Android中如何自己创造一个Cursor及MatrixCursor源码分析
我们有时在处理数据时可能用Cursor比较方便。但我们如何自己创造一个Cursor呢？
这时我们可以用Android为我们提供的MatrixCursor类，自己创造一个Cursor。
下面通过一个事例来分析，如何创造一个Cursor作为SimpleCursorAdapter的数据来源。
##1、Activity布局
android:textSize="24sp"
android:gravity="center_horizontal"
/>
##2、ListView中的数据布局
##3、模拟从服务器获取数据
public class MyDataBase {
private static int count = 0;
public static String[]Column = null;
public static String[] getColumn(){
if(null == Column){
return new String[]{"姓名","班级","学号"};
}
return Column;
}
public static Object[] getColumnData(){
count++;
return new Object[]{new Integer(count),"zhangsan"+count,"jisuanji"+count,"201210"+count};
}
}
##4、Activity
public class MainActivity extends Activity {
private ListView listView;
private SimpleCursorAdapter sca;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
listView = (ListView) this.findViewById(R.id.listView);
Cursor cursor = getCursor();
String[] from = {"姓名","班级","学号"};
int[] to = {R.id.name,R.id.banji,R.id.studentID};
sca = new SimpleCursorAdapter(MainActivity.this, R.layout.adapter_view, cursor, from, to, 0);
listView.setAdapter(sca);
}
private Cursor getCursor() {
String[] columnNames = new String[4];
int i = 0;
columnNames[i] = "_id";
for(String columnName : MyDataBase.getColumn()){
columnNames[++i] = columnName;
}
MatrixCursor cursor = new MatrixCursor(columnNames);
for(int j = 0; j<10; j++){
cursor.addRow(MyDataBase.getColumnData());
}
return cursor;
}
}
##5、运行结果
##6、MatrixCursor源码分析
其实创造一个Cursor很简单！！！我们现在需要去查看源码分析下，android工程师的大牛们是如何实现MatrixCursor的。
/**
* A mutable cursor implementation backed by an array of {@code Object}s. Use
* {@link #newRow()} to add rows. Automatically expands internal capacity
* as needed.
*/
public class MatrixCursor extends AbstractCursor {
private final String[] columnNames;
private Object[] data;
private int rowCount = 0;
private final int columnCount;
/**
* Constructs a new cursor with the given initial capacity.
*
* @param columnNames names of the columns, the ordering of which
* determines column ordering elsewhere in this cursor
* @param initialCapacity in rows
*/
public MatrixCursor(String[] columnNames, int initialCapacity) {
this.columnNames = columnNames;
this.columnCount = columnNames.length;
if (initialCapacity < 1) {
initialCapacity = 1;
}
this.data = new Object[columnCount * initialCapacity];
}
/**
* Constructs a new cursor.
*
* @param columnNames names of the columns, the ordering of which
* determines column ordering elsewhere in this cursor
*/
public MatrixCursor(String[] columnNames) {
this(columnNames, 16);
}
/**
* Gets value at the given column for the current row.
*/
private Object get(int column) {
if (column < 0 || column >= columnCount) {
throw new CursorIndexOutOfBoundsException("Requested column: "
+ column + ", # of columns: " + columnCount);
}
if (mPos < 0) {
throw new CursorIndexOutOfBoundsException("Before first row.");
}
if (mPos >= rowCount) {
throw new CursorIndexOutOfBoundsException("After last row.");
}
return data[mPos * columnCount + column];
}
/**
* Adds a new row to the end and returns a builder for that row. Not safe
* for concurrent use.
*
* @return builder which can be used to set the column values for the new
* row
*/
public RowBuilder newRow() {
rowCount++;
int endIndex = rowCount * columnCount;
ensureCapacity(endIndex);
int start = endIndex - columnCount;
return new RowBuilder(start, endIndex);
}
/**
* Adds a new row to the end with the given column values. Not safe
* for concurrent use.
*
* @throws IllegalArgumentException if {@code columnValues.length !=
* columnNames.length}
* @param columnValues in the same order as the the column names specified
* at cursor construction time
*/
public void addRow(Object[] columnValues) {
if (columnValues.length != columnCount) {
throw new IllegalArgumentException("columnNames.length = "
+ columnCount + ", columnValues.length = "
+ columnValues.length);
}
int start = rowCount++ * columnCount;
ensureCapacity(start + columnCount);
System.arraycopy(columnValues, 0, data, start, columnCount);
}
/**
* Adds a new row to the end with the given column values. Not safe
* for concurrent use.
*
* @throws IllegalArgumentException if {@code columnValues.size() !=
* columnNames.length}
* @param columnValues in the same order as the the column names specified
* at cursor construction time
*/
public void addRow(Iterable columnValues) {
int start = rowCount * columnCount;
int end = start + columnCount;
ensureCapacity(end);
if (columnValues instanceof ArrayList) {
addRow((ArrayList) columnValues, start);
return;
}
int current = start;
Object[] localData = data;
for (Object columnValue : columnValues) {
if (current == end) {
// TODO: null out row?
throw new IllegalArgumentException(
"columnValues.size() > columnNames.length");
}
localData[current++] = columnValue;
}
if (current != end) {
// TODO: null out row?
throw new IllegalArgumentException(
"columnValues.size() < columnNames.length");
}
// Increase row count here in case we encounter an exception.
rowCount++;
}
/** Optimization for {@link ArrayList}. */
private void addRow(ArrayList columnValues, int start) {
int size = columnValues.size();
if (size != columnCount) {
throw new IllegalArgumentException("columnNames.length = "
+ columnCount + ", columnValues.size() = " + size);
}
rowCount++;
Object[] localData = data;
for (int i = 0; i < size; i++) {
localData[start + i] = columnValues.get(i);
}
}
/** Ensures that this cursor has enough capacity. */
private void ensureCapacity(int size) {
if (size > data.length) {
Object[] oldData = this.data;
int newSize = data.length * 2;
if (newSize < size) {
newSize = size;
}
this.data = new Object[newSize];
System.arraycopy(oldData, 0, this.data, 0, oldData.length);
}
}
/**
* Builds a row, starting from the left-most column and adding one column
* value at a time. Follows the same ordering as the column names specified
* at cursor construction time.
*/
public class RowBuilder {
private int index;
private final int endIndex;
RowBuilder(int index, int endIndex) {
this.index = index;
this.endIndex = endIndex;
}
/**
* Sets the next column value in this row.
*
* @throws CursorIndexOutOfBoundsException if you try to add too many
* values
* @return this builder to support chaining
*/
public RowBuilder add(Object columnValue) {
if (index == endIndex) {
throw new CursorIndexOutOfBoundsException(
"No more columns left.");
}
data[index++] = columnValue;
return this;
}
}
// AbstractCursor implementation.
@Override
public int getCount() {
return rowCount;
}
@Override
public String[] getColumnNames() {
return columnNames;
}
@Override
public String getString(int column) {
Object value = get(column);
if (value == null) return null;
return value.toString();
}
@Override
public short getShort(int column) {
Object value = get(column);
if (value == null) return 0;
if (value instanceof Number) return ((Number) value).shortValue();
return Short.parseShort(value.toString());
}
@Override
public int getInt(int column) {
Object value = get(column);
if (value == null) return 0;
if (value instanceof Number) return ((Number) value).intValue();
return Integer.parseInt(value.toString());
}
@Override
public long getLong(int column) {
Object value = get(column);
if (value == null) return 0;
if (value instanceof Number) return ((Number) value).longValue();
return Long.parseLong(value.toString());
}
@Override
public float getFloat(int column) {
Object value = get(column);
if (value == null) return 0.0f;
if (value instanceof Number) return ((Number) value).floatValue();
return Float.parseFloat(value.toString());
}
@Override
public double getDouble(int column) {
Object value = get(column);
if (value == null) return 0.0d;
if (value instanceof Number) return ((Number) value).doubleValue();
return Double.parseDouble(value.toString());
}
@Override
public byte[] getBlob(int column) {
Object value = get(column);
return (byte[]) value;
}
@Override
public int getType(int column) {
return DatabaseUtils.getTypeOfObject(get(column));
}
@Override
public boolean isNull(int column) {
return get(column) == null;
}
}
通过分析我们得知：
1：Android大牛们，通过在MatrixCursor中封装一个Object[]数组保存我们每一列的值；通过一个常引用String[] columnNames来保存我们的列名。所以一旦我们的MatrixCursor对象建立，它的列名就不可改变。我们以后的数据存放都要依据我们的列名。
2：MatrixCursor通过 int rowCount = 0 来记录行数、private final int columnCount 记录列数。通过这两个数我们可以很容易的知道每一行数据在Object[]数组的区间。
3：MatrixCursor通过实现AbstractCursor中的抽象方法；当我们获取数据时调用MatrixCursor中复写的方法，来获取指定的行列值。
获取值最重要的方法是：
private Object get(int column) {
if (column < 0 || column >= columnCount) {
throw new CursorIndexOutOfBoundsException("Requested column: "
+ column + ", # of columns: " + columnCount);
}
if (mPos < 0) {
throw new CursorIndexOutOfBoundsException("Before first row.");
}
if (mPos >= rowCount) {
throw new CursorIndexOutOfBoundsException("After last row.");
}
return data[mPos * columnCount + column];
}
无非就是两个值：mPos、column
mPos：代表了我们当前的要访问的行
column：代表我们当前要访问的列
###MatrixCursor内部类：RowBuilder
RowBuilder作用就是用来给MatrixCursor添加Row数据的，不过我们经常通过MatrixCursor.addRow()实现。就像AlertDialog.Builder一样，我们既可以通过AlertDialog.Builder来构建一个对话框，也可以通过AlertDialg直接来实现。