Question

所以我有点厌倦了一遍又一遍地重写相同的查询。

repo.Query().Where(stuff => stuff.Timestamp >= minTime && stuff.Timestamp <= maxTime && ...);

我认为我应该使用名为IQueryable的方法扩展IsInDateTimeRange，然后像这样使用

repo.Query().IsInDateTimeRange(stuff => stuff.Timestamp, minTime, maxTime) ...

这对于IEnumerable来说非常简单，只有Func<T, DateTime>和两个DateTime，但对于IQueryable我需要Expression和我不确定如何使用它。

这是我的尝试，但似乎没有起作用。

public static IQueryable<TValue> IsInDateTimeRange<TValue>(
        this IQueryable<TValue> self,
        Expression<Func<TValue, DateTime>> getMember,
        DateTime minTime,
        DateTime maxTime)
    {
        return self.Where(value => minTime >= getMember(value) && maxTime <= getMember(value));
    }

Answer 1

您可以通过基于传递给方法的属性访问表达式手动构建表达式来完成此操作：

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mChart = (BarChart) findViewById(R.id.bar_chart);
        mChart.setDrawBarShadow(false);
        mChart.getDescription().setEnabled(false);
        mChart.setPinchZoom(false);
        mChart.setDrawGridBackground(false);
        // empty labels so that the names are spread evenly  
        String[] labels = {"", "Name1", "Name2", "Name3", "Name4", "Name5", ""};
        IAxisValueFormatter xAxisFormatter = new LabelFormatter(mChart, labels);
        XAxis xAxis = mChart.getXAxis();
        xAxis.setCenterAxisLabels(true);
        xAxis.setPosition(XAxis.XAxisPosition.BOTTOM);
        xAxis.setDrawGridLines(false);
        xAxis.setGranularity(1f); // only intervals of 1 day
        xAxis.setTextColor(Color.BLACK);
        xAxis.setTextSize(12);
        xAxis.setAxisLineColor(Color.WHITE);
        xAxis.setAxisMinimum(1f);
        xAxis.setValueFormatter(xAxisFormatter);

        YAxis leftAxis = mChart.getAxisLeft();
        leftAxis.setTextColor(Color.BLACK);
        leftAxis.setTextSize(12);
        leftAxis.setAxisLineColor(Color.WHITE);
        leftAxis.setDrawGridLines(false);
        leftAxis.setGranularity(2);
        leftAxis.setLabelCount(8, true);
        leftAxis.setPosition(YAxis.YAxisLabelPosition.OUTSIDE_CHART);

        mChart.getAxisRight().setEnabled(false);
        mChart.getLegend().setEnabled(false);

        float[] valOne = {10, 20, 30, 40, 50};
        float[] valTwo = {60, 50, 40, 30, 20};

        ArrayList<BarEntry> barOne = new ArrayList<>();
        ArrayList<BarEntry> barTwo = new ArrayList<>();
        for (int i = 0; i < valOne.length; i++) {
            barOne.add(new BarEntry(i, valOne[i]));
            barTwo.add(new BarEntry(i, valTwo[i]));
        }

        BarDataSet set1 = new BarDataSet(barOne, "barOne");
        set1.setColor(Color.BLUE);
        BarDataSet set2 = new BarDataSet(barTwo, "barTwo");
        set2.setColor(Color.MAGENTA);

        set1.setHighlightEnabled(false);
        set2.setHighlightEnabled(false);
        set1.setDrawValues(false);
        set2.setDrawValues(false);

        ArrayList<IBarDataSet> dataSets = new ArrayList<IBarDataSet>();
        dataSets.add(set1);
        dataSets.add(set2);
        BarData data = new BarData(dataSets);
        float groupSpace = 0.4f;
        float barSpace = 0f;
        float barWidth = 0.3f;
        // (barSpace + barWidth) * 2 + groupSpace = 1
        data.setBarWidth(barWidth);
        // so that the entire chart is shown when scrolled from right to left
        xAxis.setAxisMaximum(labels.length - 1.1f);
        mChart.setData(data);
        mChart.setScaleEnabled(false);
        mChart.setVisibleXRangeMaximum(4f);
        mChart.groupBars(1f, groupSpace, barSpace);
        mChart.invalidate();

    }


    private class LabelFormatter implements IAxisValueFormatter {

        String[] labels;
        BarLineChartBase<?> chart;

        LabelFormatter(BarLineChartBase<?> chart, String[] labels) {
            this.chart = chart;
            this.labels = labels;
        }

        @Override
        public String getFormattedValue(float value, AxisBase axis) {
            return labels[(int) value];
        }
    }

使用IsInDateTimeRange扩展IQueryable

1 个答案: