding for the shape name text
        int textPadding = 10;
        int contentWidth = shapeWidth;

        // Resolve the width based on our minimum and the measure spec
        int minw = contentWidth + getPaddingLeft() + getPaddingRight();
        int w = resolveSizeAndState(minw, widthMeasureSpec, 0);

        // Ask for a height that would let the view get as big as it can
        int minh = shapeHeight + getPaddingBottom() + getPaddingTop();
        if (displayShapeName) { 
        minh += textYOffset + textPadding;
        }
        int h = resolveSizeAndState(minh, heightMeasureSpec, 0);

        // Calling this method determines the measured width and height
        // Retrieve with getMeasuredWidth or getMeasuredHeight methods later
        setMeasuredDimension(w, h);
      }
    }

宽度和高度都是基于MeasureSpec来讨论的，一个MeasureSpec封装了父布局传递给子布局的布局要求，每个MeasureSpec代表了一组宽度和高度的要求。一个MeasureSpec由大小和模式组成。它有三种模式：UNSPECIFIED(未指定),父元素未给子元素施加任何束缚，子元素可以得到任意想要的大小；EXACTLY(完全)，父元素决定子元素的确切大小，子元素将被限定在给定的边界里而忽略它本身大小；AT_MOST(至多)，子元素至多达到指定大小的值。resolveSizeAndState()方法根据视图想要的大小和MeasureSpec返回一个合适的值，最后你需要调用setMeasureDimension()方法生效。

不同形状之间切换

如果想实现用户点击之后改变形状，需要在onTouchEvent方法里添加自定义逻辑：

public class ShapeSelectorView extends View {
      // ...
      private String[] shapeValues = { "square", "circle", "triangle" };
      private int currentShapeIndex = 0;

      // Change the currentShapeIndex whenever the shape is clicked
      @Override
      public boolean onTouchEvent(MotionEvent event) {
        boolean result = super.onTouchEvent(event);
        if (event.getAction() == MotionEvent.ACTION_DOWN) {
          currentShapeIndex ++;
          if (currentShapeIndex > (shapeValues.length - 1)) {
        currentShapeIndex = 0;
          }
          postInvalidate();
          return true;
        }
        return result;
      }
    }

现在不管什么时候视图被单击，选择的形状的下标会改变，调用postInvalisate()方法后会显示一个不同的形状，接下来更新onDraw()方法来实现更改形状的逻辑：

public class ShapeSelectorView extends View {
      // ...

      @Override
      protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        String shapeSelected = shapeValues[currentShapeIndex];
        if (shapeSelected.equals("square")) {
          canvas.drawRect(0, 0, shapeWidth, shapeHeight, paintShape);
          textXOffset = 0;
        } else if (shapeSelected.equals("circle")) {
          canvas.drawCircle(shapeWidth / 2, shapeHeight / 2, shapeWidth / 2, paintShape);
          textXOffset = 12;
        } else if (shapeSelected.equals("triangle")) {
          canvas.drawPath(getTrianglePath(), paintShape);
          textXOffset = 0;
        }
        if (displayShapeName) {
          canvas.drawText(shapeSelected, 0 + textXOffset, shapeHeight + textYOffset, paintShape);
        }
      }

      protected Path getTrianglePath() {
        Point p1 = new Point(0, shapeHeight), p2 = null, p3 = null;
        p2 = new Point(p1.x + shapeWidth, p1.y);
        p3 = new Point(p1.x + (shapeWidth / 2), p1.y - shapeHeight);
        Path path = new Path();
        path.moveTo(p1.x, p1.y);
        path.lineTo(p2.x, p2.y);
        path.lineTo(p3.x, p3.y);
        return path;
      }

      // ...
    }

现在每次点击都会显示一个不同的形状，结果如下：

接下来添加一个获取形状的方法：

public class ShapeSelectorView extends View {
      // ...
      // Returns selected shape name
      public String getSelectedShape() {
        return shapeValues[currentShapeIndex];
      }
    }

保存视图的状态

当配置发生改变的时候(比如屏幕旋转)视图需要保存它们的状态，你可以实现onSaveInstanceState()和onRestoreInstanceState()方法来保存和恢复视图状态，如下所示：

public class ShapeSelectorView extends View {
      // This is the view state for this shape selector
      private int currentShapeIndex = 0;

      @Override
      public Parcelable onSaveInstanceState() {
        // Construct bundle
        Bundle bundle = new Bundle();
        // Store base view state
        bundle.putParcelable("instanceState", super.onSaveInstanceState());
        // Save our custom view state to bundle
        bundle.putInt("currentShapeIndex", this.currentShapeIndex);
        // ... store any other custom state here ...
        // Return the bundle
        return bundle;
      }

      @Override
      public void onRestoreInstanceState(Parcelable state) {
        // Checks if the state is the bundle we saved
        if (state instanceof Bundle) {
          Bundle bundle = (Bundle) state;
          // Load back our custom view state
          this.currentShapeIndex = bundle.getInt("currentShapeIndex");
          // ... load any other custom state here ...
          // Load base view state back
          state = bundle.getParcelable("instanceState");
        }
        // Pass base view state on to super
        super.onRestoreInstanceState(state);
      }
    }

一旦你实现了这些保存和恢复的逻辑，当手机配置改变的时候你的视图能够自动保存状态。