編輯:關於Android編程
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); } }
添加視圖到布局中
<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>
接下來我們定義一個命名空間app,這個命名空間允許Android自動解析而不需要指定具體的包名。 自定義屬性 視圖可以通過XML來配置屬性和樣式,你需要想清楚要添加那些自定義的屬性,比如我們想讓用戶可以選擇形狀的顏色、是否顯示形狀的名稱,比如我們想讓視圖可以像下面一樣配置:
<com.codepath.example.customviewdemo.ShapeSelectorView app:shapeColor="#7f0000" app:displayShapeName="true" android:id="@+id/shapeSelector" ... />
為了能夠定義shapeColor和displayShapeName,我們需要在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>
對於每個你想自定義的屬性你需要定義attr節點,每個節點有name和format屬性,format屬性是我們期望的值的類型,比如color,dimension,boolean,integer,float等。一旦定義好了屬性,你可以像使用自帶屬性一樣使用他們,唯一的區別在於你的自定義屬性屬於一個不同的命名空間,你可以在根視圖的layout裡面定義命名空間,一般情況下你只需要這樣子指定:http://schemas.android.com/apk/res/<package_name>,但是你可以使用
<?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>
應用自定義屬性 在前面我們定義了shapeColor和displayShapeName兩個屬性值,我們需要提取這兩個屬性值來用在自定義的視圖中,可以使用TypedArray和obtainStyledAttributes方法來完成,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(); } } }
接下來添加一些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(); } }
當視圖屬性發生改變的時候可能需要重新繪圖,你需要調用invalidate()和requestLayout()來刷新顯示。 畫圖 假設我們要使用前面的屬性畫一個長方形,所有的繪圖都是在onDraw方法裡執行,使用Canvas對象來繪圖,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); } }
這段代碼就會根據XML裡設置的shapeColor來畫圖,根據displayShapeName屬性來決定是否顯示圖形的名稱,結果如下圖: 更多畫圖的教程可以參考這裡 Custom 2D Drawing Tutorial 計算尺寸 為了更好的理解自定義視圖的寬度和高度,我們需要定義onMeasure方法,這個方法根據視圖的內容來決定它的寬度和高度,在這裡寬度和高度是由形狀和下面的文本決定的,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); } }
寬度和高度都是基於MeasureSpec來討論的,一個MeasureSpec封裝了父布局傳遞給子布局的布局要求,每個MeasureSpec代表了一組寬度和高度的要求。一個MeasureSpec由大小和模式組成。它有三種模式:UNSPECIFIED(未指定),父元素未給子元素施加任何束縛,子元素可以得到任意想要的大小;EXACTLY(完全),父元素決定子元素的確切大小,子元素將被限定在給定的邊界裡而忽略它本身大小;AT_MOST(至多),子元素至多達到指定大小的值。resolveSizeAndState()方法根據視圖想要的大小和MeasureSpec返回一個合適的值,最後你需要調用setMeasureDimension()方法生效。 不同形狀之間切換 如果想實現用戶點擊之後改變形狀,需要在onTouchEvent方法裡添加自定義邏輯:
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; } }
現在不管什麼時候視圖被單擊,選擇的形狀的下標會改變,調用postInvalisate()方法後會顯示一個不同的形狀,接下來更新onDraw()方法來實現更改形狀的邏輯:
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; } // ... }
現在每次點擊都會顯示一個不同的形狀,結果如下: 接下來添加一個獲取形狀的方法:
public class ShapeSelectorView extends View { // ... // Returns selected shape name public String getSelectedShape() { return shapeValues[currentShapeIndex]; } }
保存視圖的狀態 當配置發生改變的時候(比如屏幕旋轉)視圖需要保存它們的狀態,你可以實現onSaveInstanceState()和onRestoreInstanceState()方法來保存和恢復視圖狀態,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); } }
一旦你實現了這些保存和恢復的邏輯,當手機配置改變的時候你的視圖能夠自動保存狀態。
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